Article

River flow trends in the west of Iran during the past 40 years: Impact of precipitation variability

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Abstract

A better understanding of hydrological changes associated with climate change can be achieved by analyzing the temporal trends in river discharge time series. We used the non-parametric Spearman's rho, Kendall's tau, Mann– Kendall and Theil–Sen approaches to evaluate annual and monthly trends in river flow data from 12 stations locat-ed in the mountainous western regions of Iran over a 40-year period spanning the hydrological years 1969–1970 to 2008–2009. The Pearson correlation analysis was also applied to explore the relationships between precipitation and river flow series. Using a 5% significance level, we found an evidence of downward trends in annual river dis-charge at Varayeneh, Omar-Abad, Saleh-Abad, Zehtaran and Bahadorbeig stations at the rates of 0.491, 1.00, 0.101, 0.044, and 0.020 m 3 /s per decade, respectively. These significant downward trends started in the hydrological years of, respectively. The greatest numbers of stations with significant river discharge trends occur in October and November, accounting for seven and six sta-tions respectively. Annual precipitation exhibited no strong directional trend, although the tendency was toward increasing annual precipitation. Monthly precipitation variability seems to be also increasing for most of the months but no overall trend pattern in monthly precipitation totals emerged. Correlation analyses indicated strong relationships between river discharge and precipitation at the annual scale and for most of the months.

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... Trend changes in precipitation, temperature, and river discharge patterns can directly affect water resources management, drinking water supplies, agriculture, hydrology, and natural environment (Bostan and Akyüre 2007). Application of statistical methods such as trend analysis and change point detection in temperature, precipitation, and discharge series has been widely used throughout the world (Ventura et al. 2002;Aziz and Burn 2006;Partal and Kahya 2006;Karpouzos et al. 2010;Zongxue et al. 2010;Tabari et al. 2012;Abghari et al. 2013). Lettenmaier et al. (1994) suggested that increasing trends in streamflow cannot be explained fully by observed trends in precipitation and temperature because of the effects of additional factors such as land use changes. ...
... Further, the magnitudes of the significant positive trends in annual T average , T max, and T min were (+) 0.412, (+) 0.452, and (+) 0.493 °C per decade, respectively. Abghari et al. (2013) have used nonparametric methods to detect annual and monthly trends of discharge data in northwest Iran and the results showed a statistically significant negative trend for the annual river discharge while no strong directional trend for the annual precipitation records. Roushangar and Alizadeh (2017) used the information theory and discrete wavelet transform in order to identifying the complexity of annual precipitation variation in Iran during 1960-2010 and obtained acceptable results to classification of the annual precipitation variation. ...
... In this area, spring is important the season due to providing the most runoff volume by snowmelt. Moreover, it is suggested that from standpoint of climate change, the mountain's watershed is the most vulnerable environments due to vulnerability to temperature changes which affect ice melt, snowfall, and rainfall (Birsan et al. 2005;Abghari et al. 2013). Other works throughout the world, for example, Guobin et al. (2004) in China and Kahya and Kalayci (2004) in Turkey are consistent with the results of current study. ...
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Trends of the three hydro-meteorological variables, including precipitation, temperature, and river discharge have been studied to detect the impacts of climate change on water resources in western Caspian Sea region, Iran. We used Kendall’s tau, Spearman’s rho, Sen’s slope estimator, and Mann–Kendall approaches to detecting the annual and seasonal trends over the period 1981–2010. The nonparametric tests detected statistically significant decreasing trends in all of the hydrometric stations. In general, magnitude of an annual downward trend in the hydrometric stations was 2.17 m³/s per decade at the 5% significance level. As the results of the Pettitt test for detecting abrupt changes (jumps), the change points (or trends started) in river flow series were found in all hydrometric stations in the years of 1994–1999 at the significance level of 0.05%. Meanwhile, the outputs exhibited that majority of the annual and seasonally precipitation trends were increasing in contrast to the river discharge trends. Furthermore, on average, magnitude of annual temperature increasing trend was found (+) 0.43 °C per decade at the 0.05 significant level. Abrupt changes in the annual temperature as same as river flow series were at the 1990s over the period 1981–2010. The Pearson correlation was used to explore the relationships between river flow, temperature, and precipitation time series. Finally, the high correlation was obtained from the area-averaged temperature and discharge series at the autumn and annual time series with the value of the (−) 59 and (−) 50%. It is concluded that decreasing the river discharge might be related to increasing temperature and decreasing precipitation and anthropogenic changes in the study area.
... Soltani et al. (2015) investigated climate extreme indices at 50 Iranian meteorological stations during 1975-2010 and determined significant trends at less than 15 % of the stations, which were not regionally systematic. Besides, there are other relevant studies on precipitation trends in Iran, e.g., Abghari et al. (2013), Shifteh Some'e et al. (2012), Tabari et al. (2012a), and Tabari et al. (2011). ...
... Performing trend analysis in 7-day high and low flows in Peninsular Malaysia during 1982-2006, Jha and Singh (2013 also showed increases in the low flows. For Iran, Abghari et al. (2013) analyzed temporal trends in river discharge and precipitation time series for a 40-year period after 1969 in western parts. They reported both increasing and decreasing trends in river discharge time series and their strong relationships with annual and monthly precipitation. ...
... Trend detection tests for hydro-meteorological time series can be classified as parametric or nonparametric (Chebana et al. 2013;Gocic and Trajkovic 2013;Ishak et al. 2013;Xu et al. 2010). Nonparametric tests do not need the time series to be normally distributed; hence, these tests are generally preferred to the parametric ones in scientific communities (Abghari et al. 2013;Gocic and Trajkovic 2013;López-Moreno et al. 2014). Accordingly, this study applied the Mann-Kendall (MK) nonparametric test for detecting statistically significant (p < 0.05) trends in the hydro-meteorological time series. ...
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This study analyses spatio-temporal trends in precipitation, temperature, and river discharge in the northeast of Iran during recent decades (1953–2013). The Pettitt, SNHT, Buishand, Box-Pierce, Ljung-Box, and McLeod-Li methods were applied to examine homogeneity in time series studied. The nonparametric Mann-Kendall and Sen’s slope estimator tests were used to detect possible significant (p < 0.05) temporal trends in hydrometeorological time series and their magnitude, respectively. For time series with autocorrelation, the trend-free pre-whitening (TFPW) method was used to determine significant trends. To explore spatial distributions of trends, their magnitudes were interpolated by the inverse distance whitening (IDW) method. Trend analysis shows that for daily, monthly, and annual precipitation time series, 12.5, 19, and 12.5 % of the stations revealed significant increasing trends, respectively. For mean temperature, warming trends were found at 38, 23, and 31 % of the stations on daily, monthly, and annual timescales, in turn. Daily and monthly river discharge decreased at 80 and 40 % of the stations. Overall, these results indicate significant increases in precipitation and temperature but decreases in river discharge during recent decades. Hence, it can be concluded that decreasing trends in river discharge time series over the northeast of Iran during 1953–2013 are in response to warming temperatures, which increase the rate of evapotranspiration. Differences between the results of our comprehensive large-scale study and those of previous researches confirm the necessity for more model-based local studies on climatic and environmental changes across the northeast of Iran.
... Tabari et al. [17] stated that mean, maximum and minimum temperatures experienced an increasing trend in south, west, and southwest of Iran. Abghari et al. [18] showed the decreasing trend of annual ow discharge during 1969{2009 in mountainous western regions of Iran. Khalili et al. [19] indicated that the annual and winter rainfalls experienced a decreasing trend during 1961-2010 in more than half of Iran (especially, south of Iran). ...
... These studies illustrate the increasing temperature and the decreasing ow discharge and precipitation in di erent regions of Iran. For example, the studies by Adib et al. [15], Somee et al. [16], Tabari et al. [17], Abghari et al. [18], Khalili et al. [19], and Zamani et al. [20] are worthy of closer consideration. ...
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This research evaluates effects of climatic change on future temperature, precipitation and flow discharge in the Karkheh watershed (a watershed in south west of Iran). For this purpose, it utilizes General Circulation Models (GCMs) and the non parametric Mann-Kendall (MK) trend test. Considered hydrometric station is the Jelogir station at the upstream of the Karkheh dam. Base time period is 1971-2014 and future time period is 2030- 2073 for prediction of meteorological and hydrometric phenomena in the Jelogir station. For GCM model, the Canadian Climate Change Scenarios Network (CCCSN) database represents data of HadCM3 model for A2 and B2 scenarios. For using in a watershed, this research applies SDSM downscaling model and introduces predicted precipitation and temperature of future time period to IHACRES model for prediction of flow discharge. Also the non parametric Mann-Kendall trend test and the Theil–Sen approach (TSA) estimator distinguishes trend of observed and predicted data. Results of scenarios A2 and B2 have not much difference. Different climatic scenarios show that temperature increases and precipitation and flow discharge decrease, also MK test and TSA estimator represent that slope of their variations will slow down in future and most of changes are related to winter and spring.
... There are several trend assessments approaches available in the literature. However, the Mann-Kendall test is the most widely used test for assessing the trends in hydroclimatic studies (Abghari et al. 2013;Piniewski et al. 2018). The Mann-Kendall test (Ahmed et al. 2017) is recommended by the World Meteorological Organization (WMO), which is often used as having several advantages ...
... This study projected water availability change at seasonal and annual scale at one of Fig. 3 Trends in the minimum annual streamflow at five study stations, following symbols: ***, **, and * represent significance at 90, 95, and 99% confidence levels the main river basins in India by revealing some practical insights for water managers and other stakeholders in order to rethink the water management policy in the Godavari river basin and beyond. The results indicate that there is a drastic decrease of water availability at some part of the Godavari basin; nevertheless, climate changes projection should be considered in the future research work in order to predict intensity, frequency and droughts severity accurately at a long-term window (Abghari et al. 2013). Several previous studies (Caloiero et al. 2018;Güçlü et al. 2019;Şen 2015) reported that trend identification constitutes one of the most substantial analyses in any water variability and climate change study. ...
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A better understanding of intra/inter-annual streamflow variability and trends enables more effective water resources planning and management for current and future needs. This paper investigates the variability and trends of streamflow data from five stations (i.e. Ashti, Chindnar, Pathgudem, Polavaram, and Tekra) in Godavari river basin, India. The streamflow data were obtained from the Indian Central Water Commission and cover more than 30 years of mean daily records (i.e. 1972–2011). The streamflow data were statistically assessed using Gamma, Generalised Extreme Value and Normal distributions to understand the probability distribution features of data at inter-annual time-scale. Quantifiable changes in observed streamflow data were identified by Sen’s slope method. Two other nonparametric, Mann–Kendall and Innovative Trend Analysis methods were also applied to validate findings from Sen’s slope trend analysis. The mean flow discharge for each month (i.e. January to December), seasonal variation (i.e. Spring, Summer, Autumn, and Winter) as well as an annual mean, annual maximum and minimum flows were analysed for each station. The results show that three stations (i.e. Ashti, Tekra, and Polavaram) demonstrate an increasing trend, notably during Winter and Spring. In contrast, two other stations (i.e. Pathgudem, Chindnar) revealed a decreasing trend almost at all seasons. A significant decreasing trend was observed at all station over Summer and Autumn seasons. Notably, all stations showed a decreasing trend in maximum flows; remarkably, Tekra station revealed the highest decreasing magnitude. Significant decrease in minimum flows was observed in two stations only, Chindnar and Pathgudem. Findings resulted from this study might be useful for water managers and decision-makers to propose more sustainable water management recommendations and practices.
... These trends help to explain why other studies have reported an increasing trend in drought conditions Bari Abarghouei et al. 2011;Golian et al. 2015;Nasri and Modarres 2009;Raziei et al. 2009). In addition, analyses of the long-term variability of river flows in southwestern and western Iran (e.g., Abghari et al. 2013;Masih et al. 2011) revealed significant upward and downward trends. ...
... They found that an increment in wintertime discharge is due to temperature changes, whereas a negative trend of low flows is attributed to the decline of precipitation in April and May. Furthermore, studying fluctuations of river flows in the western region of Iran (located at a higher latitude from the Dez River Basin) demonstrated downward trends of discharge in the studied rivers (Abghari et al. 2013). ...
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This paper investigated the variability of freshwater discharge (Q) in the Dez River Basin in Iran and its possible links to the North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO). Nonparametric statistical tests were applied to analyze fluctuations of discharge and teleconnection indexes. The results indicated statistically significant monotonic decreases of the discharge and peak flow (p-value <0.05) across most of the subbasins. Furthermore, a downward step change was detected in discharge time series around the mid-1990s from 14.7% to 43.3% with respect to long-term values. Similarly, almost concurrent shifts detected in the Oceanic Niño Index (ONI) (from +0.254 to -0.416 in 1998) and the NAO (from +0.196 to -0.155 in 1995) that suggest changing conditions toward more cold phases of ENSO and negative phases of NAO coincided with discharge and monthly peak flow depletion. Comparing the amount of discharges during different phases showed that Q(El Niño)>Q(neutral)>Q(La Niña) and Q(NAO+)>Q(NAO-). Furthermore, the Mann-Whitney U test verified statistically significant differences (p-value <0.05) between flows pertaining to different phases of both ENSO and NAO. Correlations between discharge and teleconnection indexes indicated that discharge is apparently affected by teleconnections considering the lag time.
... The advantages of the test are that it does not assume any distribution form in the data and it has similar power to its parametric competitors [13] . The World Meteorological Organization recommends the MK test and it has been widely used to assess the significance of trends in hydrometeorological time series [14][15][16][17] . ...
... In this paper, the runoff series for Lanzhou and Xiaheyan stations were restored in terms of water balance using the following formula: (17) where Q is the restored daily runoff; Q0 is the observed daily runoff; Q1 and Q2 are the inflow and outflow of Longyangxia Reservoir; Q3 and Q4 are the inflow and outflow of Liujiaxia Reservoir; and ΔQ is a correction value. ...
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River runoff is an important link in the water cycle that can be adversely affected by the construction of reservoirs. Based on the runoff data from 1950 to 2009 at eight stations in the upper Yellow River, this study investigated on the impacts of the combined operation of Longyangxia Reservoir and Liujiaxia Reservoir on river runoff. The measured series of runoff data for the period after the commencement of combined operation of two reservoirs were restored by adjusting for water balance. The intra-annual characteristics of observed and restored runoff at Lanzhou station and Xiaheyan station were analyzed using Mann-Kendall test and wavelet analysis. There was a significantly downward trend in both runoff series, with abrupt changes in 1986 in the observed data and in 1989 in the restored data. Periodic oscillations in the observed and restored runoff series occurred at temporal scales of 3–5-year, 6–9-year, 14–19-year, and 22–27-year. The observed runoff series at Lanzhou station and Xiaheyan station had periodicities of 25-year, 16-year, 8-year, and 4-year. The restored runoff series at Lanzhou station had periodicities of 23-year, 8-year, and 4-year, and at Xiaheyan station had periodicities of 24-year, 4-year, and 8-year. The results show that the combined operation of Longyangxia Reservoir and Liujiaxia Reservoir contributed to the decreasing trend of river runoff, and leaded to runoff abrupt changes and alterations in the periodic variation.
... Numerous studies have examined precipitation and flow rate changes in different sub-basins of Iran (Abghari et al. 2013;Tehrani et al. 2019;Masih et al. 2011;Khalili et al. 2016). However, there are few studies that address the issue of changes within a major catchment area. ...
... To better understand hydrological changes, the main cause of which is climate change such as changes in regional rainfall, time series analysis can use (Abghari et al. 2013). values. ...
Article
One of the characteristics of arid and semi-arid regions is low rainfall and lack of uniform distribution throughout the year, which has a direct effect on water resources in these regions. In this research, daily precipitation and flow rate data of 39 meteorological stations and 9 hydrometric stations in the period 1994–2015 have been used. To evaluate seasonal and annual changes, the average data was calculated, and also MK and SQMK tests were used to detect the type of trend and mutations of changes. Sen slope, correlation, and Pettitt tests were used to determine the slope of changes, type of relationship, and also to determine the breaking point in the data time series, respectively. The results show that in all seasons, Tashk, Bakhtegan, and Maharloo sub-basins had the highest average rainfall; however, flow rate and precipitation changing curve in this region has decreased. The time-changing curve in precipitation is decreasing in winter and spring and increasing in summer and autumn. Thus, the spatial study of the udic moisture regime shows an increase in systems' activity that affects the region in the hot (summer) and cold (autumn) seasons. The trend of leap changes in winter and summer (autumn) in 7 sub-basins with 95% confidence level has been decreasing (increasing) which has decreased (increased) the river water flow in these areas. The highest percentage of decreasing and increasing changes in precipitation is in region 9 with values of − 6.66 and 2.55. The trend of leap changes in annual precipitation and flow rate in 5 sub-basins with a 95% confidence level has been reduced. The decrease in rainfall in areas 8 and 9 have had a direct effect on flow rate and has caused a significant decrease. The correlation between rainfall and flow is positive in all sub-basins and the highest coefficient of determination is in regions 3 and 9 with 40%.
... Other studies have assessed the trend in hydrological (mostly streamflow) and meteorological variables to assess the impacts of climate change (variations) on hydrological variables. In this regard, Abghari et al. (2013) analysed the trend of monthly precipitation and streamflow in the west of Iran (Hamedan province), and revealed that there is no overall trend in precipitation in studied stations but a significant decreasing trend of 0.020 to 0.491 m 3 /s per decade in streamflow during 1969 to 2009. They concluded thatt this pattern is more likely caused by human-induced activities such as landuse and land-cover changes. ...
... Since the hydrological processes in mountainous areas are more sensitive than in other areas to temperature (Birsan et al. 2005), the large Sen's slopes in streamflow for some parts of the Alborz and Zagros mountains can be explained by increased temperature (Fig. 4a). In line with our results, Abghari et al. (2013) analysed the trends in monthly streamflow at 12 stations in Iran between 1970 and 2009, and stated that the largest negative trends are seen in the mountainous regions in western Iran. Figure 5 shows the correlations between streamflow and precipitation, and between streamflow and temperature. Unsurprisingly, there is a high correlation between streamflow and precipitation in most cases. ...
Article
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In this study, we assessed the impacts of climate variations on streamflow in 28 near-natural catchments in Iran. To this end, we analyzed the trend of annual streamflow, precipitation, and temperatures using the Mann-Kendall test and Sen’s slope between water years 1982 and 2011. We evaluated the frequency of precipitation in different classes, i.e., below 5, 5-10, 10-15, and above 15 mm/day. Our results indicate a decline in streamflow at 25 catchments with a rate of -5.66 to -0.19 mm/year. The annual precipitation amount had not decreased significantly, while the frequency of light precipitation events (<5 mm/day) increased. About 60% of the upward trend was significant. Mean temperature increased at all studied catchments with an average rate of 0.055 ℃/year. In short, our results indicate that increases in temperature and the frequency of light precipitation events are two leading factors of streamflow reduction across studied catchments.
... However, studies mostly focused on precipitation variability (Demroes et al. 1998;Dinpashoh et al. 2004;Alijani 2002) and trends detections in the observed climatic data in Iran (Modarres and da Silva 2007). Only few studies have investigated the link between streamflow trends and climate changes (Masih et al. 2011;Abghari et al. 2013;Zamani et al. 2016;Patel et al. 2020), and mostly focused on precipitation-streamflow relationship and ignore the role of temperature and it's consequent effect i.e., evaporation on streamflow, While evaporation is a big portion of water losses in arid and semi-arid areas such as Dez Basin. In Iran, approximately 70 percent of precipitation delivered is lost to evapotranspiration. ...
... These might result in hydrological alteration of runoff production processes and river flow regime. This is in agreement with the results reported in Masih et al. (2011) and Abghari et al. (2013) studies in Karkheh Basin and Hamedan province where they highlighted a higher impact of climate change on streamflow in comparison to other stressors. ...
Article
It is important to assess possible temporal trends in streamflow discharge and the factors influencing it for optimum decisions in regional water resources planning. This study aims to investigate the possible trends in river discharge, land-use changes, and climate factors and their possible linkages. Therefore, streamflow records from eight sites of the Dez River Basin, Iran, were analyzed by a modified Mann–Kendall test to examine possible trends in annual and monthly time series of streamflow. The results showed significant negative trends in the streamflow time series of the warm months (April, May, June, July, August, and September) over 34 year-long records. Streamflow reductions in sub-basins that were unaffected by irrigation indicated that land-use change cannot cause the observed trend. Although no significant trends were observed statistically for precipitation data, streamflow reductions possibly caused by overall reductions in precipitation. Besides that, there were significant increasing trends in temperature and evaporation, especially in summer season. The Pearson correlation analysis was applied to explore the relationships between evaporation and streamflow time series during the warm months (May, June, July, August, and September). The results indicated strong correlations between river discharge and evaporation at the monthly scale for six of eight sites. Besides the powerful effects of temperature on water losses through evaporation, changes in temperature also can cause some changes in precipitation forms and patterns as well as basin responses. This highlighted the role of changes in temperature that intensifies the effect of overall decreasing changes in precipitation on the streamflow trends.
... Farhangi et al. (2016) revealed a decreasing trend in mean annual precipitation series at six out of ten stations located in western Iran. The temporal changes in precipitation and other climatic variables in Iran have also been found in several studies (e.g., Rahimzadeh et al. 2009;Sheikha and Bahremanda 2011;Dinpashoh et al. 2011;Tabari et al. 2011;Kousari et al. 2011;Tabari et al. 2012a;Tabari and Aghajanloo 2013;Tabari et al. 2014;Abghari et al. 2013;Zarenistanak et al. 2015). ...
... In fact, the serial correlation significantly bears on trend detection methods (e.g., MK test) and their results. Accordingly, many recent studies have employed pre-whitened method to deal with serial correlation in meteorological parameters (e.g., Xu et al. 2010;Tabari and Hosseinzadeh Talaee 2011b;Masih et al. 2011;Zarenistanak et al. 2014b;Abghari et al. 2013;Arbabi Sabzevari et al. 2015). ...
Article
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Precipitation changes in annual and seasonal time series over the Alborz Mountains area during 1950–2014 were analyzed using historical observations from 154 rain gauge stations. The projected changes in precipitation for the twenty-first century were evaluated using three Coupled Model Intercomparison Project Phase 5 (CMIP5) datasets. Trends in the precipitation time series were detected by linear regression and its significance was tested by t test. Mann–Kendall rank test (MK test) and Sen’s slope estimator were also employed to confirm the results. Sequential Mann–Kendall test (SQ-MK test) was also applied for change point detection in annual and seasonal precipitation time series. Pre-whitening was used to eliminate the influence of serial correlation on the MK test. Future precipitation was analyzed by GFDL-CM3, HadGEM2-AO and MPI-ESM-MR models under two representative concentration pathways (RCP) RCP 4.5 and RCP 8.5 scenarios. The analysis of the historical precipitation series indicated an insignificant trend in the annual and seasonal series at most stations. The highest numbers of stations with negative significant trends occurred in winter and with positive significant trends in summer. The results of change point detection in annual and seasonal precipitation series show that most of the significant mutation points began in the 1970s. The future projections showed that precipitation may decrease according to most of the models under the RCP 4.5 and RCP 8.5 scenarios, while the decrease may not be large, except in the summer season for the end of this century.
... Coch and Mediero (2016) researched trends in low flows collected at 60 gauge stations in Spain, and their findings revealed a clear general pattern of decreasing trends. There are many other studies, such as: Lins and Slack (2005), Rice et al. (2015) and Ahn and Palmer (2016) who studied rivers in the USA; Zhang et al. (2006), Zhao et al. (2010), Hu et al. (2011), Zang and Liu (2013) and Song et al. (2015) for rivers in China; Petrow and Merz (2009) and Bormann et al. (2011) for German rivers; Panda et al. (2013) and Nune et al. (2014) for Indian rivers; Masih et al. (2011), Abghari et al. (2013) and Zamani et al. (2017) for rivers in Iran; Cunderlik and Ouarda (2009) and Burn and Whitfield (2016) for Canadian rivers; Lopez-Moreno et al. (2006) for Spanish rivers; and Kahya and Kalayci (2004), Topaloglu (2006), Gumus and Yenigun (2006) and Ceribasi and Dogan (2015) for rivers in Turkey. ...
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A comprehensive evaluation of trends in annual instantaneous maximum flows (AIMF) from 153 gauge stations located in 26 river basins in Turkey is presented. Two traditional non-parametric trend tests, the Mann-Kendall (MK) and Spearman’s rho (SR), are used to quantify the significance of trends, while Sen’s slope method is applied to determine the magnitude of trends. The traditional tests indicate that the AIMF records of 57 stations showed statistically decreasing trends, while those of six stations showed an increasing trend. Şen’s trend method, which provides more detailed assessment of the trends in different clusters (low, medium and high), was applied to the AIMF series and the results were compared with traditional tests. Şen’s trend method indicated that all flow clusters at nine stations have increasing or decreasing trends, although no significant trend was detected by the MK and SR tests.
... Salarijazi et al. (2012) investigated monthly, annual, and seasonal changes in the flow rate of SAVA River at two hydrometric stations using the Mann-Kendall test. Abghari et al. (2013) examined monthly and annual discharges of mountainous regions in the west of Iran over . They reported a significant decrease in October and November. ...
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In this study, the trend of precipitation and river flow rate changes in Lake Urmia basin (LUB) were investigated at two monthly and annual scales using the modified Mann-Kendall test (MMK) with complete removal of autocorrelation process. To this purpose, 42 rain gauge and 25 hydrometric stations were selected in Lake Urmia basin during the statistical period of 1984–2013. The results of studying the precipitation changes in the LUB indicated that this basin experienced a combination of decreasing and increasing trend during the studied statistical period. The decreasing precipitation trend was often observed at west and southwest stations of the basin. The increasing trend was observed in the southern and northeastern regions. The results of studying the changes in river flow rate of the studied stations in the LUB indicated that the trend of river flow rate is decreasing in most of the months. At an annual scale, the decreasing trend of river flow rate was also clearly observed across the basin at all stations. In addition, the decreasing trend of river flow rate is more severe in autumn and winter. The results of studying the time of changing point in the precipitation series showed that the time of changing point occurred between 1992 and 1998 at most of the stations that experienced a decreasing precipitation trend. The time of changing point of river flow rate in LUB occurred between 1994 and 1998. Two years after the reductions in precipitation, the decreasing trend of river flow rate occurred in the rivers.
... Pettitt's test is used for detecting the year at which the change was supposed to have occurred (Ghasemi 2015). All the above-mentioned trend tests were utilised to understand the trend in annual and monthly discharge time series in the mountainous western region of Iran (Abghari et al. 2013). Annual maximum daily discharge was also analysed for detecting trends (Yue et al. 2002). ...
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Measured streamflow and flood series of 43 gauging stations from 25 west flowing rivers in Kerala, India, were analysed for their descriptive characteristics to study their spatial and temporal variation. The spatial and temporal variations in streamflow are influenced by many factors including climatic and basin characteristics. Streamflow data from each station (length varies from 14 to 43 years) is analysed for their internal characteristics such as trend, stationarity, homogeneity, noise and periodicity to incorporate it in hydrological models, so that their predictions would be more accurate. The internal characteristics were studied along with the statistical analysis. For analysing each internal characteristic, more than one method of analysis has been used to have reliable result. The trend characteristic was analysed using Mann-Kendall (MK) test, Sen’s slope test, Spearman’s rank correlation coefficient and Pearson correlation coefficient methods. Stationarity characteristics have been tested using augmented Dickey-Fuller test (ADF), Phillips-Perron test and Kwiatkowski-Phillips-Schmidt-Shin (KPSS) test. For identifying homogeneous nature, Pettitt test, standard normal homogeneity (SNH) test, Buishand test and von Neumann tests were used. Noise in the streamflow was verified using Box-Peirce, Ljung-Box and McLeod-Li tests. From the study, it is found that the daily series is non-homogeneous, stationary data with white noise, whereas flood series shows mixed characteristics. Based on the variations in the time series, the daily streamflow and flood series are classified into different categories such as high, average, moderate, minor and no variation stations. In most of the river basins, daily flow shows average variation. Flood series shows average variation in 44% of stations and moderate variation in 28% of stations.
... Previously, several researchers have analyzed the discharge variation on watershed scale in different climatic and topographic regions of the world. Abghari et al. (2013) studied the trends in discharge in western Iran and found a decreasing trend in annual discharge. Tao et al. (2011) analyzed the variability of hydro-climatic variables in the Tarim Watershed in China and found significant increasing trends in annual discharge at most of the stations in the southern slope of Tianshan Mountain. ...
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This study was carried out to analyze the hydrological characteristics and assess the distinguished hydrological periods of Upper Indus Basin (UIB) Rivers of Pakistan. For this purpose, statistical analysis (variation coefficient, the auto-correlation coefficient, sequential Mann–Kendall’s test) and a proposed method for distinguishing hydrological periods (described in methodology section) were applied. The results revealed that all rivers reflect moderate variability. The results of auto-correlation displayed that the river flow observed at Astore gauging station only indicated independency, while for Gilgit, Hunza and Kachura guaging at Indus River exhibited 2, 2, 4-year lag. The mutation analysis indicated that after 1980, the change point occurred at all UIB rivers. During analysis, it was also observed that river regimes have the same hydrological periods (i.e., 4), but with different dates of occurrence. The Gilgit River showed a low high-flow hydrological period compared to Astore, Hunza and Kachora (Indus). This difference may be due to the river’s own area natural conditions. The current analysis may be helpful for planning and management of water resources, designing of hydraulic structures and to make better policies in response to agricultural water requirement downstream of UIB River.
... At the end, they found no parental distribution for the entire country. A number of statistical techniques have been applied to identify significant trends in climate variables using either parametric or non-parametric tests (e.g., Fathian et al., 2014, Tabari and Talaee, 2011, Modarres and Sarhadi, 2009, Abghari et al., 2013. east and Mediterranean Region to the south. ...
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The impacts of North Atlantic Oscillation (NAO) on hydro-climatic variables have been previously studied from different perspectives. This study, adds a new perspective, focuses on the impacts of NAO extreme phases on Western Turkish streamflow pattern. The average winter (DJF) NAO indices are obtained to determine the extreme values of NAO. A set of 12 stations with monthly streamflow data extending from 1970 to 2014 conducted in this analysis. Considering a specific multiplier of standard deviation as a measure for classifying extreme phases, a confidence interval of 90% has been selected as a criterion. For this purpose, the index values greater (lower) than the selected criteria are assumed to be a positive (negative) extreme phase. Based on this approach, the years 1989, 1992, 1993 and 2000 were chosen as positive extreme NAO phases as the years 1977, 1979 and 2010 were selected as negative extreme NAO phases. For all stations, monthly streamflow composites have been constructed according to these years to obtain annual cycles for the both NAO extreme phases to be compared with the historical seasonal cycle. The NAO signals are presented by a rate extracted from the three annual cycle characteristics. The outcomes of isohyet maps showed that Kucuk Menderes sub basin was most affected region in Western Turkey for each season. Almost all stations have experienced decrease of rates in the following seasons which reveals a weakness in the impact of extreme negative winter NAOI comparing positive period on western turkey streamflow.
... Anthropogenic global warming and its consequences, especially in the arid and semi-arid regions, received particular attention in recent years as many scholars documented the occurrence and dominance of droughts, a rise in the temperature, and an increase in the atmospheric water demand, accompanied by a reduction in the precipitation and runoff [1][2][3][4]. Additionally, in Iran, many studies support the fact that during recent decades the climate has experienced variations, mostly toward hot and dry conditions [5][6][7][8][9][10][11][12][13][14]. ...
Article
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In this paper, the impact of climate change on the climate and discharge of the Dez Dam Basin and the hydropower potential of two hydropower plants (Bakhtiari and Dez) is investigated based on the downscaled outputs of six GCMs (General Circulation Models) and three SRES (Special Report on Emission Scenarios) scenarios for the early, mid and late 21st century. Projections of all the scenarios and GCMs revealed a significant rise in temperature (up to 4.9 °C) and slight to moderate variation in precipitation (up to 18%). Outputs of the HBV hydrologic model, enforced by projected datasets, show a reduction of the annual flow by 33% under the climate change condition. Further, analyzing the induced changes in the inflow and hydropower generation potential of the Bakhtiari and Dez dams showed that both inflow and hydropower generation is significantly affected by climate change. For the Bakhtiari dam, this indicates a consistent reduction of inflow (up to 27%) and electricity generation (up to 32%). While, in the Dez dam case, the inflow is projected to decrease (up to 22%) and the corresponding hydropower is expected to slightly increase (up to 3%). This contrasting result for the Dez dam is assessed based on its reservoir and hydropower plant capacity, as well as other factors such as the timely releases to meet different demands and flow regime changes under climate change. The results show that the Bakhtiari reservoir and power plant will not meet the design-capacity outputs under the climate change condition as its large capacity cannot be fully utilized; while there is room for the further development of the Dez power plant. Comparing the results of the applied GCMs showed high discrepancies among the outputs of different models.
... Iran is located in the dessert-belt (Gat and Magaritz, 1980). As illustrated in (Abghari et al. , 2013). ...
Research
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Aquifer Vulnerability; a Review on studies in Iran
... Similarly, several studies have been conducted over Iran to analyze trends in temperature, precipitation (e.g., Ghahraman 2006;Tabari et al. 2011a, b;Karandish et al. 2017;Moghim 2018), evapotranspiration (e.g., Tabari et al. 2011a, b;Hosseinzadeh Talaee et al. 2014;Ghalami et al. 2021), aridity/moisture index (e.g., Tabari et al. 2014;Araghi et al. 2018;Nouri and Bannayan 2019), runoff/ streamflow (e.g., Abghari et al. 2013;Mousavi et al. 2019) as well as climate extremes (e.g. Zhang et al. 2005a;Rahimzadeh et al. 2009;Soltani et al. 2016; Alizadeh-Choobari and Najafi 2018; Rahimi and Hejabi 2018;Darand and Sohrabi 2018;Alavinia and Zarei 2021). ...
Article
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A comprehensive investigation of historical hydro-climatic changes at a county level is an essential prerequisite of developing any adaptation or mitigation strategies to deal with the destructive impacts of climate change. In this study, spatial distributions and trends in thirty-seven hydro-climatic mean and extreme indices across Iran were analyzed based on the state-of-the-art reanalysis datasets (ERA5-Land and AgERA5) at the county level from 1986 to 2015 using several nonparametric approaches such as multiple modified Mann–Kendall statistical tests and Sen’s Slope estimator. Their interannual oscillations were also examined using continuous wavelet transform to portray a complete picture of the temporal variability of the variables. The micro-scale of the reanalysis datasets enabled us to deal with the sparse distribution of weather stations in Iran, study various hydro-climatic variables that have received less attention, and subsequently provide a comprehensive, all-in-one reference for later policy actions and further studies. Analyzing diverse, intercorrelated variables provides a great opportunity to explore the driving forces of the temporal patterns. The results showed that trends in hydro-climatic mean and extreme indices highly depended on their geographical location and climate zones. The snow and the warm temperate climates encountered larger hydro-climatic changes compared to the arid and semi-arid climates zones. The most noticeable trends were found for temperature which significantly increased over all counties while drying trends were observed in precipitation, surface reservoir content, and runoff. Wind speed and surface albedo in most counties experienced upward and downward trends, respectively, whereas solar radiation, surface air pressure, and evaporation exhibited a high spatial variability. In terms of extreme events, Iran was faced with an increase in hot climate extremes and a decrease in cold and precipitation extremes. At last, the wavelet power spectrum analysis demonstrated the annual and seasonal dominant periods for precipitation and an annual dominant period for temperature, and there were were significant periodic variations at 2–3 years for hot and cold spell durations and intensity-based precipitation index and 4–6 years for consecutive dry and wet days. The annual fluctuation of precipitation reduced over the study period, but that of temperature remained almost constant.
... The annual trend in the Sefidchah and Gelvard sub-basins are downward (statistically significant), and in the Abelu subbasin there is a downward trend which is not significant. In contrast, Minaei and Irannezhad (2016) found no clear trend in annual streamflow in the northeast of Iran, but Abghari et al. (2013) reported significant changes in mean annual The first column of each station is the mean precipitation (mm). The second column of each station shows the Z statistic of the trend test. ...
Article
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Trend analysis of climate variables such as streamflow, precipitation, and temperature provides useful information for understanding the hydrological changes associated with climate change. In this study, a nonparametric Mann-Kendall test was employed to evaluate annual, seasonal, and monthly trends of precipitation and streamflow for the Neka basin in the north of Iran over a 44-year period (1972 to 2015). In addition, the Inverse Distance Weight (IDW) method was used for annual seasonal, monthly, and daily precipitation trends in order to investigate the spatial correlation between precipitation and streamflow trends in the study area. Results showed a downward trend in annual and winter precipitation (Z < −1.96) and an upward trend in annual maximum daily precipitation. Annual and monthly mean flows for most of the months in the Neka basin decreased by 14% significantly, but the annual maximum daily flow increased by 118%. Results for the trend analysis of streamflow and climatic variables showed that there are statistically significant relationships between precipitation and streamflow (p value < 0.05). Correlation coefficients for Kendall, Spearman’s rank and linear regression are 0.43, 0.61, and 0.67, respectively. The spatial presentation of the detected precipitation and streamflow trends showed a downward trend for the mean annual precipitation observed in the upstream part of the study area which is consistent with the streamflow trend. Also, there is a good correlation between monthly and seasonal precipitation and streamflow for all sub-basins (Sefidchah, Gelvard, Abelu). In general, from a hydro-climatic point of view, the results showed that the study area is moving towards a situation with more severe drought events.
... Analysis of predicted rainfall in the Gharesou River basin (western Iran; Zahabiyoun et al. 2013) indicated an increase of 19% in winter and a decrease in the other months, whereas the inter-model average in the present study showed an increase in winter and autumn for RCP4.5, an increase in winter and spring for RCP8.5, and a decrease during the other seasons. Abghari et al. (2013) studied the trends of observed river flow and precipitation in western Iran over a 40-year period. They reported downward trends of annual discharge for five stations, whereas annual precipitation indicated an insignificant trend. ...
Article
We evaluated the potential impacts of future land cover change and climate variability on hydrological processes in the Neka River basin, northern Iran. This catchment is the main source of water for the intensively cultivated area of Neka County. Hydrological simulations were conducted using the Soil and Water Assessment Tool. An ensemble of 17 CMIP5 climate models was applied to assess changes in temperature and precipitation under the moderate and high emissions scenarios. To generate the business-as-usual scenario map for year 2050 we used the Land Change Modeler. With a combined change in land cover and climate, discharge is expected to decline in all seasons except the end of autumn and winter, based on the inter-model average and various climate models, which illustrated a high degree of uncertainty in discharge projections. Land cover change had a minor influence on discharge relative to that resulting from climate change.
... The Mann-Kendall and modified Mann-Kendall trend tests are rank-based tests of randomness against monotonic trends (Burn et al. 2010;Abghari et al. 2013 Kibria et al. 2016). These are possibly the most widely used nonparametric tests for trend detection in hydrological and climatological studies (Mitof and Pravalie 2014;Jones et al. 2015). ...
Article
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The aim of the study is to investigate trends in selected hydroclimatic indices using novel and conventional tools, for future climate projections in the twenty-first century. Selected quasi-natural Norwegian and Polish catchments are used as a case study. The projected flows are provided by GR4J rainfall-runoff conceptual model, coupled with an ensemble of climate model projections from EURO-CORDEX initiative. The trends are analysed using conventional Mann–Kendall and modified Mann–Kendall statistical approaches, a time–frequency approach based on discrete wavelet transform (DWT) and the dynamic harmonic regression (DHR) method. Of all methods applied the DHR gives the most conservative trend estimates. Trends depend on the specific hydroclimatic character and flow regime of the catchment. The results confirmed that in catchments with a rainfall-driven flood regime, an increase in the amount of precipitation is followed by increased flows, with strong seasonal changes, whereas, in catchments with a snow-driven flood regime, despite an increase of mean annual flow, decrease in annual maximum flow is observed. Generally, positive trend is the most dominant in all catchments studied and the methods were consistent in detection of trend except in seasonal trend test.
... Long-term changes in hydrologic variables are of general interest in water resources management. Flow evaluations have been performed to understand climatic and anthropogenic drivers in North America (e.g., Bawden, Linton, Burn, & Prowse, 2014;Fox, Mongan, & Miller, 1990 Lacruz, Vicente-Serrano, López-Moreno, Morán-Tejeda, & Zabalza, 2012), and Asia (e.g., Abghari, Tabari, & Talaee, 2013;Chen & Grasby, 2009;Chen et al., 2014;He, Miao, & Shi, 2013;Jiang, Su, & Hartmann, 2007;Sharif, Archer, Fowler, & Forsythe, 2013). The Mann-Kendall (MK) test (Kendall, 1938;Mann, 1945), a common non-parametric statistical procedure to test the presence of monotonic trends, has been used extensively in hydrology trend studies (including the studies cited above) and is the primary tool employed in this analysis. ...
Article
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The San Francisco Bay-Delta estuary and its upstream watershed have been highly modified since exploration and settlement by Europeans in the mid-18th century. While these hydrologic alterations supported the growth of California's economy to the eighth largest in the world, they have been accompanied by significant declines in native aquatic species and subsequent efforts to reverse these declines through flow management. To inform ongoing deliberations on management of freshwater flows to the estuary, we examined a recent nine decade hydrologic record to evaluate seasonal and annual trends in reported Delta outflow. Statistically significant trends were observed in seasonal outflows, with decreasing trends observed in four months (February, April, May and November) and increasing trends observed in two months (July and August). Trend significance in early-to-mid autumn (September and October) is ambiguous due to uncertainty associated with in-Delta agricultural water use. In spite of increasing water use over the period examined, we found no statistically significant annual trend in Delta outflow, a result likely due to large inter-annual variability. Linkages between outflow trends and changes in upstream flows and coincident developments such as reservoir construction and operation, out-of-basin imports and exports, and expansion of irrigated agriculture are discussed. To eliminate inter-annual variability as a factor, change attribution is explored using modeled flows and fixed climatology in a companion paper.
... Previous studies have well examined the trends in stream flow and their driving factors at annual scales from the 1950s to 2005 [16]. However, stream flow changes were inconsistent among different months of a year due to the climate variability (e.g., changes in precipitation and evapotranspiration) and productive activities of humanity (e.g., irrigation, reservoir regulation) [17]. Annual stream flow changes would not well reflect the monthly stream flow changes if they did not follow the same pattern, and may mask some important changes in stream flow [18]. ...
Article
The Poyang Lake Basin has been suffering from severe water problems such as floods and droughts. This has led to great adverse impacts on local ecosystems and water resource utilization. It is therefore important to understand stream flow changes and their driving factors. In this paper, the dynamics of stream flow and precipitation in the Poyang Lake Basin between 1961 and 2012 were evaluated with the Mann-Kendall test, Theil-Sen approaches, Pettitt test, and Pearson's correlation. Stream flow was measured at the outlets of five major tributaries of Poyang Lake, while precipitation was recorded by fourteen meteorological stations located within the Poyang Lake Basin. Results showed that annual stream flow of all tributaries and the precipitation over the study area had insignificant (P > 0.1) temporal trends and change points, while significant trends and shifts were found in monthly scale. Stream flow concentration indices (SCI) at Waizhou, Meigang, and Wanjiabu stations showed significant (P < 0.05) decreasing trends with change points emerging in 1984 at Waizhou and 1978 at Wanjiabu, while there was no significant temporal trend and change point detected for the precipitation concentration indices (PCI). Correlation analysis indicated that area-average stream flow was closely related to area-average precipitation, but area-average SCI was insignificantly correlated with area-average PCI after change point (1984). El Niño/Southern Oscillation (ENSO) had greater impacts on stream flow than other climate indices, and La Niña events played a more important role in stream flow changes than EI Niño. Human activities, particularly in terms of reservoir constructions, largely altered the intra-annual distribution of stream flow but its effects on the amount of stream flow were relatively low. Results of this study provided a useful reference to regional water resource management and the prevention of flood and drought disasters.
... In the western part of the basin, although significant increases were observed in rainfall trends on annual and monthly scales, there has been a sharp decrease in the low flow rates implying excessive abstraction of surface water and groundwater resources to meet agricultural demands. These findings are consistent with the results of Abghari et al. (2013). They found that the annual average flow values of Omar-Abad, Saleh-Abad, Zehtaran, and Bahadorbeic stations (located in the western part of the basin) experienced a significant decrease at the probability level of 5% over the past four decades. ...
Article
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Low flow is very sensitive to climate change and human intervention, especially in arid regions. In this study, changes of the 7-day low flow along the most important rivers of Iran’s Namak Lake Basin were investigated using nonparametric (Mann-Kendall and modified-Mann-Kendall) tests. A significant diminishing trend was observed in 72.2% of stations during the period of 1970–2012. The northern part of the basin lacked a significant trend, while in other parts of the basin, the descending trend was distributed uniformly. On the other hand, the changes of the annual rainfall during this period showed no clear trend (a significant trend in 36% and non-significant trend in 64% of stations), and the identified pattern of its changes was complicated on the basin scale and during the year. On a monthly scale, a significant decreasing trend was observed in March as one of the most productive months of the year in 49% of the stations. In addition, rainfall reduction was significant (over 35%) over the past 15 years in more than 71% of the stations. Also, changes in the proportion of seasonal rainfall and rainfall regime were considerable. The share of winter and spring rainfall showed a diminishing trend in 90% and 82% of stations, respectively. Also, rainfall regime based on precipitation concentration index (PCI) revealed a tendency to disorder (in 53% of stations). The annual temperature and temperature of October and February indicated a strong ascending trend in 92%, 71%, and 64% of stations, respectively, which can be effective during snow melting in basins with snow-rainy regimes and increasing evapotranspiration. Groundwater level changes also showed that, in the studied plains, the average water table drawdown was between 0.31 and 1.33 m/year. Therefore, the observed trend of low flow rates in this basin reflects the impact of climate change, where both direct and indirect human interference has led to the exacerbation of this situation.
... There are several trend assessments approaches available in the literature. However, the Mann-Kendall test is the most widely used test for assessing the trends in hydro-climatic studies [18,[52][53][54][55][56]. The Mann-Kendall test [57] which is recommended by the World Meteorological Organization (WMO) often used as because it has several advantages: It does consider the data distribution, and it can cope with the outliers [58][59][60][61]. ...
Article
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Trend analysis of streamflow provides practical information for better management of water resources on the eve of climate change. Thus, the objective of this study is to evaluate the presence of possible trends in the annual, seasonal, maximum, and minimum flow of Yangtze River at Cuntan and Zhutuo stations in China for the period 1980 to 2015. The assessment was carried out using the Mann–Kendall trend test, and the innovative trend analysis, while Sen’s slope is used to estimate the magnitude of the changes. The results of the study revealed that there were increasing and decreasing trends at Cuntan and Zhutuo stations in different months. The mean annual flow was found to decrease at a rate of −26.76 m3/s and −17.37 m3/s at both stations. The minimum flow was found to significantly increase at a rate of 30.57 m3/s and 16.37 m3/s, at a 95% level of confidence. Maximum annual flows showed an increasing trend in both regions of the Yangtze River. On the seasonal scale, the results showed that stations are more sensitive to seasonal flow variability suggesting a probable flooding aggravation. The winter season showed an increasing flow trend, while summer showed a decreasing trend. The spring flow was found to have an increasing trend by the Mann–Kendall test at both stations, but in the Zhutuo Station, a decreasing trend was found by way of the innovative trend analysis method. However, the autumn flow indicated a decreasing trend over the region by the Mann–Kendall (MK) test at both stations while it had an increasing trend in Cuntan by the innovative trend analysis method. The result showed nonstationary increasing and decreasing flow trends over the region. Innovative trend analysis method has the advantage of detecting the sub-trends in the flow time series because of its ability to present the results in graphical format. The results of the study indicate that decreasing trends may create water scarcity if proper adaptation measures are not taken.
... Most studies which used MK1 test assumed that there is no serial dependence in the time series (Dinpashoh et al. 2014). However, few researchers considered the effect of lag-1 autocorrelation known as a modified form of MK1 test (Tabari et al. 2011;Abghari et al. 2013; named as MK-2 test by Kumar et al. (2009). Due to different time lags within observations of hydro-metrological series, application of MK-1 and MK-2 methods may not give valid results. ...
Article
In this study monthly, annual and seasonal groundwater variation trends of Warangal district (2000–2015) were examined for forty observation wells using four variations of non-parametric Mann Kendal (MK) methods. Magnitudes of trends were computed using the Sen’s slope estimator. Results conclude that on a monthly time series, three observation wells (31, 32 and 37) experienced significant positive trends (positive Z-statistics), whereas other wells experienced significant negative trends among forty. Wells 2, 17, 22, 33, 38, 39, 40 noticed strong significant negative trends. Using MK-1/MK-2/MK-3/MK-4 for monthly series, around 23/33/23/23% of cases exhibited positive trends (both significant and insignificant) and 77/67/77/77 % of cases exhibited negative trends. Median of trend slopes for groundwater levels (GWLs) were negative in monthly, seasonal and annual timescale. Seasonally trend slope was observed as a decreasing trend in the pre-monsoon period. Annually trend slope was varying approximately from −0.4 mm/year to +0.4 mm/year. ARIMA models were fitted for each well to predict the parameters of the models and to forecast (2015) using monthly GWLs from 2000 to 2014. ARIMA(2,1,2), ARIMA(2,0,1), ARIMA(4,0,0), ARIMA(1,1,1), ARIMA(1,0,1), ARIMA(1,0,2) and ARIMA(2,0,0) fitted for most of the Wells. Using these models GWLs were predicted and the performance of the predicted levels was analyzed using the correlation coefficient (R²) and Akaike Information Criteria (AIC) values.
... The Mann-Kendall non-parametric test was adopted in this work to assess the trends in precipitation and SPI aimed at testing the statistical distribution of the data records. There are several approaches to trend analyses available but the Mann-Kendall was the most preferred and used test for assessing the trends in hydro-climatic studies [38,47,48]. ...
... High variability of climate also caused many ecological regions and thus, the potential for a wide range of agriculture. Considering significance of precipitation in the country, a bulk amount of studies have been performed to evaluate the changes in different characteristics of precipitation in Iran (Javari 2001;Modarres 2007;Modarres and Sarhadi 2009;Nasri and Modarres 2009;Tabari et al. 2011;Kousari et al. 2011;Some'e et al. 2012;Abghari et al. 2013;Farajzadeh et al. 2014;Tabari et al. 2014;Abolverdi et al. 2014;Soltani et al. 2016;Balling et al. 2016;Javari 2016;Araghi et al. 2016;Najafi and Moazami 2016;Zarei and Eslamian 2016;Javari 2017;Alizadeh-Choobari and Najafi 2017;Asakereh 2017;Minaei and Irannezhad 2018;Zarenistanak 2018;Nazeri Tahroudi et al. 2019). Observed station data for various periods were used in most of those studies. ...
Article
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Although a significant number of studies have evaluated the trends in different characteristics of precipitation in Iran, the trends in precipitation indicators related to bioclimate are still not explored. The 0.5° spatial resolution gauge-based gridded monthly precipitation data of global precipitation climatology centre (GPCC) for the period 1901–2016 was used in this study for the evaluation of the geographical distribution of the trends of bioclimatic precipitation indicators of Iran. The trends in the indicators due to global warming were estimated using modified Mann-Kendall (MMK) trend test which can estimate unidirectional trend by separating the natural variability in climate. Obtained results were compared with that found using classical Mann-Kendall (MK) test. Besides, gridded temperature data of climate research unit (CRU) was used to identify the warm/cold periods at each grid point to assess the trends in precipitation during warm/cold periods, considering a wide spatial variation in the onset time of different seasons in Iran. The results revealed that many of the trends in some of the precipitation indicators obtained in earlier studies were due to natural fluctuation of climate. Annual precipitation in Iran was found decreasing only in the northwest semi-arid region at a rate of − 12.1 to − 14.05 mm/decade, while the precipitation in the wettest month was found increasing in a large area in the southwest semi-arid region at a rate of 3.1 to 5.3 mm/decade. The most significant changes were observed in precipitation seasonality, which was found to increase in 22.4% area, mostly in the central dry and northeast semi-dry regions and decrease in 11.3% area, mostly in the northern wetter region. The study indicates that the long-term natural variability in large-scale atmospheric phenomena that influences the precipitation of Iran may be the cause of many significant changes observed in precipitation in previous studies.
... Of course, this assessment is based exclusively on climate information, and there is no assessment of possible impact data. Nevertheless, recent studies in Iran have highlighted a certain decrease in water resources over the last few decades (Abghari et al. 2013;Fathian et al. 2014;Kazemzadeh and Malekian 2016;Masih et al. 2011), which, in addition to the changes in water demand, would be compatible with more dryness as a consequence of a higher AED. In any case, the results of our study clearly suggest that the dryness of the climate in the region has risen. ...
Article
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This study evaluates trends in climatic variables, atmospheric evaporative demand (AED) and meteorological drought between 1988 and 2018 over the southwest of Iran, where the Karkheh Basin is located. For this purpose, we used monthly time series of meteorological variables (precipitation, air temperature, wind speed and sunshine duration). The AED was estimated by the FAO-56 Penman-Monteith equation and drought severity by the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). There have been significant changes in the climate over the study domain, including a large increase in air temperature and wind speed and a decrease in relative humidity, which have driven a positive trend in the AED. The SPEI for the region has fallen, with the decrease being larger than the magnitude of change observed with the SPI. This suggests an increase in drought severity mostly associated with the rise in AED. The implications for water availability and climate change processes in the region are discussed.
... Also, in Iran, many studies have shown that warmer, drier climates increase the length of droughts and reduce the amount and intensity of rainfall. In Iran, most research on the pattern of essential variables in the hydrological cycle, such as temperature, evaporation, transpiration, and precipitation, indicates an increase in temperature J o u r n a l P r e -p r o o f and water requirements and a decrease in relative humidity and rainfall [1]. These findings are consistent with the results of other studies that have shown an increase in drought in recent years, as well as a study of the flow trend [2,3]. ...
Article
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This study has led to changes in the Dez Dam water basin area in recent decades. Non-parametric Mann-Kendal trend test and two shift change point detection tests of Pettit and Buishand were applied to the discharge time-series at the outlet of Tire, Marbore, Sazar, and Bakhtiari sub-basins to identify monotonic and abrupt changes. The Mann-Kendall test showed a significant negative (decreasing) trend of the flow in three sub-basins. Investigation of the flow of Dez Basin in the past decades shows significant monotonic and abrupt changes, which are mostly toward decreasing the basin’s potential runoff. Considering this evidences, it is likely that the basin faces discharge reduction, and results emphasize on modification of water management strategies to adapt to climate change.
... Precipitation variability matters for societal risks and impacts on all time scales: months or longer for droughts affecting crop yields, weeks for large-scale flooding, and hours or shorter for severe storms that produce field-level crop damage or loss of life. Studies suggest that increases in interannual precipitation variability may impact food security by altering streamflow (Abghari et al. 2013), snowpack (Hamlet et al. 2005;Gornall et al. 2010), drought incidence Barlow et al. (2001), and crop yields (Riha et al. 1996), while many extreme weather events are driven by short-time scale patterns (Vitart et al. 2012). On all time scales, variability changes are especially important for threshold-defined extremes, whose frequency is more sensitive to changes in the variability of a distribution than its mean (Katz and Brown 1992). ...
Article
Changes in precipitation variability can have large societal consequences, whether at the short timescales of flash floods or the longer timescales of multi-year droughts. Recent studies have suggested that in future climate projections, precipitation variability rises more steeply than does its mean, leading to concerns about societal impacts. This work evaluates changes in mean precipitation over a broad range of spatial and temporal scales using a range of models from high-resolution regional simulations to millennial-scale global simulations. Results show that changes depend on the scale of aggregation and involve strong regional differences. On local scales that resolve individual rainfall events (hours and tens of kilometers), changes in precipitation distributions are complex and variances rise substantially more than means, as is required given the well-known disproportionate rise in precipitation intensity. On scales that aggregate across many events, distributional changes become simpler and variability changes smaller. At regional scale, future precipitation distributions can be largely reproduced by a simple transformation of present-day precipitation involving a multiplicative shift and a small additive term. The “extra” broadening is negatively correlated with changes in mean precipitation: in strongly “wetting” areas, distributions broaden less than expected from a simple multiplicative mean change; in “drying” areas, distributions narrow less. Precipitation variability changes are therefore of especial concern in the subtropics, which tend to dry under climate change. Outside the tropics, variability changes are similar on timescales from days to decades, i.e. show little frequency dependence. This behavior is highly robust across models, suggesting it may stem from some fundamental constraint.
... Since streamflow is very sensitive to small-scale precipitation variability (Cigizoglu et al., 2005;Lobanova et al., 2018), it might also alter trends' altering. According to Abghari et al. (2013); Afshar et al. (2017), studying the trends of streamflow, therefore, is an important rudimentary for stakeholders and decision-makers to decide long-term management strategies. Owing to these issues, the streamflow information from various rivers worldwide was collected, including the Mekong River. ...
Article
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Precipitation variation always affects the river streamflow, and the change in discharge significantly impacts aquatic habitats, environmental amenities, recreational opportunities, irrigated agriculture production, hydropower production, and other industrial water uses. Therefore, studying the streamflow trends is essential for stakeholders and decision-makers to make a long-term and suitable management strategy. This study aimed to analyze the trend of monthly streamflow in the Prek Thnot River of the Lower Mekong Basin. Daily streamflow data of four stations recorded in the Prek Thnot River between 1997 and 2011 were assessed using the Mann-Kendall (MK) test and Sen’s slope stimator to evaluate the statistical characteristics of streamflow distributions for the trend analysis. The result highlight that these statistics of the streamflow were observed that monthly flow begins to increase from May to October and rapidly decreases from November to April. The rainy season of streamflow is higher than the dry season. From the statistical analysis in the Prek Thnot River Basin, the streamflow trend slightly decreased from 1997 to 2011. Furthermore, the annual peak and low flows occurred in 2001 and 2005, approximately 83 and 11.5 m3/s, respectively. Results Mann-Kendall test of monthly streamflow were revealed only at the significant level of 0.1 in May for Tasal Dam, while the slope magnitude estimated by Sen's Slope estimator is about 0.45. The slope was positive during the early rainy season, indicating that the monthly streamflow quantity increased during this period. The tests also revealed that most months were no trend evidence and had a sign fluctuation of streamflow throughout the period.
... Assessment of environmental change impacts on regular water resources is explicitly prominent in dry and semi-dry regions. This is because of the fragility of water resources to environmental change; a minor change in natural components may achieve critical varieties of the hydrological cycle (Abghari et al., 2013). ...
Article
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The present study is about an assessment of the Martil River restoration, which crosses Tetouan and Martil city in Northern Morocco. The evaluation compromises rainfall occurrence of 30 years, riparian habitats, and the river flow of 12 years. The rainfall analysis was carried out by the Standard Precipitation Index (SPI). Only three stations that contain continuous monthly precipitation data were considered. The river health and water quality were determined by the QBR (Riparian Habitat Quality), while the environmental flow was estimated by Tennant method. SPI results show a rainfall increment pattern with regular, extreme, and plentiful downpours causing rough deluges and floods. The ultimate flow result to support the river habitats was found 60% of the natural flow in the main channel. This study highlights the stream's unhealthy water, which falls within the low-quality water category downstream the river under anthropogenic pressure, while good to excellent quality was scored upstream the watershed. Tennant outcomes were discovered 2.40 m³/s as 30% and 5.51 m³/s as 60%, while 10% of the flow was found to be not adequate for habitats in the Martil River.
... Abghari et al. [48] found that strong relationships between river discharge and annual precipitation in Iran during the past 40 years. Abeysingha et al. [49] revealed that the significant positive correlation between streamflow and catchment rainfall in the Gomti River basin. ...
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... In the Himalayan region, analyzing the general trends in time series data of river flow and discharge is very essential to understand, quantify, and assess the effects of ongoing climate change on the water resources, and to evaluate the general policy for its management (Kundzewicz et al. 2014;Coulthard et al. 2005;Vliet et al. 2012;Abghari et al. 2013;Afshar et al. 2016). The river discharge production is known to be dependent on several factors including both natural factors such as precipitation, baseflow contribution, evaporation, glaciers and snow cover resources as well as other related anthropogenic factors (Kundzewicz et al. 2014;Mir et al. 2015Mir et al. , 2017Mir et al. , 2018 with global warming as the predominant factor (Mir et al. 2017;Saadi et al. 2017;Mir et al. 2018). ...
... The non-parametric Mann-Kendall (MK) test is generally used to detect monotonic trends in a series of environmental data, climate data, and hydrological data (Mann, 1945;Abghari et al., 2013;Pohlert, 2018). The MK test has several advantages, such as (1) its low sensitivity to abrupt breaks due to inhomogeneous time series (Tabari et al., 2011) and (2) its ability to consider the data distribution and cope with the outliers (Sonali and Kumar, 2013;Şen, 2017;Nashwan and Shahid, 2019). ...
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Abstract Over recent decades, Iran and especially its western regions have suffered from devastating floods. On the other hand, planning and designing hydraulic structures require information from flood magnitude corresponding to a given occurrence probability. Hence, in this study, we aimed to determine flood homogeneous regions and regional growth curves, using L-moments and flood index method, for annual maximum flow of 62 sub-basins in Great-Karoon and Karkhe basins. Moreover, the impacts of droughts and hydraulic structures on the stationary and stochastic nature of floods were investigated using Mann-Kendall trend test, Pettitt’s change point detection test (CPD) and graphical interpretations. It was found that there are five hydrological homogeneous regions in study area. The goodness of fit tests results showed that the Generalized Pareto (GPA) distribution for the first and second regions, the Generalized Normal (GNO) distribution for the third region and the Generalized Logistic (GPA) distribution for the fourth and fifth regions are the most appropriate regional statistical distributions. Also, maximum daily flows corresponding to 2, 5, 10, 25, 50, 100, 500 and 1000 year return periods were estimated for all study sub-basins. Regional flood frequency analyzes revealed that for 100-year return period the fifth region’s sub-basins, with 4060 m3/s discharge on average, have the highest maximum daily flow. The average 100-year maximum daily flows in the regions I, II, III and IV were estimated around 632, 682, 460 and 429 (m3/s), respectively.
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In this study, long-term annual and monthly trends in mean maximum, mean minimum and mean temperature are investigated at 35 synoptic stations in Iran. The statistical significance of trends is assessed by the Mann–Kendall test. Most stations, especially those in western and eastern parts of country, had significant positive trends in monthly temperature time series in summer season. However, the maximum number of stations with the positive trend were observed in April (30 stations), and then in August (29 stations) while the negative trends were seen in February (16 stations) and March (15 stations). On annual scale, most stations in western and southern parts of Iran had significant positive trend. Overall, about 71%, 66% and about 40% of stations had statistically significant trends in mean annual temperature, mean annual minimum temperature and in mean annual maximum temperature, respectively. These results, however, indicate that the climate in Iran is growing warmer, especially in summer.
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Streamflow characteristics in the Yukon River Basin of Alaska and Canada have changed from 1944 to 2005, and some of the change can be attributed to the two most recent modes of the Pacific Decadal Oscillation (PDO). Seasonal, monthly, and annual stream discharge data from 21 stations in the Yukon River Basin were analyzed for trends over the entire period of record, generally spanning 4–6 decades, and examined for differences between the two most recent modes of the PDO: cold-PDO (1944–1975) and warm-PDO (1976–2005) subsets. Between 1944 and 2005, average winter and April flow increased at 15 sites. Observed winter flow increases during the cold-PDO phase were generally limited to sites in the Upper Yukon River Basin. Positive trends in winter flow during the warm-PDO phase broadened to include stations in the Middle and Lower Yukon River drainage basins. Increases in winter streamflow most likely result from groundwater input enhanced by permafrost thawing that promotes infiltration and deeper subsurface flow paths. Increased April flow may be attributed to a combination of greater baseflow (from groundwater increases), earlier spring snowmelt and runoff, and increased winter precipitation, depending on location. Calculated deviations from long-term mean monthly discharges indicate below-average flow in the winter months during the cold PDO and above-average flow in the winter months during the warm PDO. Although not as strong a signal, results also support the reverse response during the summer months: above-average flow during the cold PDO and below-average flow during the warm PDO. Changes in the summer flows are likely an indirect consequence of the PDO, resulting from earlier spring snowmelt runoff and also perhaps increased summer infiltration and storage in a deeper active layer.
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Air temperature variability and trends in Romania were analysed using monthly, seasonal, and annual datasets. Temperature data of winter wheat season were also analysed. The Mann-Kendall test, Sen’s slope estimate, the sequential version of the Mann-Kendall test, the Pettitt test and spatial and temporal hierarchical cluster analyses were used. First, the datasetswere checked for changing points. The 106-year period was divided into two long periods of 100 years each to verify the importance of a very short interval in changing of general trends; after that it was divided into three shorter periods of 35–36 years each. The main conclusions are as follows: the 6 years making up the difference between the two long periods are very important in the context of the recent globalwarming; the three shorter periods analysis indicate some fluctuations rather than continuous warming. The latest short period is the most relevant for global warming. Spatial hierarchical cluster analysis indicated the existence of two distinctive groups. One of them, which includes stations in the south-east part of the country, seems to be influenced by the Black Sea surface temperature. Temporal hierarchical cluster analysis reveals that annual data series have the closest relation with the summer data series. Further, the impact of temperature changes on winter wheat phenology was determined using a phenology simulation performed with the model from theDecision Support System for Agrotechnology Transfer v. 4.0.2.0 platform. Earlier occurrences of anthesis and maturity were noticed for several regions in the country.
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In the last decades, droughts are a recurrent phenomena in many regions of the world, especially in the subtropics and mid-latitudes, affecting more and more the society. Aridity indices are often used to identify regions prone to that phenomenon. In this paper, we used data recorded in 30 locations in the extra-Carpathian areas of Romania over the period 1961–2007. The De Martonne aridity index (IDM) and the Pinna combinative index (IP) were employed in order to identify critical areas in the most important agricultural regions of the country. Monthly, seasonal, annual, and winter wheat and maize growing season datasets of IDM and annual values of IP were calculated. The trends were identified using the Mann–Kendall test and Sen’s slope, while ordinary Kriging technique was employed for interpolation. The main findings are that the most vulnerable to semi-aridity are the southeastern regions, especially during the warm period of the year, and that for Romania, the use of IDM is more appropriate compared to IP.
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The characteristics of hydro-climatic changes in the Tarim River Basin were analysed based on data collected at 39 weather stations and 29hydrological stations for the period 1961–2008 and 1952–2008, respectively. A non-parametric trend test on basin scale for annual data shows an increasing trend of precipitation, relative humidity, vapour pressure, and the aridity index since 1986. Surface temperature started increasing in 1996. A decreasing trend of sunshine started in 1990. The potential evapotranspiration (ET) is calculated by the Penman–Monteith equation, and points out decreasing trend of potential evapotranspiration since 1985. This negative trend can also be detected for wind speed in both the same time scale and spatial extent. The stations with significant increasing trends in annual streamflow are mainly distributed at the southern slope of Tianshan Mountain, which can only be explained by climatic changes. The detected negative runoff trend of the main stream of the Tarim River can be explained by anthropogenic activities (such as irrigation and domestic water use) and climatic changes. A quantitative assessment reveals that local human activities since the 1970s led to a decrease of the water volume diverted into the main stream of the Tarim River Basin, which has been aggravated in the 2000s.Research highlights► The temporal and spatial characteristics of climate change of the Tarim River Basin are analysed based on different meteorological variables. ► Change points are detected for all meteorological variables. ► The impacts of Climate change on headstream runoff are positive during past 50 years. ► The detected negative runoff trend of the main stream of the Tarim River can be explained by anthropogenic activities and climatic changes.
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The highly variable timing of streamflow in snowmelt-dominated basins across western North America is an important consequence, and indicator, of climate fluctuations. Changes in the timing of snowmelt-derived streamflow from 1948 to 2002 were investigated in a network of 302 western North America gauges by examining the center of mass for flow, spring pulse onset dates, and seasonal fractional flows through trend and principal component analyses. Statistical analysis of the streamflow timing measures with Pacific climate indicators identified local and key large-scale processes that govern the regionally coherent parts of the changes and their relative importance. Widespread and regionally coherent trends toward earlier onsets of springtime snowmelt and streamflow have taken place across most of western North America, affecting an area that is much larger than previously recognized. These timing changes have resulted in increasing fractions of annual flow occurring earlier in the water year by 1–4 weeks. The immediate (or proximal) forcings for the spatially coherent parts of the year-to-year fluctuations and longer-term trends of streamflow timing have been higher winter and spring temperatures. Although these temperature changes are partly controlled by the decadal-scale Pacific climate mode [Pacific decadal oscillation (PDO)], a separate and significant part of the variance is associated with a springtime warming trend that spans the PDO phases.
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In this study, long-term annual and monthly trends in rainfall amount, number of rainy days and maximum precipitation in 24 h are investigated based on the data collected at 33 synoptic stations in Iran. The statistical significance of trend and climate variability is assessed by the Mann-Kendall test. The Linear trend analysis and the Mann-Kendall test indicate that there are no significant linear trends in monthly rainfall at most of the synoptic stations in Iran. However, the maximum number of stations with negative trends have been observed in April (29 station), and then in May (21 stations) and February (21 stations) and with positive trends in December (26 stations) and July (24 stations). The significant linear trends, with a significant level of 0.05, in annual rainfall have been noticed only at five stations. The monthly number of rainy days does not show any significant linear trend for most areas in Iran. The maximum number of stations with monthly negative trends in rainy days has also been observed in April with the minimum in December. In April, out of 24 stations with negative trends, 12 stations have a significant negative trend. Contrary to that, in October there is no significant linear trend. Most stations have positive trends in annual number of rainy days. Also, the monthly maximum precipitation in 24 h does not show any significant linear trend for most areas in Iran. The maximum number of stations with monthly negative trends in maximum precipitation has also been observed in February with the minimum in December. In spite of that, there are almost no significant precipitation variations in Iran during the last 50-odd years, the tendency of decreasing rainfall amount in April and increasing rainfall amount in December and July could indicate an eventual climate change in this area in the future.
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This paper examines trends in streamflow and their links with local climate in the Karkheh River and its major tributaries, which originate from the Zagros Mountains, Iran. Streamflow records from five mainstream stations for the period 1961–2001 were used to examine trends in a number of streamflow variables. The studied variables were mean annual and monthly flows, 1 and 7days maximum and minimum flows, timing of the 1-day maxima and minima, and the number and duration of high and low flow pulses. Similarly, the precipitation and temperature data from seven climate stations for the period from 1950s to 2003 were used to examine trends in climatic variables and their correlation with the streamflow. The Spearman Rank test was used for the detection of trends and the correlation analysis was based on the Pearson method. The results reveal a number of significant trends in streamflow variables both increasing (e.g. December flows) and decreasing (e.g. May flows) for all stations. However, some trends were not spatially uniform. For example, decline in low flow characteristics were more significant in the upper parts of the basin, whereas increasing trends in floods and winter flows were noteworthy in the middle parts of the basin. Most of these trends could be attributed to precipitation changes. The results show that the decline in April and May precipitation causes the decline in the low flows while the increase in winter (particularly March) precipitation coupled with temperature changes lead to increase in the flood regime. The observed trends at the Jelogir station on the Karkheh River reflect the combined effect of the upstream catchments. The significant trends observed in a number of streamflow variables at Jelogir, 1-day maximum, December flow and low pulse count and duration, point to the changes in hydrological regime of the entire Karkheh River system and are attributed to the changes in climatic variables.
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Air temperature variability and trends in Romania were analysed using monthly, seasonal, and annual datasets. Temperature data of winter wheat season were also analysed. The Mann-Kendall test, Sen’s slope estimate, the sequential version of the Mann-Kendall test, the Pettitt test and spatial and temporal hierarchical cluster analyses were used. First, the datasets were checked for changing points. The 106-year period was divided into two long periods of 100years each to verify the importance of a very short interval in changing of general trends; after that it was divided into three shorter periods of 35–36years each. The main conclusions are as follows: the 6 years making up the difference between the two long periods are very important in the context of the recent global warming; the three shorter periods analysis indicate some fluctuations rather than continuous warming. The latest short period is the most relevant for global warming. Spatial hierarchical cluster analysis indicated the existence of two distinctive groups. One of them, which includes stations in the south-east part of the country, seems to be influenced by the Black Sea surface temperature. Temporal hierarchical cluster analysis reveals that annual data series have the closest relation with the summer data series. Further, the impact of temperature changes on winter wheat phenology was determined using a phenology simulation performed with the model from the Decision Support System for Agrotechnology Transfer v. 4.0.2.0 platform. Earlier occurrences of anthesis and maturity were noticed for several regions in the country.
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Streamflow series of five hydrological stations were analyzed with aim to indicate variability of water resources in the Tarim River basin. Besides, impacts of climate changes on water resources were investigated by analyzing daily precipitation and temperature data of 23 meteorological stations covering 1960–2005. Some interesting and important results were obtained: (1) the study region is characterized by increasing temperature, however, only temperature in autumn is in significant increasing trend; (2) precipitation changes present different properties. Generally, increasing precipitation can be detected. However, only the precipitation in the Tienshan mountain area is in significant increasing trend. Annual streamflow of major rivers of the Tarim River basin are not in significant trends, except that of the Akesu River which is in significantly increasing trend. Due to the geomorphologic properties of the Tienshan mountain area, precipitation in this area demonstrates significant increasing trend and which in turn leads to increasing streamflow of the Akesu River. Due to the fact that the sources of streamflow of the rivers in the Tarim River basin are precipitation and melting glacial, both increasing precipitation and accelerating melting ice has the potential to cause increasing streamflow. These results are of practical and scientific merits in basin-scale water resource management in the arid regions in China under the changing environment.
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The paper presents techniques that are suitable for the exploratory analysis of monthly water quality data for monotonic trends. The first procedure described is a nonparametric test for trend applicable to data sets with seasonality. The second procedure, the seasonal M. G. Kendall slope estimator, is an estimator of trend magnitude. The third procedure provides a means for testing for change over time in the relationship between constituent concentration and flow.
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Drought, a normal recurrent event in arid and semiarid lands such as Iran, is typically of a temporary nature usually leaving little permanent aftermath. In the current study, the rainfall and drought severity time series were analyzed at 10 stations in the eastern half of Iran for the period 1966–2005. The drought severity was computed using the Standardized Precipitation Index (SPI) for a 12-month timescale. The trend analyses of the data were also performed using the Kendall and Spearman tests. The results of this study showed that the rainfall and drought severity data had high variations to average values in the study period, and these variations increased with increasing aridity towards the south of the study area. The negative serial correlations found in the seasonal and annual rainfall time series were mostly insignificant. The trend tests detected a significant decreasing trend in the spring rainfall series of Birjand station at the rate of 8.56mm per season per decade and a significant increasing trend in the summer rainfall series of Torbateheydarieh station at the rate of 0.14mm per season per decade, whereas the rest of the trends were insignificant. Furthermore, the 12-month values of the standardized precipitation index decreased at all the stations except Zabol during the past four decades. During the study period, all of the stations experienced at least one extreme drought which mainly occurred in the winter season.
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The annual and seasonal trends of seven meteorological variables were analyzed for twelve weather stations in Serbia during 1980–2010. The non-parametric Mann-Kendall and Sen's methods were used to determine whether there was a positive or negative trend in weather data with their statistical significance. The occurrence of abrupt changes was detected using cumulative sum charts and bootstrapping. In the present study, the increasing trends were indicated in both annual and seasonal minimum and maximum air temperatures' series. The relative humidity decreased significantly in summer and autumn, while the vapor pressure had a significant increasing trend in spring, summer and autumn. Besides, no significant trends were detected in summer and winter precipitation series. In general, the results of using the Mann-Kendall and Sen's tests demonstrated the good agreement of performance in detection of the trend for meteorological variables.
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Precipitation is a principal element of the hydrological cycle and its temporal variability is important from both the scientific and practical point of view. The annual and seasonal precipitation trends of 41 stations in Iran for the period 1966-2005 have been analyzed using the Mann-Kendall test, the Sen's slope estimator and the linear regression. The effective sample size method was applied to eliminate the effect of serial correlation on the Mann-Kendall test. The results indicated a decreasing trend in annual precipitation at about 60% of the stations. The decreasing trends were significant at seven stations at the 95% and 99% confidence levels. The magnitude of the significant negative trends in annual precipitation varied from (-)1.999 mm/year at Zanjan station to (-)4.261 mm/year at Sanandaj station. The spatial distribution of the annual precipitation trends showed that the significant negative trends occurred mostly in the northwest of Iran. On the seasonal scale, the trends in the spring and winter precipitations time series were mostly negative. The highest numbers of stations with significant trends occurred in winter while no significant positive or negative trends were detected by the trend tests in autumn precipitation. The significant negative trends ranged between (-)0.283 mm/year at Zahedan station and (-)0.807 mm/year at Sanandaj station in winter season. In addition, the highest and lowest significant increases of precipitation values were obtained over Semnan and Mashhad in summer at the rates of (+)0.110 mm/year and (+)0.036 mm/year, respectively.
Article
Observational studies of Sacramento Basin annual mean streamflow response to precipitation and temperature indicate that streamflow amounts in the basin are strongly sensitive to precipitation, but virtually insensitive to mean seasonal temperature. This result is in accord with studies of the climate mean sensitivity of streamflow amount to changes in climatological mean precipitation and temperature as simulated by conceptual hydrological models of the basin. Simpler regression models of the Sacramento Basin show a strong dependence of streamflow amount on temperature, which is not evident in the observation-based annual mean streamflow sensitivity. The interannual variability in Sacramento Basin streamflow response to precipitation exhibits substantial nonlinearity in that the partitioning of precipitation to runoff strongly depends on the precipitation volume. During very wet years in the basin, streamflow response to precipitation exhibits a proportionally greater change (increase) than during normal or dry years. For most dry years in the basin the streamflow response to precipitation is fairly linear. However, on the tails of droughts the streamflow response to precipitation is diminished relative to other dry years. This implies that a shift to a much drier or more drought-prone climate would result in a more substantial reduction in streamflow amounts than might at present be expected from observations of streamflow responses in isolated dry years. This conclusion assumes no changes in the features controlling water retention in the basin, which is an important issue to be addressed in basin climate change studies.
Article
SUMMARY The assessment of river flows over an area requires a wider investigation of the hydrology than the study of measured flows at single sites; such studies of mountainous and semi-arid areas present problems which do not arise in flatter humid areas. An investigation of the surface water resources of a river basin in a mountainous region of Iran is used to illustrate the problems involved. Because of the topography the precipitation is extremely variable, but the use of a rainfall-elevation relationship enables the residual effect of position to be deduced. For many hydrological purposes for such terrain it is preferable to map equivalent precipi­ tation at a constant altitude rather than produce a complex isohyetal map. The effect of altitude on potential and actual evaporation is discussed and a relation­ ship between rainfall and runoff is derived from the records.
Article
Secular trends in streamflow are evaluated for 395 climate-sensitive streamgaging stations in the conterminous United States using the non-parametric Mann-Kendall test. Trends are calculated for selected quantiles of discharge, from the 0th to the 100th percentile, to evaluate differences between low-, medium-, and high-flow regimes during the twentieth century. Two general patterns emerge; trends are most prevalent in the annual minimum (Q0) to median (Q50) flow categories and least prevalent in the annual maximum (Q100) category; and, at all but the highest quantiles, streamflow has increased across broad sections of the United States. Decreases appear only in parts of the Pacific Northwest and the Southeast. Systematic patterns are less apparent in the Q100 flow. Hydrologically, these results indicate that the conterminous U.S. is getting wetter, but less extreme.
Article
There is the need to evaluate changes in the spatial and temporal patterns of rainfall in order to improve water management strategies of a given region. In this study, standard tests are used to examine the existence of trend in annual and monthly rainfall of Nigeria over the last century. Rainfall variability index was estimated as standardized rainfall departure while autocorrelation spectral analysis is used to obtain the periodicities inherent therein. Rainfall spatial distribution was highly latitudinal dependent (r2 > 0.90) and had no clearly linear relations with the longitude. Rainfall variability index showed that 1950s was the wettest decade (+0.84) while 1980s was the driest (−1.19), with the two decades between 1970 and 1990 being drier than any other comparable period in the last century. Observed rainfall changes varied between −3.46 and +0.76 mm yr−2. About 90% of the entire landscape exhibited negative trends but only 22% showed significant changes at 5% level. There was a sharp difference between changes in rainfalls in 1931–1960 and 1961–1990 periods. Annual precipitation reduced by 7% between the two periods. While more than 90% of the landscape showed no significant rainfall change in the first period, about 57% of Nigeria showed a significant (P < 0.05) decrease in the second. The dominant peaks can be classified into four distinct rainfall cycles with periods 2–3, 5–7, 10–15 and 30 yr. These cycles may be associated with the stratospheric Quasi-Biennial Oscillation (QBO), the El-Nino Southern Oscillation (ENSO); the sunspot cycles and the Atlantic Multi-Decadal Oscillation (AMO) sea surface temperature, respectively.
Article
The suspected impact of climate warming on precipitation distribution is examined in the Yangtze River Basin. Daily precipitation data for 147 meteorological stations from 1961–2000 and monthly discharge data for three stations in the basin have been analyzed for temporal and spatial trends. The methods used include the Mann–Kendall test and simple regression analysis. The results show (1) a significant positive trend in summer precipitation at many stations especially for June and July, with the summer precipitation maxima in the middle and lower Yangtze River basin in the 1990s; (2) a positive trend in rainstorm frequency that is the main contributor to increased summer precipitation in the basin; and (3) a significant positive trend in flood discharges in the middle and lower basin related to the spatial patterns and temporal trends of both precipitation and individual rainstorms in the last 40 years. The rainstorms have aggravated floods in the middle and lower Yangtze River Basin in recent decades. The observed trends in precipitation and rainstorms are possibly caused by variations of atmospheric circulation (weakened summer monsoon) under climate warming.
Article
The least squares estimator of a regression coefficient β is vulnerable to gross errors and the associated confidence interval is, in addition, sensitive to non-normality of the parent distribution. In this paper, a simple and robust (point as well as interval) estimator of β based on Kendall's [6] rank correlation tau is studied. The point estimator is the median of the set of slopes (Yj - Yi)/(tj-ti) joining pairs of points with ti ≠ ti, and is unbiased. The confidence interval is also determined by two order statistics of this set of slopes. Various properties of these estimators are studied and compared with those of the least squares and some other nonparametric estimators.
Article
Regression analysis is usually carried out under the hypothesis that one of the variables is normally distributed with constant variance, its mean being a function of the other variables. This assumption is not always satisfied, and in most cases difficult to ascertain.
Article
Monthly, seasonal and annual trends in mean temperatures have been analysed in this study using data from 37 weather stations from the Pakistan Meteorological Department with records from 1952 to 2009. Statistical tests including Sen's slope and Mann‐Kendall were applied to each of the 37 stations in order to determine the sign and slopes of trends and their statistical significance. The study reveals that the temperature has generally increased in Pakistan at all time scales analysed over the past few decades. March and the pre‐monsoon season were the periods with the highest number of weather stations showing statistical significance, and also with the highest magnitudes of trends. Mean annual temperature increased around 0.36 °C/decade. This rise in temperature is slightly higher than other results found for Pakistan. The association between temperatures and certain teleconnection patterns, as well as the influence of the urban effect, might be among the causes of the trends found in this study. The largest number of correlations between mean temperatures and teleconnection patterns was found in March, April and May with NAO, ENSO and NCP, respectively. The North Atlantic Oscillation (NAO) may also have an influence on the temperatures of certain months in the monsoon season and particularly in August. At a seasonal resolution, NAO and NCP may control temperatures in the pre‐monsoon season. The lowest number of sites exhibiting correlations was found in winter and in the post‐monsoon seasons. Copyright © 2012 Royal Meteorological Society
Article
Monitoring the temporal variations of reference evapotranspiration (ETo) and quantifying any trends offer valuable information for regional hydrology, agricultural water requirements and water resources management. This study aimed to examine the temporal trends in the Penman–Monteith ETo in the west and southwest of Iran by using the Kendall and Spearman tests after eliminating the influence of significant lag-1 serial correlation from the ETo time series. The magnitudes and starting years of significant ETo trends were determined by the Mann-Kendall rank statistic and the Theil–Sen’s estimator, respectively. For the study period of 1966–2005, a significant positive lag-1 serial correlation coefficient was observed at almost all the stations. The existence of the positive serial correlation in the ETo series increased the possibility of the Kendall and Spearman tests to reject the null hypothesis of no trend while it is true. It was found that the Kendall test was more sensitive than the Spearman test to the existence of the positive serial correlation in the ETo series. After removing the serial correlation effect with pre-whitening method, only three significant increasing ETo trends were obtained at Khorram-Abad, Shahrekord and Zanjan stations at the rates of 0.16, 0.06 and 0.06 mm/day per decade, respectively. The significant increasing ETo trends of Khorram-Abad, Zanjan and Shahrekord stations started in 1997, 1994 and 1998, respectively. The stepwise regression method showed that wind speed was the most dominating variable affecting on the significant changes of ETo.
Article
The analysis of trends in hydroclimatic parameters and assessment of their statistical significance have recently received a great concern to clarify whether or not there is an obvious climate change. In the current study, parametric linear regression and nonparametric Mann–Kendall tests were applied for detecting annual and seasonal trends in the relative humidity (RH) and dew point temperature (T dew) time series at ten coastal weather stations in Iran during 1966–2005. The serial structure of the data was considered, and the significant serial correlations were eliminated using the trend-free pre-whitening method. The results showed that annual RH increased by 1.03 and 0.28 %/decade at the northern and southern coastal regions of the country, respectively, while annual T dew increased by 0.29 and 0.15°C per decade at the northern and southern regions, respectively. The significant trends were frequent in the T dew series, but they were observed only at 2 out of the 50 RH series. The results showed that the difference between the results of the parametric and nonparametric tests was small, although the parametric test detected larger significant trends in the RH and T dew time series. Furthermore, the differences between the results of the trend tests were not related to the normality of the statistical distribution.
Article
The spatiotemporal trends of aridity index in the arid and semi-arid regions of Iran in 1966–2005 were in-vestigated using the Mann–Kendall test and Theil–Sen's slope estimator. The results of the analysis showed negative trends in annual aridity index at 55 % of the stations, while just one site had a statistically significant (α00.1) negative trend. Furthermore, the positive trends in the annual aridity index series were significant at the 95 % confidence level at Bushehr and Isfahan stations. The significant negative trend in the annual aridity index was obtained over Mashhad at the rate of −0.004. In the seasonal series, the negative trends in the spring and winter aridity index were larger compared with those in the other seasonal series. A noticeable de-crease in the winter aridity index series was observed mostly in the southeast of the study area. In the summer and autumn aridity index, two significant positive trends were found.
Article
An important characteristic of a river flow regime type is the time of year when high and low flows are likely to occur. How likely is it, however, to observe an identified seasonal pattern each individual year? Stability is an often neglected property of a flow regime, though shifts in the seasonal behaviour of flows affect both environmental and economic activities. An approach to characterize objectively the stability of a flow regime type, based on the concept of entropy, is presented. The stabilities of river flow maxima and minima are studied separately to investigate their respective contributions to the stability character of a particular regime type. A quantitative “instability index” permits a study of the development of a flow regime's stability in time, especially important in the context of a possible climate change. The method is presented using the example of a quantitative flow regime classification developed for Scandinavia and western Europe.
Article
An analysis of spatial and temporal trends of precipitation in Beijiang River basin, Guangdong Province, China during 1959–2003 was performed using 17 time series (including monthly, annual, wet season, dry season, early flood period and late flood period totals) both on station based and sub-basin based data sets. Two nonparametric methods (Mann–Kendall and Sen's T) were used for data analysis. The results showed that (1) downward trends of temporal distribution were mostly detected during the early flood period, especially in May, while upward trends were observed in July and the dry season; (2) downward trends of spatial distribution were mostly detected in the southern Beijiang River basin, while upward trends were observed north of this area. Our results indicated a delayed rainy season and a northward trend of the precipitation belt compared to recent years. Copyright © 2007 John Wiley & Sons, Ltd.
Article
Since the late 1950s a series of soil conservation practices have been implemented in the Loess Plateau. It is important to assess the impact of these practices on hydrology at the catchment scale. The Jialuhe River catchment, a tributary of the Yellow River, with a drainage area of 1117 km2 in the Loess Plateau, was chosen to investigate the hydrological responses to conservation practices. Parametric and non-parametric Mann–Kendall tests were utilized to detect trends in hydrological variables or their residuals. Relationships between precipitation and hydrological variables were developed to remove the impact of precipitation variability. Significant linear decreasing trends in annual surface runoff and baseflow were identified during the treated period from 1967 to 1989, and the rate of reduction was 1·30 and 0·48 mm/year, respectively. As result, mean annual surface runoff and baseflow decreased by 32% over the period of 1967 to 1989. Seasonal runoff also decreased during the treated period with the greatest reduction occurring in summer and the smallest reduction in winter. The response of high and low daily flow to conservation practices was greater than average flows. Copyright © 2004 John Wiley & Sons, Ltd.