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The Hydrology of the Mekong River

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Abstract

The Mekong rises on the Tibetan Plateau at an altitude of about 5200 m and flows 4800 km southeast to the South China Sea, through six developing countries: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. The Mekong catchment has an unusual shape. Most catchments tend to have a dendritic form, with the width of the catchment gradually decreasing downstream, producing a characteristic teardrop shape. The Mekong catchment, by contrast, progressively widens down valley so that its widest point is immediately upstream of its delta. The hydrology of the Mekong River is characterized by a huge mean annual discharge; concentrated in an extremely regular wet-season peak. The size of the wet-season peak and its highly predictable timing are the defining characteristics of large tropical monsoonal rivers. In the upper part of the Lower Mekong system, at Vientiane, the flow originating from China and Burma, the so-called Yunnan Component, not only provides most of the dry-season flows, but in addition, most of the floodwater during the majority of years. Even though floods can cause major devastation along the Mekong River, the peak discharge of the largest floods tends to be only about double the size of the bankfull discharge.
... The Mekong River is one of the world's ten most significant rivers both in terms of its discharge and sediment load (MRC, 2005 (Adamson, Rutherfurd, Peel, & Conlan, 2009;He, Lu, Li, & Li, 2009). The Mekong River can be divided into two main parts (Binh et al., 2020). ...
... (Adamson, 2006) divided the year into four distinct flow seasons in the Mekong River Bassin (MRB): a dry season from late November to late May, a flood season from late June to early November and two transitional seasons in between. This seasonal separation was later re-used by (Adamson et al., 2009). Meanwhile, other studies agreed that there are two main seasons: a dry season from November to May and a flood season from June to October (Fan, He, & Wang, 2015;D. ...
... As we can see fromFigure 4, the high-flow or flood season consists of July, August, September and October, meanwhile, the low-flow or dry season consists of six months (December-May). June and November are the two transitional months.This seasonal separation results in a very similar seasonal pattern as that proposed byAdamson (2006) andAdamson et al. (2009) for the MRB. ...
Article
The Langcang‐Mekong River Basin is the most important transboundary river basin in Asia. However, over the recent decades, dam construction has had increasingly profound effects on hydrological processes and aquatic, particularly riparian, ecosystems. Understanding these impacts is critical for the foundation of sustainable runoff surface management. In this study, different methods, based on both graphical and statistical techniques, were applied to assess the effects of the dams on annual, seasonal and monthly runoff and to detect hydroclimatic trends in the Upper Mekong Basin during the period 1960–2020. The results reveal two change points with respect to seasonal and annual flow regimes; that is, 2003 for the flood season and annual flows and 2013 for the dry season flow. The duration of the flood season and the volume of annual discharges have both significantly decreased since 2003 and the dry season discharge has significantly increased since 2013 (with both p‐values <.05). The quantitative assessment suggests that, due to the effect of dams, the monthly discharges increased by 10–450 m3/s during the dry season (December–May), while the flood season's monthly flows decreased significantly, by 1,028–2,150 m3/s, from July to October at Chiang Saen station. The study of hydrological changes in the Mekong watershed is expected to be a significant contribution towards a better understanding of large watersheds in which the hydrological effects are influenced not only by climate change at large spatial and temporal scales but also by changes in the physical environment due to the construction of dams.
... Large rivers are exceedingly diverse, reflecting the impact of various climatic and geological imprints on their processes and forms over time as seen in the role they play in various hydro-climatological processes (Mallen-Cooper and Zampatti 2018;Wohl 2007;Adamson et al. 2009). Large rivers have been defined as being several kilometres long and draining vast areas of land (Potter 1978). ...
Article
Complex systems such as river flow do not obey the law of linearity but most often than not behave in a nonlinear manner. In this study, the dynamics of the River Niger discharge along two stations located in Nigeria were investigated using the methods of phase space reconstruction, correlation dimension and Lyapunov exponent. The analysis was carried out by using the daily data for three different periods — 1914–1939 (period 1), 1940–1964 (period 2), and 1966–1991 (period 3) spanning before and after the Kainji dam construction. Time delay embedding values at Baro and Lokoja for the three time periods were used to reconstruct the phase space of the river discharge at both locations. The embedding dimension represents the number of variables to completely describe the system. The presence of chaos was confirmed with positive values of Lyapunov exponents in both locations at the three different periods considered. The Lyapunov exponents at Baro were in the range 0.0014–0.0150 while the range at Lokoja was 0.007–0.0145. Significant correlation dimension values obtained at Baro (2.11–2.81) and Lokoja (2.28–4.51) are indicative of low dimensional chaos. A possible explanation for this could be the temporal sequence in the dam installation. The forecast horizon, the inverse of the largest Lyapunov exponents, gives a prediction boundary on a chaotic time series. In this study, the forecast horizon for both locations is expected to be in the range of 40–58 days into the future.
... Eyler [32] decided to come up with a book after traveling along the Mekong River and talking to the community members along this river. Eyler [32], provides one very important observation that due to massive dams which have been constructed by the Chinese on the Mekong before it leaves China on its flow route via Myanmar, Laos, Thailand, Cambodia and finally into Vietnam [33,34]; the Great Mekong which is the source of 20% of the world's freshwater fish catch is heavily dependent on the seasonal monsoon and flow of the river. However, Eyler [32] observes that it is heavily impaired by the Chinese dam construction (Figure 8). ...
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Deltas are landforms, which come into existence when sediment carried by river or stream empties its load into another water body with slow flow rates or stagnant water. Sometimes, a river may empty its sediment load on land, although this is uncommon. The world’s deltas are amongst the most productive and in some cases more populated than even land. This chapter reviews the formation of deltas, the ecology and habitats of deltas as well as the biodiversity in coastal habitats and delta habitats. Additionally, the chapter looks at recent advances in deltas such as the loss of sediment and other stressors currently facing deltas with a focus on anthropogenic activities in the Mekong River Delta (MRD) that is amongst the most resource rich deltas in the world. The Mekong River Delta (MRD) is currently known to be in peril due to anthropogenic activities such as dam construction for hydropower and irrigation, overfishing, agricultural production amongst many others. Additionally, demographical trends like population increase have also been scrutinized to see the impacts on the MRD. The results of the review process have shown that at least 85% of the deltas in the world are subsiding and losing their fertility to the sea. Finally, the chapter has endeavored to come up with suggestions on how best to overcome some of these stressors resulting from the anthropogenic activities.
... In the warmest months of March and April, the average temperature ranges from 30 • C to 38 • C. Rainy season means temperatures decrease significantly from south to north, from 26 • -27 • C in Phnom Penh to 21 • -23 • C in Thailand northern part. The Mekong River's average discharge to the sea is about 15,000 m 3 /s (Adamson et al., 2009;Gupta and Liew, 2007). ...
Article
The Mekong River in Asia is one of the world's longest rivers. Although it has some of the highest levels of biodiversity and productivity in Asia, the water quality in the basin has recently deteriorated as a consequence of land use changes, dam reservoir construction, population growth, and climate change. For the first time, this study estimates the interannual and monthly variabilities of nutrient fluxes (nitrate - NO3⁻) and total phosphorus - TP) in the lower Mekong River and Tonle Sap River in Cambodia, and assesses the nutrient linkage between the them. Long-term monitoring data were obtained from Kratie station (in the upper reach of the Mekong River), Chroy Changva station (just upstream of the lower Mekong River–Tonle Sap River confluence), and Prek Kdam station (on the Tonle Sap River ~40 km upstream of the confluence and 70 km downstream of Tonle Sap Lake). From 1995 to 2017, the estimated interannual flux of NO3⁻ was 364 ± 45 kt/y at Kratie and 557 ± 109 kt/y at Chroy Changva. From 2005 to 2017, the estimated interannual flux of TP was 100 ± 16 kt/y at Kratie and 73 ± 19 kt/y at Chroy Changva. Considerable seasonal differences were observed in both fluxes in the Mekong River, with 80–90% of the annual NO3⁻ flux occurring from May to October. The results of the nutrient exchange budget indicated that the NO3⁻ and TP fluxes from the Mekong River into Tonle Sap Lake were ~ 35.8 ± 12.5 kt/y and ~ 8.7 ± 3.3 kt/y, respectively, while these were 34.0 ± 13.8 kt/y and 6.6 ± 1.4 kt/y, respectively, from Tonle Sap Lake into the Mekong system. The results demonstrate that the Mekong River is a vital nutrient source, especially during the flood season, to Tonle Sap Lake and its floodplain.
... The seasonal flood pulse of the Lower Mekong River Basin is ecologically important, facilitating dispersal of sediment, nutrients, and biota throughout the system (Rainboth, 1996;Poulsen and Valbo-Jørgensen, 2000;MRC, 2005;Adamson et al., 2009;Kondolf et al., 2018). More specifically, dispersal of larval fish, also called 'larval drift', is a remarkable event in the Mekong (Poulsen et al., 2002;Halls et al., 2013b). ...
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The endangered striped catfish, Pangasianodon hypophthalmus is a large-bodied, migratory catfish, and one of the most cultured catfishes in the world. The early life history of wild catfish has been estimated from hatchery-raised fish without validation from wild-caught samples. This study examines the otoliths of drifting wild-caught and hatchery-raised larvae of striped catfish to validate daily ring counts, calculate fish age, and estimate spawning time and spawning location to improve our understanding of the species’ life history. We conducted daily sampling of larval/juvenile striped catfish in the Mekong mainstream in Phnom Penh in 2015 and at a fish hatchery in Cambodia. Daily water levels and river distances at and between four hydrological gauges over the Mekong River reach between Phnom Penh and Cambodia-Lao border were also collected. Standard length and otolith ring counts were examined in both hatchery-raised (150) and wild-caught (390) samples. This study revealed that (i) mass larval drift of striped catfish at the Mekong River in Phnom Penh took place from July 13th to July 23rd, 2015 corresponding with increasing flow, (ii) otolith rings are formed daily in hatchery-raised larvae, and can be used to determine the age of wild larvae/juveniles, (iii) otolith ring increments are strongly correlated with standard length in hatchery-raised larvae, but not in wild-caught larvae, (iv) mean standard length and age of striped catfish larvae collected in Phnom Penh are 14.65 mm (se = 0.07) and 27.6 days (se = 0.13), respectively, and (v) most striped catfish likely spawn in June and early July between the full and new moon, and the spawning grounds are most likely located along the Cambodian Mekong between Stung Treng and Kratie and, in 2015, travel time from spawning grounds to Phnom Penh was approximately 27 days. Upper Cambodian Mekong River is therefore likely very necessary for the persistence of all striped catfish populations and fisheries in the lower floodplains of Cambodia and Mekong delta. The existing and proposed dams in the upper Mekong River in Cambodia and beyond would drastically affect spawning sites and dry season refuge habitats and disconnect the upstream spawning from the downstream rearing habitats of the species.
... The 3SRB is formed by three main rivers: Sekong, Sesan, and Srepok, and divided into three sub-basins, namely Sekong River Basin (Sekong RB), Sesan River Basin (Sesan RB), and Srepok River Basin (Srepok RB) (Fig. 1). The 3SRB has a total area of approximately 78,650 km 2 (accounting for approximately 10% of the Mekong River Basin) and contributes approximately 20% of Mekong's total annual flow (Adamson et al. 2009), 30% of the annual nitrate load (Oeurng et al. 2016), and large amounts of sediment with 10-25 Mt/year (Kondolf et al. 2014). Moreover, the 3SRB has been habitat of 40% of Mekong biodiversity. ...
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This paper aimed at examining the climate variability and land-use change effects on streamflow and pollutant loadings, namely total suspended sediment (TSS), total nitrogen (T-N), and total phosphorus (T-P), in the Sesan, Sekong, and Srepok (3S) River Basin in the period 1981–2010. The well-calibrated and validated Soil and Water Assessment Tool (SWAT) was used for this purpose. Compared to the reference period, climate variability was found to be responsible to a 1.00% increase in streamflow, 2.91% increase in TSS loading, 11.35% increase in T-N loading, and 19.12% reduction in T-P loading for the whole basin. With regard to the effect of land-use change (LUC), streamflow, TSS, T-N, and T-P loadings increased by 0.01%, 3.70%, 10.12%, and 10.94%, respectively. Therefore, the combination of climate variability and LUC showed amplified increases in streamflow (1.03%), TSS loading (7.09%), and T-N loading (25.05%), and a net effect of decreased T-P loading (10.35%). Regarding the Sekong and Srepok River Basins, the streamflow, TSS, T-N and T-P showed stronger responses to climate variability compared to LUC. In case of the Sesan River Basin, LUC had an effect on water quantity and quality more strongly than the climate variability. In general, the findings of this work play an essential role in providing scientific information to effectively support decision makers in developing sustainable water resources management strategies in the study area.
Chapter
The lower Mekong area is part of mainland Southeast Asia and includes four countries: Cambodia, Lao PDR, Thailand, and Vietnam. In this region, diarrheal illnesses are frequent and can lead to deaths especially among children under five. Due to its availability, accessibility, and low cost, traditional medicine is commonly used to treat diarrheal illnesses in the region. The first part of this chapter opens up with the background of the sociocultural importance of traditional medicine for the treatment of diarrheal diseases in the lower Mekong area. This is followed by the most important pharmacological models used to validate the ethnomedical use of plants for diarrhea. The chapter concludes with the most commonly used plant species for the treatment of diarrhea in the lower Mekong basin (i.e., Cambodia, Lao PDR, Thailand, and Vietnam) and discusses the current knowledge on their pharmacological and toxicological activities as well as their therapeutic potential in humans.
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Water infrastructure development is considered necessary to drive economic growth in the Mekong region of mainland Southeast Asia. Yet the current understanding of hydrological and flood pattern changes associated with infrastructural development still contains several knowledge gaps, such as the interactions between multiple drivers, which may have serious implications for water management, agricultural production, and ecosystem services. This research attempts to conduct a cumulative assessment of basin-wide hydropower dam construction and irrigation expansion, as well as climate change, implications on discharge, and flood changes in the Cambodian Mekong floodplain. These floodplains offer important livelihoods for a considerable part of the 6.4 million people living on them, as they are among the most productive ecosystems in the world – driven by the annual flood pulse. To assess the potential future impacts, we used an innovative combination of three models: Mekong basin-wide distributed hydrological model IWRM-VMod, with the Mekong delta 1D flood propagation model MIKE-11 and 2D flood duration and extent model IWRM-Sub enabling detail floodplain modelling. We then ran scenarios to approximate possible conditions expected by around 2050. Our results show that the monthly and seasonal hydrological regimes (discharges, water levels, and flood dynamics) will be subject to substantial alterations under future development scenarios. Projected climate change impacts are expected to decrease dry season flows and increase wet season flows, which is in opposition to the expected alterations under development scenarios that consider both hydropower and irrigation. The likely impact of decreasing water discharge in the early wet season (up to −30 %) will pose a critical challenge to rice production, whereas the likely increase in water discharge in the mid-dry season (up to +140 %) indicates improved water availability for coping with drought stresses and sustaining environmental flows. At the same time, these changes would have drastic impacts on total flood extent, which is projected to decline by around 20 %, having potentially negative impacts on floodplain productivity and aquaculture, whilst reducing the flood risk to more densely populated areas. Our findings demonstrate the substantial changes that planned infrastructural development will have on the area, potentially impacting important ecosystems and people's livelihoods, calling for actions to mitigate these changes as well as planning potential adaptation strategies.
Chapter
The filling of the Grand Ethiopian Renaissance Dam in 2020 raised the age-old confrontation between upstream Ethiopia and downstream Egypt to a new level. In this study, a geographic approach is used to ascertain whether Ethiopia could survive without touching the Blue Nile water by investigating the balance between quality of land resources and demographic pressures against the backdrop of the geohyropolitics in the Mekong River. We used a set of geospatial databases acquired from measured and modeled scientific data sources. DEM, slope, and contours were generated from the terrain data. Rasterized population data were used to discern the temporal trend and plot population density across topographic gradients. Global Climate Model (GCM) generated temperature, and precipitation data, averaged for 2020–2040, 2040–2060, 2060–2080, and 2080–2100, were extracted from five sample locations in each river basin and analyzed for December and June, representing Winter and Summer seasons, respectively. To explore alternative water sources for Egypt, multiple buffer zones were created from the Red Sea and Mediterranean Seas. Results revealed (1) In light of intensifying demographic pressure, dwindling carrying capacity of land resources, warming atmospheric temperature, and growing uncertainty of rainfall patterns, Ethiopia’s vulnerability to food and economic insecurity is bound to worsen in the foreseeable future, which ultimately calls for equitable and reasonable utilization of the Blue Nile water. (2) Over 75% of settlements and over 97% of Egypt’s population are seated within a 300-km distance from the two seas where desalinated water could be harvested for domestic and agricultural uses. The study concludes that Egypt should come to terms with the biophysical and demographic dynamics of the Nile Basin and diversify its water sources rather than hoping to cling to obsolete treaties of the Nile, which the upper riparian countries were not a party to.
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Field data on plastic pollution is extremely limited in Southeast Asian rivers. Here we present the first field measurements of plastic transport in the Mekong, based on a comprehensive monitoring campaign during the monsoon season in the confluence of the Mekong, Tonle Sap, and Bassac rivers around Cambodia’s capital (Phnom Penh). For improved accuracy in the estimation of plastic loads and distribution, we combined Neuston net multipoint cross-sectional water sampling with Acoustic Doppler Current Profiler high resolution measurements. During the wet season, around 2.03 x 105 kg·day^-1 of plastic were released from Phnom Penh into the Mekong, equivalent to 89 g·day^-1 capita^-1, or 42% of all plastic waste generated in the city. Most plastic mass moved downstream at the surface. A smaller portion of plastics is mixed deep into the water column, potentially retained in the rivers, breaking down and resuspending over time. Overall, plastic waste from Phnom Penh and transported by the Mekong is a significant contribution to Southeast Asia’s plastic release into the ocean. This pollution represents a crucial risk to people in the region, as their livelihoods depend on fisheries from these water bodies.
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Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May. then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs. Finally, the timing of the onset of the Asian monsoon in 1989 was explored. It was shown that the onset of the Asian monsoon occurs when the warm or rising phase of different low-frequency oscillations reach the "East Asian monsoon area" (EAMA) concurrently. These include the warm phase of the eastward propagating two-to three-week oscillation (TTO) of the upper-layer temperature in middle latitudes, the rising phase of the northward propagating Madden-Julian oscillation of the southern tropical divergence, and the rising phase of the westward propagating TTO of the western Pacific divergence. It was concluded that the timing of the Asian monsoon onset is determined when the favorable phases of different low-frequency oscillations are locked over the EAMA.
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Seasonal summer monsoon (June–September) data for 120 stations over East Asia (China, Japan, Mongolia, Korea) varying from 1881 to 1998 are utilized to understand their interannual and climate characteristics, and to investigate their teleconnections with South Asian (in particular India's) monsoon rainfall. Contemporaneous relations on an interannual time-scale reveal that the rainfall variations over north China (southern Japan) are in-phase (out-of-phase) with South Asian rainfall.Based on the instrumental data available, regional rainfall anomaly time series for the 118-year period for the two coherent regions, over north China and southern Japan are prepared. All the three series (India, China, Japan) have been subjected to statistical tests. Results reveal that while there are year-to-year fluctuations, the Mann–Kendall rank statistic suggests no significant long-term trends. However, the application of Cramer's statistic to study the short-term climate variability depicts decadal variability with certain epochs of above and below normal rainfall over each region. The epochs tend to last for about three decades over India and China, and about five decades over Japan. The turning points for China follow those of India about a decade later.The relationships of South and East Asian monsoon rainfall exhibit secular variations. The inter-connections between the monsoon-related events (rainfall over South Asia, rainfall over East Asia, Northern Hemisphere circulation, tropical Pacific circulation) appear to strengthen (or weaken) around the same time, implying that the monsoon related events over geographically separated regions seem to get linked (or delinked) around the same time. Copyright © 2001 Royal Meteorological Society
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1] Workshops to identify transboundary and basin-wide environmental issues and a diagnostic study by consultants identified priority environmental concerns of resource managers in the lower Mekong River basin. The issues identified, in priority order, were water quality, reduction in dry season flows, sedimentation, fisheries decline, wetland degradation, and flooding. An analysis of the available data found no evidence that water quality was poor except in the delta, where nutrient levels were high and increasing. Dry season flows have not decreased, and in the immediate future they are more likely to increase. Suspended sediment levels in the river are not high, and there is no indication that sediment loads are substantially increasing. Fish catch per unit effort has declined over the past decades, as have catches of large fish, but total fish catch has increased. Flooding does not appear to have increased in frequency or extent. There is no reliable quantitative information available on changes in wetland extent or condition, although it is reasonable to assume that both have declined. Reasons for the mismatch between perceptions and the data may include a failure by management agencies to analyze and publish data and provide adequate responses to issues raised in the popular press. This results from a lack of capacity in many government agencies and the Mekong River Commission, where there are high staff turnover rates and a dependence on short-term experts with limited experience in the basin.
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Changes in the climatology of precipitation, evapotranspiration, and soil moisture lead also to changes in runoff and streamflow. The potential effects of global warming on the hydrology of 23 major rivers are investigated. The runoff simulated by the Canadian Centre for Climate Modeling and Analysis (CCCma) coupled climate model for the current climate is routed through the river system to the river mouth and compared with results for the warmer climate simulated to occur towards the end of the century. Changes in mean discharge, in the amplitude and phase of the annual streamflow cycle, in the annual maximum discharge (the flood) and its standard deviation, and in flow duration curves are all examined. Changes in flood magnitudes for different return periods are estimated using extreme value analysis. In the warmer climate, there is a general decrease in runoff and 15 out of the 23 rivers considered experience a reduction in annual mean discharge (with a median reduction of 32%). The changes in runoff are not uniform and discharge increases for 8 rivers (with a median increase of 13%). Middle- and high-latitude rivers typically show marked changes in the amplitude and phase of their annual cycle associated with a decrease in snowfall and an earlier spring melt in the warmer climate. Low-latitude rivers exhibit changes in mean discharge but modest changes in their annual cycle. The analysis of annual flood magnitudes show that 17 out of 23 rivers experience a reduction in mean annual flood (a median reduction of 20%). Changes in flow duration curves are used to characterize the different kinds of behavior exhibited by different groups of rivers. Differences in the regional distribution of simulated precipitation and runoff for the control simulation currently limit the application of the approach. The inferred hydrological changes are, nevertheless, plausible and consistent responses to simulated changes in precipitation and evapotranspiration and indicate the kinds of hydrological changes that could occur in a warmer climate.
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The present study sets out to investigate the sensitivity of water availability to climate change for a large western Himalayan river (the Satluj River basin with an area of 22 275 km2 and elevation range of 500 to 7000 m), which receives contributions from rain, snow and glacier melt runoff. About 65% of the basin area is covered with snow during winter, which reduces to about 11% after the ablation period. After having calibrated a conceptual hydrological model to provide accurate simulations of observed stream flow, the hydrological response of the basin was simulated using different climatic scenarios over a period of 9 years. Adopted plausible climate scenarios included three temperature scenarios (T + 1, T + 2, T + 3 °C) and four rainfall scenarios (P − 10, P − 5, P + 5 and P + 10%). The effect of climate change was studied on snowmelt and rainfall contribution runoff, and total stream flow. Under warmer climate, a typical feature of the study basin was found to be reduction in melt from the lower part of the basin owing to a reduction in snow covered area and shortening of the summer melting season, and, in contrast, an increase in the melt from the glacierized part owing to larger melt and an extended ablation period. Thus, on the basin scale, reduction in melt from the lower part was counteracted by the increase from melt from upper part of the basin, resulting in a decrease in the magnitude of change in annual melt runoff. The impact of climate change was found to be more prominent on seasonal rather than annual water availability. Reduction of water availability during the summer period, which contributes about 60% to the annual flow, may have severe implications on the water resources of the region, because demand of water for irrigation, hydropower and other usage is at its peak at this time. Copyright © 2004 John Wiley & Sons, Ltd.
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Question: Can recent satellite imagery of coarse spatial resolution support forest cover assessment and mapping at the regional level? Location: Continental southeast Asia. Methods: Forest cover mapping was based on digital classification of SPOT4-VEGETATION satellite images of 1 km spatial resolution from the dry seasons 1998/1999 and 1999/2000. Following a geographical stratification, the spectral clusters were visually assigned to land cover classes. The forest classes were validated by an independent set of maps, derived from interpretation of satellite imagery of high spatial resolution (Landsat TM, 30 m). Forest area estimates from the regional forest cover map were compared to the forest figures of the FAO database. Results: The regional forest cover map displays 12 forest and land cover classes. The mapping of the region's deciduous and fragmented forest cover remained challenging. A high correlation was found between forest area estimates obtained from this map and from the Landsat TM derived maps. The regional and sub-regional forest area estimates were close to those reported by FAO. Conclusion: SPOT4-VEGETATION satellite imagery can be used for mapping consistently and uniformly the extent and distribution of the broad forest cover types at the regional scale. The new map can be considered as an update and improvement on existing regional forest cover maps.
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1] We assess the spatial distribution of the largest rainfall-generated streamflows from a database of 35,663 flow records composed of the largest 10% of annual peak flows from each of 14,815 U.S. Geological Survey stream gaging stations in the United States and Puerto Rico. High unit discharges (peak discharge per unit contributing area) from basins with areas of 2.6 to 26,000 km 2 (1–10,000 mi 2) are widespread, but streams in Hawaii, Puerto Rico, and Texas together account for more than 50% of the highest unit discharges. The Appalachians and western flanks of Pacific coastal mountain systems are also regions of high unit discharges, as are several areas in the southern Midwest. By contrast, few exceptional discharges have been recorded in the interior West, northern Midwest, and Atlantic Coastal Plain. Most areas of high unit discharges result from the combination of (1) regional atmospheric conditions that produce large precipitation volumes and (2) steep topography, which enhances precipitation by convective and orographic processes and allows flow to be quickly concentrated into stream channels. Within the conterminous United States, the greatest concentration of exceptional unit discharges is at the Balcones Escarpment of central Texas, where maximum U.S. rainfall amounts apparently coincide with appropriate basin physiography to produce many of the largest measured U.S. floods. Flood-related fatalities broadly correspond to the spatial distribution of high unit discharges, with Texas having nearly twice the average annual flood-related fatalities of any other state.
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The impact of doubled CO2 concentration on the Asian summer monsoon is studied using a coupled ocean-atmosphere model. Both the mean seasonal precipitation and interannual monsoon variability are found to increase in the future climate scenario presented. Systematic biases in current climate simulations of the coupled system prevent accurate representation of the monsoon-ENSO teleconnection, of prime importance for seasonal prediction and for determining monsoon interannual variability. By applying seasonally varying heat flux adjustments to the tropical Pacific and Indian Ocean surface in the future climate simulation, some assessment can be made of the impact of systematic model biases on future climate predictions. In simulations where the flux adjustments are implemented, the response to climate change is magnified, with the suggestion that systematic biases may be masking the true impact of increased greenhouse gas forcing. The teleconnection between ENSO and the Asian summer monsoon remains robust in the future climate, although the Indo-Pacific takes on more of a biennial character for long periods of the flux-adjusted simulation. Assessing the teleconnection across interdecadal timescales shows wide variations in its amplitude, despite the absence of external forcing. This suggests that recent changes in the observed record cannot be distinguished from internal variations and as such are not necessarily related to climate change. Copyright © 2007 Royal Meteorological Society
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The Tonle Sap, the ‘Great Lake’ of central Cambodia, is the central component of wetland ecosystems in the lower Mekong River basin, and is of enormous conservation value. The lake's unusual hydraulic relationship with the Mekong River, and its consequent sensitivity to monsoon variability, makes the Tonle Sap sensitive to climate change. Exploring the dynamics and development of this system under different climate regimes of the past offers a perspective on possible future impacts, which is critical for sound management. Biostratigraphic and sedimentological data derived from cores of lake sediment indicate that during the period >7000 to ca. 5500 14C years Before Present the lake was less variable than present in terms of depth during the annual cycle of flood, and may have been strongly influenced by saline tidal waters associated with higher-than-present seas levels. As regional environments became drier and more seasonal in the late Holocene, more sediment was re-suspended during the increasingly marked dry season lake level minimum, lowering the effective sediment accumulation rate. Contrary to current interpretations of the history of the lake and associated wetland ecosystems, the data presented here imply that regional hydraulic connections between the lake and the Mekong River existed from at least the early Holocene.
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An irrigation scheme, based on simulated soil moisture deficit, has been included in the variable infiltration capacity macroscale hydrologic model. Water withdrawals are taken from the nearest river, or, in periods of water scarcity, from reservoirs. Alternatively, water can be assumed freely available. The irrigation scheme successfully simulates crop consumptive water use in large river basins. In general, irrigation leads to decreased streamflow and increased evapotranspiration. The locally significant increases in evapotranspiration (or latent heat) results in lower surface temperatures, and hence decreased sensible heat flux. Simulations performed for a 20-year period for the Colorado and Mekong river basins indicate irrigation water requirements of 10 and 13.4 km3 year−1, respectively, corresponding to streamflow decreases of 37 and 2.3%. The increase in latent heat flux is accompanied by a decrease in annual averaged surface temperatures of 0.04 °C for both river basins. The maximum simulated increase in latent heat flux averaged over the three peak irrigation months for one grid cell is 63 W m−2, where surface temperature decreases 2.1 °C. Simulated actual water use is somewhat less than simulated irrigation water requirements; 8.3 and 12.4 km3 year−1 for the Colorado and Mekong river basin, respectively.