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Trend analysis of annual and seasonal rainfall to climate variability in North-East region of India

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Global warming, climate change and its consequences are major threat for the global agriculture. The agriculture in the NorthEast region of India is supposed to more in danger due to its topographic features. Agriculture in the state of Arunachal Pradesh is dependent on rainfall and variability in rainfall due to climate change is expected to threaten the food production in future. This study examines the impact of climate change on rainfall using the trend analysis technique for the four districts of Arunachal Pradesh. For this purpose temporal trends in annual and seasonal rainfall were detected using nonparametric Mann-Kendall test at 5% significance level. The daily time series rainfall data for the period 1971-2007 were analyzed statistically for each district separately. The results of Mann Kendall test showed decreasing trend in annual mean rainfall in east Siang, upper Siang and lowers Dibang valley and no trend in the west Siang district over the period of 1971-2007. In case of east Siang, upper Siang and lower Dibang valley districts, decreasing trend of rainfall was observed in the post monsoon season with slope magnitude of 3.01 mm/yr, 3.32 mm/yr and 3.95 mm/yr respectively. Decreasing pattern of rainfall in post monsoon season may affect the vegetable and fruit production in the winter season.
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Journal of Applied and Natural Science 6 (2): 480-483 (2014)
Trend analysis of annual and seasonal rainfall to climate variability in
North-East region of India
G. T. Patle
*
and A. Libang
Department of Soil and Water Engineering, College of Agricultural Engineering and Post Harvest Technology
(Central Agricultural University), Gangtok-737135, INDIA
*
Corresponding author. E-mail: gtpatle77@gmail.com
Received: February 08,2014 ; Revised received: July 07, 2014 ; Accepted: October 13, 2014
Abstract Global warming, climate change and its consequences are major threat for the global agriculture. The
agriculture in the North-East region of India is supposed to more in danger due to its topographic features. Agriculture in
the state of Arunachal Pradesh is dependent on rainfall and variability in rainfall due to climate change is expected
to threaten the food production in future. This study examines the impact of climate change on rainfall using the
trend analysis technique for the four districts of Arunachal Pradesh. For this purpose temporal trends in annual and
seasonal rainfall were detected using nonparametric Mann-Kendall test at 5% significance level. The daily time
series rainfall data for the period 1971-2007 were analyzed statistically for each district separately. The results of
Mann Kendall test showed decreasing trend in annual mean rainfall in east Siang, upper Siang and lowers Dibang
valley and no trend in the west Siang district over the period of 1971-2007. In case of east Siang, upper Siang and
lower Dibang valley districts, decreasing trend of rainfall was observed in the post monsoon season with slope magnitude
of 3.01 mm/yr, 3.32 mm/yr and 3.95 mm/yr respectively. Decreasing pattern of rainfall in post monsoon season may
affect the vegetable and fruit production in the winter season.
Keywords: Climate change; Mann-Kendall; Rainfall; Seasonal; Trend analysis
INTRODUCTION
Climate is the driving force for the several human
activities including agriculture. Global warming and its
consequences have made the changes in the global and
regional climate and can affect land ecosystems
especially water cycle. Climate change and variability
in the climatic parameters may adversely affect the
agriculture and water resources of agrarian country
like India (Sinha et al., 1998; Kumar et al., 2004; Mall
et al., 2006;). Agricultural crop production system is
greatly influenced by the climate and climatic
parameters namely temperature and rainfall. Rainfall is
useful in the planning and management of agriculture
and irrigation projects and any change in this variable
can influence the sustainable food production and
water availability for the agriculture. Uncertainty of
rainfall and uneven temporal and spatial distribution in
one hand, creating flooding and of the other hand
longer dry spells evoking drought conditions. Increase
in precipitation trends can also result in an increase in
the frequency of floods and could thereby affect water
quality. On the other hand, a decrease in rainfall trend
could imply an increase in instances of drought.
Climate change studies are mainly focused on the
probable changes in the climatic parameter such as
rainfall or temperature and variability of these parameters
ISSN : 0974-9411 (Print), 2231-5209 (Online) All Rights Reserved © Applied and Natural Science Foundation www.ansfoundation.org
is important. Several climate change models (physical
models) are being used to study the long term changes
in climate parameters on temporal and spatial scale
(IPCC, 2001; INCCA, 2010; Krishna Kumar et al.,
2011).
Temporal and spatial variability of climatic parameters
can also be studied using statistical approach through
the analysis of long term climatic data (Patle et al.,
2013). Climatic variability of time series data can also
be investigated using parametric and non-parametric
statistical methods. The parametric method considers
that data is normally distributed and is free from
outliers but non-parametric methods are free from any
such assumption (Hamed and Rao, 1998). The most
widely used non-parametric method for analyzing the
trend in the time series is the Mann-Kendall test
(Mann, 1945; Kendall, 1955). Several researchers have
widely used this method for different
hydro-meteorological parameters (Aziz and Burn,
2006; Mondal et al., 2012).
Arunachal Pradesh is one of the hilly states of the
northeast region of India. The region is characterized
by difficult terrain, wide variations in slopes and
altitudes, land tenure systems and indigenous cultivation
practices and about 70% of the peoples in the state are
dependents on the agriculture and forests for their
livelihood. Shifting cultivation is a main feature in the
481
state. The variation in rainfall due to climate change
may affect the agriculture and water resources in future.
In view of above, this study was undertaken to detect
the temporal trends in the annual and seasonal time
series of rainfall which will be helpful for agricultural
planning and in devising the location specific climate
change mitigation and adaptation strategies.
MATERIALS AND METHODS
Study area comprises the four districts of Arunachal
Pradesh namely east Siang, west Siang, upper Siang
and lower Dibang valley. Arunachal Pradesh is located
between 26.28° N and 29.30° N latitude and 91.20° E
and 97.30° E longitude and having a total geographical
area of 83,743 sq. km. The climate varies from hot and
humid in the Shivalik range with heavy rainfall.
Arunachal Pradesh receives heavy rainfall of 2,000 to
4,100 mm during the months of May and September.
The land use statistics in the state shows that 61.54%
of the area is under forest and only 2.31% of the total
geographical area under different crops. Agriculture is
a mainstay and source of income for most of the
peoples. The total area under agricultural operation in
the state was 3, 43,719 ha in 2005-06 and net sown
area was 203600 ha. The daily time series data of
rainfall were obtained from NICRA project for the
period of 1971-2007. The data were transformed to
annual and seasonal time series of rainfall and were
statistically analyzed. Three seasons were considered
viz., pre monsoon, monsoon and post monsoon season.
Variability in rainfall over the period of 37 years was
assessed by non parametric Mann Kendall and Sen’s
slope estimator. The Mann-Kendall test, a
non-parametric method was used for trend analysis of
time series data (Mann, 1945; Kendall, 1975).
Monotonic trend (increasing or decreasing) in the time
series of annual and seasonal rainfall was tested based
on the normalized Z statistics value. Negative value of
the Z statistics represents the decreasing trend and
positive value of Z statistics shows the increasing trend
of rainfall. The trends were detected at 5% level of
significance and the probability density function (pdf)
was computed. The trend is said to be significant if the
computed pdf is greater than the level of significance.
Non parametric Sen’s slope estimator (Sen, 1968) was
used to calculate the change per unit time. The slope of
the trend gave the rate of increase or decrease in the
annual and seasonal trend and the direction of change
(Choudhury et al., 2012).
RESULTS AND DISCUSSION
Trends in annual mean rainfall: In case of annual
mean rainfall, summary statistics revealed that
coefficient of variation was 29.04%, 29.97%, 26.36%
and 31.50% for east Siang, upper Siang, west Siang
and lower Dibang valley respectively. From this it was
observed that there was more variation in the annual
rainfall within the districts. Negative value of Z statistics
revealed decreasing trend in annual mean rainfall in all
four districts. Results of the non-parametric Mann–Kendall
test at the 5% significance level shows the significant
decreasing trend in east Siang, upper Siang and lower
Dibang valley districts. West Siang showed the
decreasing trend in the annual mean rainfall which was
not statistically significant and considered as no trend.
From the Sen’s slope estimator revealed that the rate of
decrease in the annual rainfall was more in the lower
Dibang valley (-42.96 mm/yr) followed by the upper
Siang (-38.98 mm/yr) and the east Siang (34.80mm/yr)
over the period 1971-2007 (Table 1 and Fig.1). From
this it may be concluded that annual mean rainfall has
decreased from 1971 to 2007 for the three districts and
is expected to follow the same trend in the future. As
all three districts are of hilly terrain with few water
storage facilities, water availability for agriculture is a
major concern. Many studies have attempted to
G. T. Patle and A. Libang
/ J. Appl. & Nat. Sci. 6 (2): 480-483 (2014)
Table 1. Statistics and Mann-Kendall analysis of annual mean rainfall.
Where, NT represents no trend and FT shows falling trend
District mean SD CV Z Q Trend
East siang 3349.2 972.5 29.04 -2.03 -34.80 FT
Upper siang 3466.5 1038.9 29.97 -2.11 -38.98 FT
West siang 3174.3 836.7 26.36 -1.87 -25.70 NT
Lower Dibang valley 3541.4 1115.7 31.50 -2.13 -42.96 FT
Table 2. Mann-Kendall analysis of seasonal rainfall.
Where, NT represents no trend and FT shows falling trend
District Pre-monsoon Monsoon Post-monsoon
Test Z Q Trend Test Z Q Trend Test Z Q Trend
East siang -1.69 -7.55 NT -1.50 -24.18 NT -2.13 -3.01 FT
Upper siang -1.43 -6.76 NT -1.58 -25.81 NT -1.90 -3.32 FT
West siang -1.77 -7.17 NT -1.48 -17.66 NT -0.94 -1.78 NT
Lower Dibang valley -1.48 -6.71 NT -1.58 -28.46 NT -2.34 -3.95 FT
482
determine the trend in rainfall at different scales
namely country, regional or district levels and the
similar results have been reported by the several re-
searchers for monthly, seasonal and annual series of
rainfall. Yue and Hashino (2003) studied long term
trends in annual precipitation of in Japan and found
significant negative trends. Partal and Kahya (2006)
also reported a negative trend in annual precipitation
for the majority of stations in Turkey at the 5% level of
significance. Decreasing trend in the annual mean rainfall
was also observed by Sarangi and Kumar (2009) for
New Delhi and Soman et al. (1988) for stations in the
hilly terrain of Kerala. Kumar and Jain (2010) also
observed decreasing trend in annual rainfall for Srinagar,
Kulgam, Handwara stations in Kashmir valley of
India.
Trends in seasonal rainfall : Table 2 shows the trends
in seasonal rainfall patterns for pre monsoon, monsoon
and post monsoon seasons. Negative value of Z
statistics revealed non-significant decreasing trend in
the pre monsoon and monsoon season in all four districts
namely east Siang, upper Siang, west Sing and lower
Dibang valley over the period 1971 to 2007. In case of
post monsoon significant decreasing trend was
observed for east Siang, upper Siang and lower Dibang
valley with slope magnitude of -3.01 mm/yr, -3.32
mm/yr and -3.95 mm/yr respectively. Krishnakumar et
al. (2009) studied the temporal variability of seasonal
and annual rainfall over Kerala for 1871–2005 and
reported significant decrease in south-west monsoon
rainfall and an increase in post-monsoon season
whereas in our study significant decrease in post
monsoon rainfall is observed. Similarly, decrease in
the monsoon rainfall and non significant increase in
post monsoon rainfall in Umiam of Meghalaya state
was reported by Choudhury et al. (2012). Kothyari and
Singh (1996) also found decreasing trend in monsoon
rainfall in the Ganga basin. Rainfalls during the post
monsoon season play a significant role in the crop
intensification, particularly in rainfed agriculture of
north east India (Choudhury et al., 2012) and may
affect the vegetable and fruit production in the winter
season.
Conclusion
Trend analysis of seasonal and annual mean rainfall
(1971-2007) of east Siang, upper Siang, west Siang
and the lower Dibang valley districts of Arunachal
Pradesh was carried out using non parametric Mann
Kendall and Sen’s slope test. From the analysis it was
concluded that annual mean rainfall had decreasing
trend in east Siang, upper Siang and lowers Dibang
valley districts. No trend was observed in the annual
mean rainfall for west Siang district over the period
1971-2007. In case of east Siang, upper Siang and
lower Dibang valley districts, decreasing trend of rainfall
was observed in the post monsoon season with slope
magnitude of -3.01 mm/yr, -3.32 mm/yr and -3.95
mm/yr respectively. Decreasing pattern of rainfall in
post monsoon season may affect the vegetable and
fruit production in the winter season. Thus, temporal
variability of seasonal and annual rainfall may affect
the agricultural crop production and future water
availability in the hilly state of Arunachal Pradesh.
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Attempts were made to study temporal variation in monthly, seasonal and annual rainfall over Kerala, India, during the period from 1871 to 2005. Longterm changes in rainfall determined by Man-Kendall rank statistics and linear trend. The analysis revealed significant decrease in southwest monsoon rainfall while increase in post-monsoon season over the State of Kerala which is popularly known as the “Gateway of summer monsoon”. Rainfall during winter and summer seasons showed insignificant increasing trend. Rainfall during June and July showed significant decreasing trend while increasing trend in January, February and April. Hydel power generation and water availability during summer months are the concern in the State due to rainfall decline in June and July, which are the rainiest months. At the same time, majority of plantation crops are likely to benefit due to increase in rainfall during the post-monsoon season if they are stable and prolonged.
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The study has analysed seasonal and annual rainfall and rainy days at five stations namely Srinagar, Kulgam, Handwara, Qazigund and Kukarnag to decipher rainfall trends over the Kashmir Valley. Rainfall data collected by the India Meteorological Department (IMD) were used. Due to the varying length of the available data, analysis was performed for two common periods: 1903–1982 (80 years) at three stations and 1962–2002 (41 years) at three stations. The 102 years of data at Srinagar were also analysed to examine the trends for last century. Time series of annual and seasonal rainfall/rainy days were examined for trends by analysis of anomalies and application of statistical tests.During the period 1903–1982, Srinagar, Kulgam and Handwara stations experienced a decreasing trend in annual rainfall; the maximum decrease was found for Kulgam (−20.16% of mean/100 years) and minimum for Srinagar (−2.45% of mean/100 years). All three stations showed a decreasing trend in monsoon and winter rainfall and an increasing trend in pre-monsoon and post-monsoon seasonal rainfall. The decreasing trend in winter rainfall was found to be statistically significant (95% confidence) at Kulgam and Handwara, whereas none of the increasing trend in the pre-monsoon and post-monsoon season was significant. Srinagar and Handwara witnessed a decreasing (non-significant) trend in annual rainy days, whereas Kulgam experienced the opposite trend. All the stations experienced a decreasing trend in monsoon and winter rainy days.Qazigund and Kukarnag experienced decreasing annual rainfall, and Srinagar showed increasing annual rainfall during the period 1962–2002. Pre-monsoon and post-monsoon rainfall decreased at all three stations. None of the increasing/decreasing trends were found to be significant. Annual, pre-monsoon, post-monsoon and winter rainfall increased (non-significant) whereas monsoon rainfall decreased (non-significant), at Srinagar during the last century.