Trend detection in Temperature and Rainfall over ...

Asian Journal of Research in Social Sciences and Humanities Vol. 5, No. 2, February 2015, pp. 12-26.

Asian Journal of Research in Social Sciences and Humanities

ISSN 2249-7315

www.aijsh.org

Asian Research Consortium

Trend detection in Temperature and Rainfall over Rajasthan during the Last Century Abira Dutta Roy* *Centre for the Study of Regional Development, School of Social Sciences-III, Jawaharlal Nehru University, New Delhi, India.

DOI NUMBER-10.5958/2249-7315.2015.00022.2

Abstract Climate change studies in Rajasthan have been very few. This inhibits decision making and sustainable risk reduction strategies. The present paper intends to fill up some of the research lacunas. It analyses the spatio-temporal variation of seasonal maximum, minimum temperatures and rainfall conditions with an assemblage of monthly data for the years 1901-2002 in Rajasthan obtained from the CRU TS2.1 dataset. It also uses Mann-Kendall trend test to identify climate change scenarios existing in the region. The result shows significant increase in temperatures and decrease in monsoon rainfall in most parts of the state. The analysis also showed a delayed onset of monsoon resulting in increase in post monsoon rainfall. The analysis was mapped to assist in decision making and risk reduction measures.

Keywords: Mann-Kendall, drought frequencies, spatio-temporal trends, variability, climate change, IDW. ________________________________________________________________________________

1. Introduction With the global climate change affecting large number of pockets all across the world and resulting in water scarcity and desertification as has been studied by (Houérou, 1996, Sellers et.al.,2008; Costa and Soares, 2012; Hillel and Rosenzweig, 2002; Sivakumar 2007; Odorico et. Al., 2012) it becomes imperative to look into the climatic conditions of the already environmentally stressed arid parts of India, especially Rajasthan which is prone to acute and frequent droughts. A handful of literatures that are available on climate studies of Rajasthan do not provide any commonly accepted statement on the prevailing temperature and rainfall conditions. Old studies by Banerjee and Upadhya (1975), with the use of climatic data of 1869- 1969 looked into the aspect of occurrences

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Roy (2015). Asian Journal of Research in Social Sciences and Humanities, Vol. 5, No.2, pp. 12-26.

of droughts in the region. They identified that the variability of rainfall increased from east to west, resulting in an increase in the frequency of drought in a similar spatial pattern. Pant and Hingane, (1987) dealt with the annual surface temperature and annual rainfall conditions over the north western region of India encompassing Rajasthan. They concluded that during the study period (1901-1982) annual surface temperatures declined and annually as well as monsoonal rainfall conditions increased, thus contradicting the research done by Winstanley (1973). Rathore (2005) analysed the frequencies of droughts over the past hundred years and have deciphered that severe and very severe droughts are more likely to occur in the south and western parts of the state with a probability of 47%. Ranade et.al, (2008) studied climate change trends over pan India from which decreasing annual rainfall pattern was seen between the period 1976- 2006. RAPCC, (2010) too has studied the frequencies of drought over the past century using the IMD (India Meteorological Department) 1º x 1º gridded data and categorized districts which are more vulnerable to the frequent droughts. The study for SPACC in addition looked into the erratic nature of the rainfall and the spatio-temporal variability of the dry spells. Because of the paucity of research, this study attempts towards a documenting a comprehensive study on the prevailing climate change scenario. For this purpose several accepted statistical techniques were applied on the collected climate data to look into the spatial variability of temperature and rainfall within 1901-2002. Accepted climate change detection methods have also been used. 1.1. Study Area Situated in the western fringe of India, Rajasthan extends from 23º30´ and 30º 12’ North latitude and 69º 30’ and 78º 17’ East longitude and occupies an area of about 342,239 square kilometers. Known as the desert state of India, the physiography of the state is broadly divided in four major categories. The Western Sandy Plains, the Aravalli Range and the Borat Plateau, the Eastern Plains of the Banas and Chappan Basins and the south eastern plateau comprising of the Deccan Lava Plateau and Hadoti Plateau. The Aravalli runs parallel to the cloud bearing winds and fails to create any orographic rainfall in the state during the monsoon months. The Tropic of Cancer passes through the southern districts of the state resulting in high temperatures during the summer. The arid and semi-arid parts of the state experience high diurnal and seasonal variations in temperature. The summer temperature averages around 26º C to 46 ºC. In winter the temperatures range from 8 ºC to 28 ºC. The annual rainfall totals to only 100mm in the western parts and around 600mm in the eastern and south eastern parts. On the basis of the variations in climatic, soil and physiographic conditions Rajasthan has also been categorized into various agro climatic regions: Arid Western Plain, Irrigated North Western Plains, Transitional Plain of Inland Drainage, Semiarid Eastern Plain, Flood Prone Eastern Plains, Subhumid Southern Plains & the Aravalli Hills which determine the agro-pastoral livelihood of the people, the cropping pattern and water demand conditions.

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Figure 1: Administrative Map of Rajasthan 2. Materials and Methods Monthly rainfall data and monthly minimum and maximum temperatures for 32 districts of Rajasthan were procured from the Climate Research Unit (CRU) TS2.1 dataset for 100 years 1901 to 2002. 2.1. Coefficient of Variation and IDW In order to fill the research gaps, analysis of monthly temperature and rainfall variability throughout the state over the past century was done. The monthly maximum and minimum temperature data at the district level were clubbed to form seasonal data. The data from June to September were averaged to generate the data for monsoon months, October to January for Post monsoon and February to May for the pre monsoon months. Similar techniques were used to generate seasonal precipitation data from the monthly data. A centennial average of maximum temperature, minimum temperature and rainfall were computed on a seasonal basis for all the districts in Rajasthan. The seasonal averages were then mapped in GIS environment using IDW (Inverse Distance Weighted) interpolation technique for comprehending the spatial dynamics. In order to analyse the variability of the climatic conditions the coefficient of variation were computed over the century for the three seasons and mapped using the IDW interpolation technique. 14


CV =

σ ∗ 100 μ

CV stands for co efficient of variation, σ for the standard deviation of the data and μ for the mean of the data. 2.2. Mann Kendall Trend Test For determining climate change patterns Mann Kendall trend test has been used. Though the aspects of climatic variability and climate change has been studied through various statistical analyses like linear and non-linear trend analysis (Fiona, 2008; Schneider, 2004; Antonio, 2010), climate change models (Blenkinsop, 2007; Jian et.al, 2006), the Mann-Kendall trend test (Khaled, 2008; Dileep et.al, 2007; Xu, 2003; Yonghui et.al, 2009), emerges out to be the most widely used technique for climate change detection. The trend test comprises of listing the monthly observations in temporal order, and computing differences that may be formed between the current reading and early reading. Consecutively the test statistic (S) is computed which is the difference between the number of positive and negative differences. If the result shows a positive value, then the trend is considered to be rising and if negative value is shown, the trend is a declining one. +1 if Xj− Xk > 0 sgn Xj − Xk =

0 if Xj − Xk = 0 −1 if Xj − Xk < 0

S=

n−1 k=1

n j=k+1 sgn(X j

Var S = n n − 1 2n + 1 −

− Xk )

t(t − 1)(2t + 5)/18 t

After computation of S its variance is calculated. In the above equations X j and Xk are the actual values of temperature or precipitation for the j th and kth year, n is the total number of years. In the next step the standard normal variate Z is calculated using the following formula S−1

Z=

if S > 0 √Var(S) 0 if S = 0 S+1 if S < 0 Var(S)

As per the students t-test table if the value of Z amounts to be more than +1.65 for an observation of over 100 years, it is considered to be definitely a rising trend at the 99 % level of significance and if the Z value is less than -1.65 it is surely a significant negative trend, hence a situation of climate change is cognizable. The Z values with lesser significance are categorized in table 1 under their respective level of significance as per the students’ t test table values. After the Z values were

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computed, the climate change conditions were mapped on the basis of the degree of significance from the Mann Kendall Z values for both the temperature and rainfall condition across the districts.

Table 1 Categories of Mann-Kendall Z values Z Values > 1.65 1.64 to 0.84 0.83 to 0.52 0.52 to 0.00 0.00 to -0.52 -0.52 to -0.83 -0.84 to -1.64 < -1.65

Significance of the Trend Increasing trend with >90% level of significance Increasing trend with 90-80% level of significance Increasing trend with 80-70% level of significance Increasing trend but insignificant Decreasing trend but insignificant Decreasing trend with 80-70% level of significance Decreasing trend with 90-80 % level of significance Decreasing trend with >90% level of significance

Climate change can strongly affect the existing drought conditions either by worsening or by improving the present scenario. Hence it is necessary to identify drought conditions, its severity and the frequency. Indian Meteorological Department (IMD) defines meteorological drought as occurring when the seasonal rainfall received over an area is less than 75% of its long-term average value. It is further classified as moderate drought if the rainfall deficit is between 26-50% and severe drought when the deficit exceeds 50% of the normal. For this study the years where the rainfall has been below 50% of the normal rainfall and rainfall within 26-50% were counted and mapped to analyse the district level variability of drought occurrences for all the seasons.

3. Result and Discussion 3.1. Temperature Variability The computed seasonal averages of maximum and minimum temperatures for over a period of 100 years were mapped through IDW interpolation technique and can be seen in figure 1. In the pre monsoon months the maximum temperatures are seen to range around 36 C especially in the western and south western districts of Jaisalmer, Jalore, Barmer, Sirohi, Dungarpur and Udaipur. Such temperature conditions also prevail in and around Bundi in the south eastern parts of the state. In the districts Ganganagar, Hanumangarh, Churu, Junjhunu, Sikar and Alwar the pre monsoon maximum temperature ranges to about 32C. Thus a gradual decrease in temperature towards the north can be observed from the map. The tropic of cancer passes through the southern districts and the direct rays of the sun during the summer months explain the reason behind such spatial variation. The pre monsoon maximum temperatures do not show much temporal variation as the co-efficient of variation (CV) calculated over the period of 100 years varies between 2-8%. Thus, in terms of predictability such range of temperatures has a high probability of occurrence. The variability pattern too is seen to increase from the southern to the northern districts.

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a

d

b

c

e

f

Figure 2: Average Seasonal Maximum Temperature (a) pre monsoon, (b) monsoon, (c) post monsoon and Seasonal Variability(d) pre monsoon, (e) monsoon, (f) post monsoon

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The monsoon months also experience higher ranges of temperature, ensuring prolonged summers. During these months the temperatures spatially vary from 30C to 38C. The desert districts of Jaisalmer, Bikaner in the west and Ganganagar, Hanumangarh, Churu, and parts of Jhunjhunu, Nagaur, Sikar, Barmer and Jodhpur experience very high temperatures ranging between 35C to 38C. The temperatures decline spatially towards the south and south eastern region to about 30C. The temporal variability is negligible. The CV is around 1.2% in the desert region increasing towards the south eastern districts to about 2.2%. During the post monsoon months the temperatures follow a similar spatial pattern as the one observed in the summer months, but the temperature conditions are 5-6C cooler in the post monsoons. March, April and May experience relatively higher temperatures as a result of the approaching summer months. The CV declines from 2.6% in the south eastern districts to a low of 2.1% in the western and northern districts. Throughout the state the pre monsoon season experience minimum temperatures ranging between 16 to 20º C. The seasonal minimum temperatures follow a similar spatial trend like the seasonal maximum. But here the spatial pattern of the minimum temperatures is highly influenced by the desert topography because of which the western districts experience the highest maximum temperature but the lowest minimum temperatures. Extraterrestrial radiation during the night leads to the drop in minimum temperatures to a great extent. Hence forth the districts of Jaisalmer, Bikaner, Jodhpur, Hanumangarh and Ganganagar experience the lowest minimum temperature ranging between 18ºC to 16ºC. The southern and south eastern districts experience relatively higher minimum temperatures, for example, districts around Udaipur and Kota observe a minimum temperature of about 20 ºC. The temporal variability in the pre monsoon minimum temperatures is inconsequential. The CV ranges to a 3.2% to 5%. Temporal variability is around 5%, in the northern districts which gradually declines towards the southern districts. The spatial variability is vivid in figure 2. The minimum temperatures during the monsoon months range between 22ºC to 27ºC. The diurnal variation is around 10ºC in most of the districts. Hanumangarh, Ganganagar, Churu and parts of western Rajasthan experience higher temperatures around 25º-27ºC. But due to the desert conditions the minimum temperature drops significantly in Jaisalmer, Bikaner, Jodhpur and Barmer in contrast to the maximum temperatures. The southern districts experience lower minimum temperatures around 20 to 22ºC. The CV ranges between 1-3% gradually decreasing from east to west.

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a

d

c

b

e

f f

Figure 3: Average Seasonal Minimum Temperature (a) pre monsoon, (b) monsoon, (c) Post monsoon and Seasonal Variability(d) pre monsoon, (e) monsoon, (f) Post monsoon During the post monsoon months the minimum temperatures fall to around 10ºC to 14ºC. The temporal variability during the post monsoon months ranges within 4.6% to 6.2% over the century. It can be said that the conditions of temperature do not show high temporal variability and hence can be very well predicted during different seasons. 3.2. Rainfall Variability Analysis of rainfall pattern and its variability is crucial for this region. Thus detailed analysis of the seasonal rainfall trends for 32 districts of Rajasthan from 1901-2002 were done. Figure 3 explains the spatial and temporal variability of the average seasonal precipitation pattern. The pre monsoon rainfall ranges between 2-9 mm. The northern districts of Hanumangarh, Churu, Ganganagar, Jhunjhunu and Sikar receive the highest rainfall during this season. A gradual decline in rainfall amount is observed towards south. The CV varies from 61.6% to 125%, which illustrates that the temporal variability of rainfall is very high during this season. The variability is highest towards the south and south western districts of Jalore, Sirohi, Barmer, Jaisalmer, Jodhpur, 19


Udaipur, Dungarpur and Banswara while it declines towards north, north eastern and eastern districts. During the monsoon months the average monsoon rainfall is around 225mm in the southern and south eastern districts especially around Udaipur and Kota. Parallel to the stretch of the Aravalli ranges the rainfall amount decreases to its west and increases to its east. To the west of the Aravalli ranges in the rain shadow districts of Bikaner, Barmer, Jaisalmer, Jodhpur and Nagaur the rainfall ranges within 60-40mm. As the rainfall amount spatially decreases the temporal variability and unpredictability of rainfall amount increases. The CV in the western side of the Aravalli Mountains is a little above 20%, but it increases to around 52 % in the desert districts further west. The post monsoon season receives an average rainfall ranging between 2mms to 30mms. The districts of Bahartpur, Dhaulpur in the north eastern parts and Jhalawar and Kota receives around 30 mm. During this season also the rainfall pattern runs parallel to the Aravallis

a f

d f

b f

c f

e f

f f

Figure 4: Average Seasonal Rainfall (a) pre monsoon, (b) monsoon, (c) post monsoon and Seasonal Variability(d) pre monsoon, (e) monsoon, (f) post monsoon

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and is least in the west. The CV computed on the data of 100 years shows a high temporal variation of 56% in the eastern and north eastern districts which further increases towards west upto 122%. Such high inter-seasonal and inter annual variability in rainfall is a matter of concern as it hampers in drought predictions and necessary policy determination. 3.3. Drought Frequencies High temperatures and low rainfall conditions lead to drought. Further analysis of rainfall data were done to understand drought scenarios based on IMDs description of drought The severe drought conditions were considered to be those with less than 50% of the

a f

b f

c f

Figure 5. Seasonal Frequencies of severe droughts(a) pre monsoon, (b) monsoon, (c) post monsoon mean rainfall. Moderate drought conditions were those where rainfall ranged within 26- 50% of the mean rainfall. After computation of the number of years under severe drought and moderate drought conditions at district level for the three seasons it was mapped through choropleth. Figure 4 shows the frequency of severe droughts occurring in pre monsoon, monsoon and post monsoon months. During the pre monsoon months it is seen that the districts of Hanumangarh, Churu and Bundi has experienced around 20-25 years of severe drought within a span of 100 years. The districts in the eastern half of Rajasthan experienced severe droughts for around 15-20 years. Central and western districts of Rajasthan observed severe droughts for 10-15 years. Nagaur, Jalore and Pali district faced severe droughts in less than 10 years. During the monsoon months severe droughts were observed in central districts of Nagaur and Ajmer, for over 25 years. The districts neighbouring Nagaur and Ajmer and western desert districts to have observed severe drought for about 20-25 years. The rest of the districts observed at least 10 years of severe drought within 1901-2002. In the post monsoon season, none of the districts observed less than 50% of the mean post monsoon rainfall for more than 25 years. Ganganagar, Jaisalmer, Dhaulpur, Sikar and Tonk district observed severe drought conditions for about 20-25 years. Most of the districts in the 21


northern part observed 15-20 years of less than 50% of mean rainfall. Only Jalore, Barmer and Pali experienced less than 10 years of severe drought conditions. Out of this scenario, it emerges out that most parts of northern and western district are chronically prone to severe droughts. The only exceptions are couple of districts of Jalore and Pali which less frequently experience such low rainfall.

a

b f

c f

Figure 6: Seasonal frequencies of moderate droughts (a) pre monsoon, (b) monsoon, (c) Post monsoon A further mapping of the moderate drought conditions is shown in figure 6. It shows that during the monsoon season moderate drought conditions prevailed in the entire state for less than 10 years. During the post monsoon months the central and southern districts observed moderate drought for more than 60 years. During the same season the rest of the districts except for Hanumangarh experienced moderate droughts for 40-60 years. Pre monsoon months on the other hand observed moderate droughts in the western, central, southern and south eastern districts for 20-40 years while the rest of the districts in the north and eastern half received rainfall within 26-50% of the mean for around 20 years. Such an analysis showed that all the districts in Rajasthan have experienced drought for at least 10 years in the last century. The western zone and central parts in the rainshadow of the Aravalli ranges have been the ones which were severely and frequently affected. 3.4.

Temperature Trends

In order to look into the climate changes occurring in the state Mann-Kendall trend test was conducted for monthly maximum and minimum temperatures as well as rainfall. On the basis of student t test table values, the significance levels of trends were identified and mapped. This enabled the identification of spatial variations in degrees of climate change occurring in Rajasthan. Figure 7 shows the Mann- Kendall Z values of maximum and minimum temperatures being mapped for the different districts in the three categorized seasons. The threshold Mann Kendall Z

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value of ± 1.65 has been considered to be the value where the rise or fall in temperature and rainfall can be said to be definitely occurring with more than 90% of confidence.

a f

d f

c f

b f

e f

f f

Figure 7: Mann Kendall Z Values of Maximum Temperature (a) pre monsoon, (b) monsoon, (c) post monsoon and minimum temperature (d) pre monsoon, (e) monsoon, (f) post monsoon During the pre monsoon months entire Rajasthan has experienced a rising trend in maximum temperature as well as minimum temperature. The level of significance of the trend is above 90%. The highest of the Mann Kendall Z values are observed in the western desert districts ranging from 3.02 to 3.78. In the southern tip of Rajasthan the district of Banswara observed the maximum rise with a Mann Kendall Z value of 4.21. The western side of Aravalli ranges has experienced a rather steeper temperature rise than the eastern half. The Mann Kendall Z values in the eastern districts range within 1.68 to 3. The similar spatial pattern is seen in the case of minimum temperature during this season. During the monsoons the maximum temperatures show a significant declining trend, especially in the northern and central parts. The trend is reflected from the Mann Kendall Z values which vary from -3.42 to -2.05. None of the districts during monsoons have experienced significant rising trends in maximum and minimum temperatures. Only Jaisalmer in the extreme west and Banswara 23


in extreme south showed a rising trend but an insignificant one. Their Mann Kendall Z values are 0.05 and 0.14 respectively. The significant decreasing trend in maximum and minimum temperatures during this season has been observed in Jodhpur, Pali, Jalore, Sirohi, Bhilwara and Bundi. The level of significance of trend in these districts ranged within 80-90% and the Mann Kendall Z values within -0.89 to -1.24. The districts Bikaner, Barmer, Chittaurgarh, Jhalawar, Rajsamand, Kota and Baran experienced declining maximum temperatures but the significance levels are low. The monsoon minimum temperatures follow an almost similar spatial variation excepting the district Alwar where insignificant rising trend has been noticed. During the post monsoon months within the last century, most parts of Rajasthan are seen to have experienced an increasing trend in maximum temperature conditions except for Jaipur. It has observed a declining trend with a Mann Kendall Z value of -0.6, the significance of this trend is 80%. Jhunjhunu, Alwar, Sikar and Dausa have observed a rise in maximum temperatures, but the confidence level of such a trend is low. Temperature rise is seen in all the districts during post monsoon season. Jodhpur and Jaipur have observed insignificant decrease with a Mann Kendall Z value of 0 and -0.5. 3.5.

Rainfall Trends

Rising temperature in most parts of the year throughout the state is a definite indication of occurrence of climate change. A look into the rainfall trends would also be of decisive importance in order to estimate the graveness of the situation. Figure 8 shows the Mann Kendall Z values of rainfall for pre monsoon, monsoon and post monsoon season.

a f

c f

b f

Figure 8 Mann Kendall Z Values of rainfall (a) pre monsoon, (b) monsoon, (c) post monsoon During pre monsoon months the districts of Banswara, Dungarpur, Udaipur, Rajsamand and Sirohi have experienced declining rainfall. 80-90% of confidence can be expressed in the conspicuous declining trend. All the eastern districts and only Jhunjhunu in the north have experienced a considerable rise in rainfall, the significance of which is more than 90%. The rest of the districts in the north of the state too have experienced the rising trend in rainfall but the significance level

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varies from 80-90%. Even the desert districts of Barmer, Bikaner, Jaisalmer have experienced minor rise in rainfall during this season. An analysis of the monsoon rainfall trends which contributes the maximum share of precipitation shows are rather different spatial scenario. The expanse of declining rainfall trends along the southern districts has enlarged in comparison to the pre monsoon season. The south west and south eastern districts, to experience a decreasing rainfall. The decrease is fairly significant with Mann Kendall Z values ranging from -0.02 to -1.26. The western districts of Bikaner and Jaisalmer too experienced similar decreasing rainfall trends. Only Alwar and Sikar observed significant rise in rainfall. The rest of the northern districts too experienced increase in rainfall. The Mann Kendall Z values in these areas ranged from 0.67 to 0.58. Insignificant rise is seen in Bikaner, Chittaurgarh, Pali, Kota and Rajsamand. Unlike the former two seasons the post monsoon months show a rising trend in rainfall in all the districts. Significant rising trends are observed in the districts of Barmer, Jalore, Sirohi and Jodhpur, their respective Mann Kendall Z values being 1.68, 2.58, 2.47 and 1.70. Except for Jaisalmer, Chittaurgarh and Jhalawar and the rest of Rajasthan experienced rising rainfall pattern of 70-90 % level of significance.

4. Conclusion The high variability in onset of monsoon rains have resulted in rising rainfall trends in most of the districts in the non-monsoon period. Moreover the spatial analysis shows the concentration of high and rising rainfall amounts in the eastern districts and acute scarcity of rains in the southern districts. With such a condition and added rising temperatures in most parts of the year can worsen the situation of availability of water. The desert districts and those in the south would be badly affected. These results can act as an eye opener and lead to sustainable water management practices and climate resilient agriculture in the identified vulnerable districts.

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