CLIMATE RESEARCH AND SERVICES INDIA METEOROLOGICAL DEPARTMENT MINISTRY OF EARTH SCIENCES PUNE Observed Rainfall Variability and Changes over Rajasthan State Met Monograph No.: ESSO/IMD/HS/Rainfall Variability/22(2020)/46 Pulak Guhathakurta, Divya Surendran, Preetha Menon, Ashwini Kumar Prasad, Neha Sangwan and S C Advani GOVERNMENT OF INDIA MINISTRY OF EARTH SCIENCES INDIA METEOROLOGICAL DEPARTMENT Met Monograph No.: ESSO/IMD/HS/Rainfall Variability/22(2020)/46 Observed Rainfall Variability and Changes Over Rajasthan State Pulak Guhathakurta, Divya Surendran, Preetha Menon, Ashwini Kumar Prasad, Neha Sangwan and S C Advani INDIA METEOROLOGICAL DEPARTMENT PUNE - 411005 1 DOCUMENT AND DATA CONTROL SHEET 1 Document Title Observed Rainfall Variability and Changes Over Rajasthan State 2 Issue No. ESSO/IMD/HS/Rainfall Variability/22(2020)/46 3 Issue Date 4 Security Unclassified Classification 5 Control Status Uncontrolled 6 Document Type Scientific Publication 7 No. of Pages 27 8 No. of Figures 42 9 No. of References 3 10 Distribution Unrestricted 11 Language English 12 Authors Pulak Guhathakurta, Divya Surendran, Preetha Menon, Ashwini Kumar Prasad, Neha Sangwan and S C Advani 13 Originating Climate Research Division/ Climate Application Division/ Group & User Interface Group/ Hydrometeorology 14 Reviewing and Director General of Meteorology, India Approving Meteorological Department, New Delhi Authority 15 End users Central and State Ministries of Water resources, agriculture and civic bodies, Science and Technology, Disaster Management Agencies, Planning Commission of India 16 Abstract India is in the tropical monsoon zone and receives plenty of rainfall as most of the annual rainfall during the monsoon season every year. However, the rainfall is having high temporal and spatial variability and due to the impact of climate changes there are significant changes in the mean rainfall pattern and their variability as well as in the intensity and frequencies of extreme rainfall events. The report brings the result of the analysis based on the recent 30 years of data (1989-2018) on the mean spatial rainfall pattern as well as mean spatial pattern of different rainfall events, trends and variability as well as extreme rainfall events during the monsoon months and annual for the state. 17 Key Words Rainfall trend, variability, extreme events, dry days 2 1. Introduction The state of Rajasthan is located in the northwestern part of India. It is the largest state covering an area of 3,42,239 square kilometers, bounded by 23°30'North and 30°11'North latitude and 69°East and 78°East longitude. The state shares its west and northwestern boundary with Pakistan which is about 1070 km. Gujarat and part of Madhya Pradesh on its south, rest of Madhya Pradesh and Uttar Pradesh on its east and Punjab and Haryana on its north and northeast. This state has a type of rhomboid shape and stretches lengthwise 869 km from west to east and 826 km from north to south. The tropic of Cancer passes through its southern tip in the Banswara district. This state is practically free from maritime to influence. Orographic features and absence of maritime influence affect the climate of the state to a large extent. The distinguishing feature of the state is the Aravalli range, the strong barrier which divides the plains of Marwar (lying between 24°37' and 27°42'North Latitudes and 70°05' and 75°22' East Longitude) from the high table land of Mewar (lying between 23°49' and 25°28'North Latitude and 73°01' and 74°49'East Longitude). The range which commences at the ‘Ridge’ at Delhi, comes into prominence near the town of Ajmer, where it appears in a succession of parallel hills. The highest point, on which is perched the fort of Taragarh, rises immediately above the city of Ajmer to a height of about 870 metres above sea level between 395 and 425 metres above the valley at its base. The 'Nagpahar' or serpent hill, 5 km west of Ajmer city, attains a slightly lower elevation. About 16 km from Ajmer, the hills disappear for a short distance but in the neighbourhood of Beawar form a compact double range which approach each other southward and finally meet near Todgarh, from where a succession of hills and valleys extends to the farthest extremity of the Merwara. Thence the range gradually becomes bolder and more precipitous, till it finally terminates in the southeast corner of the Sirohi district near Mount Abu. The plateau on which the town of Ajmer stands, marks the highest point in plains of India and from the hills, which surround the land slopes away on every side range of hills between Ajmer and Nasirabad marks the dividing watershed of the Indian sub-continent. The entire portion of West Rajasthan formerly known as Marwar (lying 24o37'and 27°42' North Latitude and 70°05' and 70o22' East Longitude) is sterile, sandy inhospitable but improves gradually from a mere desert in the far west and north comparatively fertile and habitable lands in the northeast, east and southeast in neighbourhood of the Aravalli hills. The 'great desert', forming the whole of the West Rajasthan - Sind frontier, extends from the edge of the Rann of Kutch beyond Luni river northward and between it and what has been called the "little desert on the east is a zone of absolutely barren country, consisting of a rocky land cut off by limestone ridges, which to some extent protect it from the desert sands. These places are covered with sand hills, shaped generally in long straight ridges, which seldom meet but run in parallel lines. Some of these ridges may be 3 kilometres long. 3 Many studies are available on the observed trends and variability of rainfall and also extreme rainfall events, but all the studies are based on past 100 years or more data and also the recent years are not included (Guhathakurta et al, 2015; Guhathakurta et al, 2011; Guhathakurta & Rajeevan, 2008 etc). In the present report all the analysis of observed rainfall patterns, trends and variability have been done based on recent past 30 years (1989-2018) that will help to have idea of the recent changes for climate change adaptation and management by the state authorities. 2. Data and Methodology Daily Rainfall data from 1989 to 2018 is considered for analysis of trend variability and mean rainfall patterns. From the daily rainfall data monthly rainfall series of each station are computed and then monthly district rainfall series has been constructed by considering arithmetic average of all the station rainfall values within the district. The monthly rainfall series of the state has been computed by using area weighted rainfall values of all the districts within the state. The analysis has been done in two parts. For identification of the spatial pattern mean rainfall and variability and observed trends we have used district rainfall series and results have been brought out for four southwest monsoon months viz. June, July, August, September, for the southwest monsoon season and also for annual. Fig.1 gives the location of the districts of the state. For identification of mean pattern and trends of intensities of various rainfall events we used the station daily rainfall data. From mean and standard deviation (SD), the coefficient of variation (CV) is calculated as follows: Standard Deviation Coeffcient of variation (CV) = × 100 Mean Fig. 1 Location of the districts of Rajasthan 4 3. State rainfall mean and variability and trend Table 1 shows the mean rainfall (mm) and coefficient of variation of the state for the monsoon months, southwest monsoon season and annual during the period 1989-2018. It can be seen that the state gets highest rainfall (38%) of south west monsoon rainfall in July month while the August month get 35% of the south west monsoon rainfall. June and September receive 12% and 15% of south west monsoon rainfall. Also more than 91% of annual rainfall receives during the southwest monsoon season only. The variability of monsoon or annual rainfall is also very less (24-25%). June July August September JJAS Annual Mean 51.5 156.1 144.7 61.9 414.2 454.9 CV 53.5 38.5 47.3 60.8 25.5 24.0 Table 1 Mean rainfall (mm) and coefficient of variation of the state for the monsoon months, southwest monsoon season and annual Fig. 2 and 3 show the time series of rainfall in mm for the months of June, July, August, September and southwest monsoon season, annual respectively. The trend lines are also displayed for each of the series. Neither monthly rainfall nor seasonal or annual rainfall shows any significant increasing/decreasing trend. All the monsoon months as well as southwest monsoon and annual rainfall shows increasing trend ( nonsignificant). During the last 30 years highest rainfall of June (128.9 mm) received in 1996, July (262.4 mm) received in the year 2015 while highest rainfall of 278.1mm in August received in the year 2016 and of 138mm in September received in the year 2011. The highest annual rainfall of 618.4 mm and highest southwest monsoon rainfall of 573.7 mm received in the year 2013 and 1994 respectively. 5 Fig. 2 Time series of rainfall in mm for the months of June, July, August, September and trends Fig. 3 Time series of rainfall in mm for the southwest monsoon season and annual trends 6 4. District rainfall mean, variability and trend 4.1 Mean and coefficient of variation Table 2 gives the rainfall statistics for the districts of Rajasthan for the four monsoon months, southwest monsoon season and annual while Fig. 4 to 5shows the spatial pattern of these statistics. It can be seen that south-eastern districts viz.