Tripura State
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CLIMATE RESEARCH AND SERVICES INDIA METEOROLOGICAL DEPARTMENT MINISTRY OF EARTH SCIENCES PUNE Observed Rainfall Variability and Changes over Tripura State Met Monograph No.: ESSO/IMD/HS/Rainfall Variability/26(2020)/50 Pulak Guhathakurta, Sudeep Kumar B L , 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/26(2020)/50 Observed Rainfall Variability and Changes Over Tripura State Pulak Guhathakurta, Sudeep kumar B L, 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 Tripura State 2 Issue No. ESSO/IMD/HS/Rainfall Variability/26(2020)/50 3 Issue Date January 2020 4 Security Unclassified Classification 5 Control Status Uncontrolled 6 Document Type Scientific Publication 7 No. of Pages 23 8 No. of Figures 42 9 No. of References 3 10 Distribution Unrestricted 11 Language English 12 Authors Pulak Guhathakurta, Sudeep kumar B L, 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 Tripura state is located in the northeastern region of India. The state lies approximately between latitude 220 56’N and 240 32’N and longitude 91009’E and 92010’E and covers an area of about 10492 Km2. It is bordered by Bangladesh on the north, west, south and southeast, by Assam and Mizoram on the northeast. Tripura is a land locked state and its aerial distance to Bay of Bengal is about 100 km. Tripura has small hills and low lying land with a numerous deep rivers and valleys. The hills of the state run from north to south and parallel to one another till they disappear in the plains of Sylhet in Bangladesh. The principal hill ranges are the Baramura-Deotamura ranges, Atharamura ranges, Langtari ranges, Sakhan ranges, and the Jampui hill ranges from the east. The highest peak of the state is known as Betling Shiv around 935 metres above mean sea level in the Jampui hill range. The hills are mostly of low elevation and are covered by thick forests. Many studies available on the observed trends and variability of rainfall and also extreme rainfall events over India, 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). Also, there are limited studies on district rainfall trends and variability of West Bengal state. 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 stations 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. Fig.1 gives the location of the districts of the state. The objective of the analysis is to: 1. Identify the spatial pattern of the mean rainfall 2. Understand district wise observed rainfall trend and variability in annual and SW monsoon season (June, July, august and September). Daily station rainfall data is utilized for identification of the mean spatial patterns and rainfall intensity trends. From mean and standard deviation (SD), the coefficient of variation (CV) is calculated as follows: 3 Standard Deviation Coeffcient of variation (CV) = × 100 Mean Fig. 1 Location of the districts of Tripura 3. State rainfall mean, 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 (31% of south west monsoon rainfall) in June month followed by July (27% of the south west monsoon rainfall). August and September receive 23% and 19% of south west monsoon rainfall, respectively. About 60% of annual rainfall receives during the southwest monsoon season. The variability for monsoon and annual rainfall is about 19% each. June July August September JJAS Annual Mean 436.0 385.4 330.9 270.3 1422.6 2380.4 CV 35.5 37.4 26.5 26.9 19.1 18.6 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 4 series. There is no significant trend in the rainfall for monthly, southwest monsoon season or annual series. Although there is an insignificant decreasing trend in the southwest monsoon season and annual rainfall. In southwest monsoon months, an insignificant decreasing trend in June, July and September rainfall and an insignificant increasing trend in the August rainfall is seen. In last 30 years, the highest rainfall of June, July, August and September is received in the year 2003 (819.4 mm), 1998 (724 mm), 1995 (554.7 mm) and 2004 (459.4 mm) respectively. Highest southwest monsoon rainfall (2088.7 mm) and annual rainfall (3485.7 mm) are received in 1993. Lowest rainfall of June, July, August and September received in 2005 (192.2 mm), 2000 (169.7 mm), 2003 (154.3 mm) and 1998 (159.8 mm) respectively. Lowest rainfall of southwest monsoon season and annual rainfall is received in 1994 (1062.3 mm) and 2006 (1764.5 mm) respectively. 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 5 4. District rainfall mean, variability and trend 4.1 Mean and coefficient of variation Table 2 gives the rainfall statistics for the districts of Tripura for the four monsoon months, southwest monsoon season and annual, and Fig. 4-5 show the spatial pattern of these statistics. South Tripura receives the highest rainfall in the southwest monsoon season (1549.1 mm) and North Tripura receives the highest rainfall in the annual (2542.5 mm). Maximum amount of rainfall in June, July, August and September are received in Dhalai (466.7 mm), South Tripura (475.4 mm), South Tripura (364.2 mm) and North Tripura (305.4 mm) respectively. The lowest rainfall received in West Tripura in southwest monsoon (1260.9 mm) as well as annual (2103.3 mm). West Tripura receives the lowest rainfall in June (391.1 mm), August (275.6 mm) and September (237.7 mm) and Dhalai(352.4 mm) receives lowest rainfall in July. The districts exhibiting maximum and minimum variability of mean rainfall are Dhalai (51.6%) and North Tripura (34.8%) for June, Dhalai (56.3%) and South Tripura (39.9%) for July, Dhalai (47.8%) and North Tripura (30.4%) for August and West Tripura (40%) and North Tripura (29.2%) for September. Dhalaishows the maximum variability of rainfall during the southwest monsoon (30.7%) and annual (34.7%). South Tripura (20.8%) and North Tripura (19.1%) shows the minimum variability during the monsoon and annual respectively. JUNE JULY AUGUST SEPTEMBER MONSOON ANNUAL DISTRICT MEAN CV MEAN CV MEAN CV MEAN CV MEAN CV MEAN CV NORTH TRIPURA 449.1 34.8 370.0 48.4 351.5 30.4 305.4 29.2 1476.0 22.3 2542.5 19.1 DHALAI 466.7 51.6 352.4 56.3 349.1 47.8 274.6 36.0 1442.8 30.7 2501.7 34.7 WEST TRIPURA 391.1 40.9 356.6 46.4 275.6 32.9 237.7 40.0 1260.9 24.4 2103.3 20.4 SOUTH TRIPURA 452.6 36.7 475.4 39.9 364.2 33.4 256.9 39.2 1549.1 20.8 2407.9 21.8 Table 2. Rainfall statistics for the districts of Tripura for the four monsoon months, southwest monsoon season and annual a b 6 c d e f Fig.4 Mean rainfall pattern over districts of Tripura for (a) June, (b) July, (c) August, (d) September, (e) southwest monsoon and (f) annual.