Trends in Rainfall Patterns over the Tamarabarani Basin in Tamil Nadu, India

Rainfall is the key hydro-climatic variable that governs the regional hydrologic cycle and availability of water resources. Recent studies have analysed the changes in rainfall patterns at global as well as regional scales around the world. These studies have also suggested that any analysis of hydro-climatic variables should be performed at the local scale rather than at a large or global scale because the trends and their effects may be different from one location to the other. The Tamarabarani River basin is one of the important basins in Southern Tamil Nadu with rich surface water resources. The basin is mainly agriculture based and people are engaged in the cultivation of various crops like paddy, cotton, sugarcane, groundnut, etc. The Tamarabarani River provides consistent irrigation to a large agricultural area within the basin. The Tamarabarani River and its tributaries have many dams, with reservoirs providing water for irrigation and power generation. The Papanasam and Manimuthar dams located on the western part of the basin are two of the major reservoirs that cater to the water requirements of the district. The basin receives a majority of its rainfall during the northeast monsoon, which is during the months of October, November and December (with November having the maximum rainfalls). Thus, step changes and trends in rainfall for these stations and during the monsoon months would especially be of interest to the farmers, irrigators and water resource managers in the basin. Since no studies on hydro-climatic trend analysis were found specific to this basin, this study performs a spatiotemporal trend analysis on long-term rainfall records (1971-2000) at 14 measuring stations within the basin. The Mann-Kendall test was used to detect trends and the cumulative summation (CUSUM) technique was used to identify the trend beginning year for monthly rainfall data. The results of the CUSUM test indicated that the two stations of Papanasam and Dam Camp located in the Western Ghats had a statistically significant step-up change during the late nineteen seventies and early eighties. The trend analysis results indicated that increasing trends were observed (for most of the months) at 3 stations, which are Dam Camp, Papanasam and Cheranmadevi. The Cheranmadevi station exhibited the maximum number of months (seven out of twelve) with statistically significant increasing trends. For Papanasam and Cheranmadevi, the month of November exhibited statistically significant increasing trends. This is encouraging from the irrigation point of view, since these stations are located on the forested western part of the basin, which forms the catchment area for the major reservoirs. Thus, there is an increase in potential for harvesting water from this part of the basin. On the other hand, it is observed that five stations have exhibited predominantly decreasing trends for most of the months during the past three decades. These stations are Tirunelveli, Manimuthar, Nanguneri, Kovilpatti and Ayyikudi. Most of these stations are located in the central part of the river basin and these areas would need special attention as over exploitation of surface as well as ground water resources in these regions need to be restricted to conserve water for future use.