Peak Rainfall Seasonality and Its Changes Influenced by Monsoons in India

ABSTRACT Seasonality of peak rainfall finds its application in several water resource activities such as deciding the timing of agriculture, regulation of water control structure, predicting the timing of floods; and understanding the lag between rainfall and floods. In a country like India, rainfall (which is the primary source of water) is influenced by different monsoon patterns, causing a disparity in its seasonality, thus, understanding the timing of the peak rainfall and its changes becomes crucial for water resources management. Here, the circular statistical approach is employed to understand the characteristics of peak rainfall (PRF) seasonality in India. It is observed that the western coast and central parts are influenced by the southwest monsoon (June–September) yielding strong seasonality. While, dominance of the northeast monsoon (October–December) is observed along the eastern coast with moderate to strong seasonality moving from north to south. The region lying between the west and the east coast is influenced by both southwest and northeast monsoons yielding bimodal seasonality of PRF. Also, northeast India is found to be influenced by both monsoon patterns. A weak seasonality of PRF is observed in the northern parts that are influenced by several factors like southwest monsoon, northeast monsoon, and snowfall (December–April). Three existing tests along with the proposed modified versions of Pettitt and Mann–Kendall tests are applied to detect changes in the seasonality of PRF. Grids identified with changes at 5% and 10% significance levels considering these tests showed significant differences in mean date and resultant length, and gradual shift in the dates of PRF. The analysis demonstrates the necessity of distinct water resource planning for different Indian regions. Also, changes in the timings of PRF implicate the effect of climate change, which demonstrates the need for a revision in water resources planning and management.