Proportional Trends of Continuous Rainfall in Indian Summer Monsoon

A comprehensive study on the Indian summer monsoonal rainfall (ISMR) is performed in the light of decadal changes in the continuous rainfall events and the number of rainy days using 68 years (1951-2018) of gridded rain gauge data. Non-parametric Mann-Kendall's test is applied on total rainfall amount, the number of rainy days, number of continuous rainfall events, and rainfall magnitude to find trends over different climatic zones of India for the two periods, 1951-1984 and 1985-2018. Our results found a decreasing trend for more than 4-days of continuous rainfall events during the recent 34 years (1985-2018) compared to 1951-1984. The rate of increase/decrease in extreme/continuous rainfall events does not follow a similar trend in number of continuous rainfall events and magnitude. Moreover, the rainfall is shifted towards a lesser number of continuous rainfall days with higher magnitudes during 1985-2018. During the crop's sow season (i.e., the first 45 days from the onset date of Indian monsoon), the total number of rainy days decreased by a half day during the last 34 years. Over the Central and North East regions of India, the number of rainfall days decreased by similar to 0.1 days/yr and similar to 0.3 days/yr, respectively, during 1985-2018. Overall, the decreasing trends in continuous rainfall days may escalate water scarcity and lead to lower soil moisture over rain-fed irrigated land. Additionally, an upsurge in heavy rainfall episodes will lead to an unexpected floods. On a daily scale, rainfall correlates with soil moisture and evaporation up to 0.87 over various land cover and land use regions of India. Continuous light-moderate rainfall seems to be a controlling factor for replenishing soil moisture in upper levels. A change in rainfall characteristics may force the monsoon-fed rice cultivation period to adopt changing rainfall patterns.

Automated summary

The study revealed a decreasing trend in 4-day or longer continuous rainfall events and a reduction in the number of rainy days in the Indian summer monsoon over the past 34 years. This change may lead to water scarcity, reduced soil moisture, and an increased risk of flooding from heavy rainfall events.