Trend analysis of total, seasonal and extreme rainfall data for Ajay River Basin, West Bengal.

Rainfall is an essential climatic parameter for any region, and it can have a significant socioeconomic impact on society. In this study, the trend analysis of rainfall data of the Ajay River Basin was performed for daily rainfall data from the APHRODITE dataset. It is a gridded dataset with a resolution of 0.25*0.25 degree latitude and longitude with 1951 to 2007 long time series for Asia. The non-parametric Mann-Kendall test was used to detect the monotonic trend in the rainfall time series and the Theil-Sen estimator to look at the magnitude of the change. The quantile perturbation method is used for extreme rainfall analysis. The study reveals that total annual rainfall and the monsoon period (June, July, August, September) have increased over the basin's southern part at a 5% significance level. In the pre-monsoon period (March, April, May) rainfall has increased all over the basin area at the 5% significance level. Extreme rainfall anomalies were found in most of the basin region, but some periods had very high perturbation. In the 1950-1960s, the northern area of the basin showed statistically significant negative anomalies, while the southern region showed significant positive anomalies. The 1970-1980s was the period of the highest significant positive anomalies, with up to 110% change. Significant negative anomalies dominated most of the southern basin from 1980 to 2000. The study concluded that although total rainfall has increased, extremes have decreased in the region.