Analysis of Rainfall Trends and its Spatial Patterns During the Last Century over the Gangetic West Bengal, Eastern India

Abstract

Long-term spatial and temporal trends of rainfall at monthly, seasonal, and annual scales have been studied for 12 meteorological stations of the Gangetic West Bengal located in Eastern India during 1901–2002 using 102 years of rainfall data. The non-parametric Mann-Kendall test and Sen’s slope estimator were used to detect trends and their slope. The changes are calculated in percentage over the time period. The results highlight a marked increase in post-monsoon (33.87%), the overall increase in annual (2.61%), a considerable decrease in winter (14.83%) as well as pre-monsoon (4.03%), and an inconsequential increase in monsoonal (1.21%) rainfall. In the annual and monsoonal series, the trend is positive in the southern half but negative in the northern counterpart. A considerable decrease in rainfall during June and August at most stations signifies that monsoon is losing in the early monsoonal months with an occurrence of mid-season dry spells. The increase in rainfall during September (13.80%) and October (34.38%) reveals that the monsoon is shifting toward these late monsoon and post-monsoon months, respectively. Both the decrease of rainfall in June (early monsoonal month), as well as an increase in rainfall in September (late monsoonal month) and October (start of post-monsoonal month), suggest that the monsoon is being delayed on its onset and withdrawal. In the Rarh region, monsoon rainfall is reducing whereas post-monsoon rainfall is increasing. In the Deltaic region, both monsoon and post-monsoon rainfalls are increasing. Such altering patterns of rainfall call for reviewing the agricultural practices and water use in this region.