Long-period trend analysis of annual and seasonal rainfall in West Bengal, India (1901–2020)

Location-specific information on annual and seasonal rainfall trends has immense utility in devising crop planning as well as water resource management in West Bengal. We assessed long-period (1901–2020) trends and magnitudes of seasonal and annual rainfall across districts of West Bengal. The non-parametric Mann-Kendall (MK) test, sequential Mann-Kendall (SQMK) test, and Sen’s slope estimator were applied to the gridded (0.25° × 0.25°) rainfall dataset. Results revealed that both the annual and seasonal rainfall of West Bengal increased non-significantly (p>0.05), except for the winter season, which experienced a non-significant decrease. Annual rainfall across a majority of the districts in sub-Himalayan West Bengal (SHWB) declined non-significantly. On the contrary, a significant (p<0.05) increase in annual rainfall was observed across most of the districts of Gangetic West Bengal (GWB) at the rate of 1.8 mm year−1 to 2.9 mm year−1. Monsoon rainfall increased significantly over Gangetic West Bengal (GWB) at 1.4 mm year−1 to 2.1 mm year−1, while it declined significantly in Dakshin Dinajpur district at 3.2 mm year−1 of SHWB. Post-monsoon rainfall increased significantly over GWB at the rate of 0.5 mm year−1 to 0.9 mm year−1. The winter rainfall decreased non-significantly across all the districts. Normalized rainfall anomaly (NRA) revealed that 17% of the study period experienced surplus (NRA>1) rainfall, while another 17% received deficit (NRA>−1) rainfall. Nearly 70% year received normal monsoon rainfall (NRA = ±1) and 16% year deficit and 14% year surplus rainfall. The SQMK test for seasonal rainfall showed a general trend change point around 1960 and 1970 in both the district and state-level annual rainfall.