Long-Term Shifts in Monsoon Patterns: A Study of Ken and Betwa Basins in India

Abstract

The spatial and temporal variability of monsoon rainfall frequently leads to droughts, dry spells, and excessive rainfall, affecting crop yields and production stability. Among these factors, the onset of monsoon is particularly crucial as it determines sowing schedules in rainfed agricultural systems. The delays or irregularities in the onset of monsoon can disrupt timely planting leading to substantial yield losses and potentially affecting subsequent cropping cycles which can have broader implications for farm income and food security. This study examined the Number of Rainy Days (NRD) and Onset of Effective Monsoon (OEM) in Ken basin and Betwa basin, where agricultural activities depend mainly on monsoonal rainfall. The long-term daily rainfall data from 1971 to 2019 for 49 years were used for this analysis. The OEM and NRD were determined using Ashok Raj's approach, which defines agriculturally effective monsoon onset in terms of rainfall adequacy and continuity. The Mann-Kendall (MK) test was used to identify trends in a time-series dataset without requiring the data to conform to a particular distribution. The relative change was calculated to quantify the overall variation during the study period. The result indicated that monsoon variation has changed noticeably in both basins. The onset of effective monsoon exhibited no trend in Ken Basin with total change ranging from 17 to -4 days, whereas in Betwa basin, the onset is generally delayed with total change ranging from 17 to -32 days. The number of rainy days showed a predominantly decreasing trend in both basins. In Ken basin, the total change in rainy days ranges from 8 to -12 days, while in Betwa Basin, the decreased is more significant, with total change ranging from 11 to -16 days. The decreased in rainy days, along with a delay in monsoon onset indicating extended dry spells and lower early-season soil moisture availability. The irregularity of rainfall is rising due to a concurrent reduction in rainy days and a postponement in the start of the effective monsoon. These alterations have important agro-hydrological impacts, such as increased irrigation needs, limited water supply at the start of the growing season, and greater vulnerability to seasonal droughts. The findings highlight the importance for farmers, planners, and policymakers to promote climate-resilient agricultural methods to improve soil moisture retention and basin-wide adaptation approaches.