Detecting Shifts of Monsoon Precipitation Patterns and a Large Increase in Soil Erosion Potential During 1979–2020 in Nepal

Abstract

Highlights: What are the main findings? TPHiPr outperformed seven other gridded precipitation datasets in capturing Nepal’s complex rainfall patterns, especially at high altitudes. Rainfall-runoff erosivity (R-factor) is rising nationwide due to increased frequent and intense extreme precipitation, notably in high-altitude regions. What are the implications of the main findings? Soil erosion risk is increasing across Nepal, including areas with declining total rainfall, requiring targeted soil conservation in vulnerable zones. Watershed management policies must address both intensified monsoon erosion/flooding in central/eastern Nepal and elevated dry-season water scarcity in western Nepal. Nepal is highly vulnerable to severe soil erosion driven by monsoonal rainfall and rugged terrains. Limitations in ground observation networks have hindered comprehensive, high-resolution national assessment of precipitation and rainfall-runoff erosivity (R-factor) across Nepal. This study systematically evaluated eight global gridded precipitation datasets (GPDs) against data from 152 weather stations, identifying the optimal precipitation dataset (TPHiPr) representing Nepal’s complex topography. Based on this high-quality dataset, we provided the first independent, long-term (1979–2020), high-resolution national-scale assessment of precipitation and the R-factor for Nepal. Our analysis reveals that 1996 marked a turning point in nationwide precipitation trends: annual precipitation shifted from a decreasing to an increasing one in the humid eastern and central regions, while the drier western region transitioned from an increasing to a decreasing trend, particularly during the dry season. A clear spatial divergence was observed between total precipitation and the R-factor, highlighting the dominant role of precipitation frequency and intensity. Extreme precipitation events intensified significantly (e.g., days with ≥25 mm rainfall increased by 0.2 days yr−1, and the 95th percentile precipitation threshold increased by 0.4 mm yr−1, p < 0.01), driving a nationwide increase in the R-factor (6.3 MJ mm ha−1 h−1 yr−2, p < 0.01), with high-altitude areas experiencing the most pronounced effects. We conclude that soil erosion risk has intensified nationwide due to increasing precipitation extremes. Watershed management must develop elevation-specific adaptation strategies that integrate climate science with practical solutions to address the dual challenges of intensified monsoon-driven erosion and growing dry-season water scarcity.