Spatial-Temporal Variability of Rainfall over Chitwan District, Nepal

This study analyzes rainfall data (2014–2024) from 10 meteorological stations across Chitwan District to assess spatial-temporal variability. Monsoon (June–September) contributes 78% of annual rainfall, with peaks in July (mean: 435 mm). Pre-monsoon (March–May), post-monsoon (October–November), and winter (December –February) contribute 15%, 4.5%, and 2.5%, respectively. Spatial analysis reveals higher rainfall in northern hills (e.g., Madi: 2,150 mm/yr) versus southern plains (e.g., Bharatpur: 1,400 mm/yr). A significant decreasing trend (-0.82 mm/yr) in annual rainfall was observed. Mann-Kendall tests show 8 stations with declining trends (2 significant). These findings highlight climate vulnerability in a key agricultural region. This study investigates the spatial-temporal variability of rainfall across Chitwan District, Nepal, over the period 2014–2024 using data from ten ground-based meteorological stations. Seasonal and annual rainfall distributions were analyzed to assess long-term changes in precipitation patterns. The results indicate pronounced intra-annual and inter-annual variability, with July recording the highest mean monthly rainfall (644.6 mm) and November the lowest (7.9 mm). The monsoon season (June–September) accounted for approximately 83% of the total annual precipitation, followed by the pre-monsoon (10.7%), post-monsoon (4.1%), and winter (2.2%) seasons. Spatial interpolation using the Inverse Distance Weighted (IDW) method revealed significant heterogeneity in rainfall distribution, with southern forested regions such as Madi and Kalika consistently receiving higher rainfall compared to northern and urban municipalities like Bharatpur and Khairahani. Trend analysis using the non-parametric Mann-Kendall test and Sen’s slope estimator identified a statistically significant decreasing trend in annual rainfall in several central and northern stations, with an average annual decline of 1.03 mm/year across the district. These findings underscore the increasing hydrological vulnerability of Chitwan’s lowland ecosystems and emphasize the need for region-specific water resource planning and climate adaptation measures.