Unraveling spatio-temporal rainfall variability in the Tlawng watershed, Mizoram under a tropical monsoon regime

This study provides a comprehensive assessment of the spatiotemporal characteristics and long-term trends in rainfall across the Tlawng watershed in Mizoram, Northeast India, over a 40-year period (1984–2023). Rainfall variability and distribution patterns were evaluated using multiple indices, including the Precipitation Concentration Index (PCI), Percent of Normal Precipitation Index (PNPI), Rainfall Anomaly Index (RAI), and Rainfall Seasonality Index (RSI). To detect trends and quantify their magnitude, non-parametric statistical techniques namely Innovative Trend Analysis (ITA), Percent Bias (PBIAS), and Sen’s Slope Estimator (Q) were employed. The statistical significance and reliability of the observed trends were assessed using the Mann-Kendall (MK) test, Spearman’s Rho test, and the Modified Mann-Kendall (MMK) test. The results indicate a predominantly irregular rainfall distribution throughout the watershed. The PCI and RSI values suggest that precipitation is largely concentrated during the monsoon months, confirming the seasonal dependency of the region’s hydro-meteorological regime. Notably, the analysis revealed a higher frequency of wet years (61.25%) compared to dry years (38.75%), as determined by RAI and PNPI metrics. Consistent positive trends in rainfall were identified across all observation stations, as evidenced by ITA, Q, and PBIAS outputs. Furthermore, the MK (α < Z), MMK (α < Zc), and Spearman’s Rho (α < Zsr) tests confirmed statistically significant increasing trends at the 95% confidence level (α ≥ 0.05) across all locations. Monthly trend analysis indicated that the period from May to October exhibited significantly increasing rainfall trends. On a seasonal scale, both the warm and monsoon seasons demonstrated significant positive trends, as confirmed by Zc and Zsr values. “Rainfall showed an increase of + 1.8 mm yr⁻¹ during the monsoon and a decline of − 0.9 mm yr⁻¹ during the pre-monsoon season, while winter rainfall increased by + 0.6 mm yr⁻¹. These findings underscore the growing importance of integrated watershed-scale hydrological planning in the face of climatic variability. The insights derived from this study are expected to support policymakers, watershed management authorities, and researchers in formulating strategies for sustainable water resource management, particularly in relation to irrigation planning and water supply enhancement in the region.