Characterizing natural drivers of water-induced disasters in a rain-fed watershed: Hydro-climatic extremes in the Extended East Rapti Watershed, Nepal

Extreme climatic and hydrological events result in water-induced disasters and associated loss and damage of lives, livelihoods, and properties. It is related with various climatic, topographical, and anthropogenic factors, and therefore, expected to vary widely across the watersheds. This study characterized historical and projected future trends in climatic extremes, their spatial variations, hydrological extremes, and linkage between hydro-climatic extremes for a rain-fedExtended East Rapti (EER)watershed in Central-Southern Nepal. The watershed feeds intothe Gangesin the South Asia through the Gandakiriver. A set of 14 climate extreme indices, seven related to precipitation and seven to temperature, and eight hydrological indices were selected to characterize the extremes. Climatic and hydrological extreme indices were computed using RClimDex and IHA (Indicators for Hydrological Alteration) tools, respectively. Trends were calculated using the Modified Mann-Kendall test and Sens’s slope estimator. Relationship between hydrological and climatic extremes was evaluated by checking dates for rainfall extreme, hydrological extreme, and reported cases of flooding during that period. Results showed increasing trends in both precipitation and temperature extremes for the historical period (1980–2005) with a rate of 10–35% increase in RX1day (monthly maximum 1-day precipitation), 10–50% increase in R95p (very wet days precipitation amount) and 15–60% increase in warm nights from the base period until the mid-century. Hydrological alterations in terms of increasing extremes are also clearly visible in maximum flows, minimum flows as well as the shift in the day of maximum flow. Since, hydro-climatic extremes bear a direct relationship, future hydrological extremes, primarily floods, are expected to increase in future.