Monsoon-Driven Hydrological Pathways of Flood Generation in Semi-Arid River Basins

Abstract

Hydrological processes that drive basin-scale flood generation in semi-arid river systems remain poorly constrained, limiting effective basin management under an increasingly variable monsoon regime. Using a historic monsoon flood in a regulated semi-arid basin, we show that the synchronisation of hydrometeorological perturbations (or extremes) with reservoir operations activates distinct hydrological pathways responsible for extreme downstream flooding in semi-arid river basins. A systematic change in rainfall intensity distribution from light to moderate and heavy events as the monsoon progressed altered the basin-scale hydrological balance. The rainfall sequencing, from widespread early-season events to intense, localised late-season extremes, transitioned the basin from an infiltration-dominated behaviour to a runoff-dominated response. These transitions produced an atypical dual-peak reservoir discharge pattern–an unprecedented hydrological response in a semi-arid basin, where high reservoir outflows are rare and, when they occur, are confined to a single seasonal peak. Consequently, basin buffering capacity was progressively reduced during the staggered first discharge peak, promoting runoff concentration during the subsequent highly synchronised peak release. The late-season occurrence of dynamically driven mesoscale convective systems intensified this hydrological response by delivering concentrated rainfall when antecedent storage and hydrological loading were already elevated. The conceptual framework developed from the extreme monsoon event demonstrates that flood generation in regulated semi-arid basins is also governed by the spatio-temporal organisation of rainfall, antecedent hydrological pressure and the transition of reservoir systems from buffering to synchronising behaviour. These previously undocumented characteristics of the regional hydrometeorology and flood response serve as a basis for improved process-level understanding of semi-arid river basin hydrology. The findings of this study provide actionable insights for anticipatory flood management, adaptive reservoir operations and climate-resilient basin-scale planning in dryland river systems experiencing intensifying hydroclimatic extremes.