Thresholds for Hydrological Stability Under Forest Loss and Urban Expansion in the Western Himalayas

Abstract

Deforestation profoundly alters watershed hydrology, degrading infiltration capacity, reducing soil moisture retention, amplifying runoff dynamics, increasing flood risks and decreasing groundwater recharge. This study compared hydrological implications in two climatically similar but ecologically distinct basins in the Western Himalayas: the Kunhar watershed, characterised by stable forest cover, and the Swat Watershed, with progressive forest loss coupled with rapid urban expansion. Using the Digital Watershed Model (DWM) integrated with the Ensemble Kalman Filter (EnKF) for data assimilation, the hydrological behaviours were quantified across these basins. Results identified a critical threshold of 62% forest cover (with an uncertainty range of ±5%), below which hydrological stability is compromised. While applying EnKF data assimilation significantly improved model accuracy, reducing peak flow errors by 52% in Kunhar and 38% in Swat. Monte Carlo simulations ensured statistically robust identification of critical forest cover thresholds that influence watershed stability. This study provides new insights into the hydrological consequences of forest loss in the Himalayas and proposes actionable thresholds for maintaining forest cover to ensure watershed stability.