Forest Composition Controls on Water Security in Nepal's Middle Hills

Montane watersheds in the Himalayan region are becoming increasingly vulnerable to hydroclimatic extremes that cause floods, landslides, droughts and seasonal water scarcity. These risks are compounded by ecohydrological transformation resulting from shifts in forest composition, notably the widespread establishment of pine plantations, as well as climate variability and intensified land‐use activities. Understanding how forest composition influences watershed hydrological response remains a key challenge. Here, we use long‐term data and process‐based modelling of forest canopy trends to simulate ecohydrological dynamics in a headwater catchment supplying water to the Kathmandu Valley. The model is used to assess the hydrological performance of four forest composition scenarios: status quo (current forest cover), monoculture‐pine, monoculture‐broadleaf and managed‐mixed forest. The process‐based model, RHESSys, was used to evaluate seasonal evapotranspiration (ET), groundwater depth response, streamflow and canopy dynamics across scenarios. Given the uncertainty in climate forcing data and the strong topographic variability, the model reproduced observed daily streamflow reasonably well. Daily NSE, LNSE, KGE and PBIAS were 0.55, 0.65, 0.60 and 24% in calibration and 0.57, 0.72, 0.61 and 23% in validation, respectively. Monthly values were 0.61, 0.67, 0.64 and 24% in calibration and 0.68, 0.78, 0.66 and 23% in validation, respectively. Results show that pine‐dominated forests exhibit elevated year‐round water demand, and therefore lower low flows (lowest 1% of daily flows) by 7.7%. In contrast, broadleaf forests can reduce high flows (highest 1% of daily flows) by 4% while increasing low flows by 29% and reducing ET during water‐limited periods. However, the managed‐mixed forest scenario delivers the most balanced and sustainable hydrological performance, reducing high flows by 5% and increasing low flows by 16%. These findings highlight the critical role of forest type and management in shaping watershed‐scale water balance and that strategically designed, topographically informed mixed‐species restoration is essential to sustaining water availability under increasing climate pressure. This study offers a case study–based modelling framework with potential transferability to similarly vulnerable montane landscapes across the Himalayan region.