Forest Hydrology in High Tropical Mountains: Contrasting Eco-Hydro-Geomorphological Processes in Temperate Broadleaved and Sub-Alpine-Conifer-Mixed Tropical Montane Forests of Eastern Himalaya, India

Ecohydrological processes controlling key water and carbon services remain least understood in the high tropical mountains, such as the Himalaya. In a first, the study compares the hydrological functioning of the world's highest non-glaciated headwater catchments (<25 km2) in the Eastern Himalaya using a physically-based, data-driven mechanistic baseflow-separation filter developed by Furey and Gupta (2001), discharge variability and lag regression analysis. The five catchments cover gradients of elevation (3750-4900 mm), catchment size (0.2-13.4 km2), stream-order (first-tothird), precipitation (3700-4900 mm) dominated by temperate broad-leaved (henceforth temperate) and sub-alpine conifer-mixed (henceforth alpine) forests. We empirically derive coefficients of overland flow, evapotranspiration, and groundwater recharge using the filter, which performed better in temperate than in alpine streams. The overland and rapid sub-surface flow varied considerably with antecedent moisture, catchment properties (size, bed-slope, and geomorphology), and vegetation. The overland flow and groundwater recharge coefficients were significantly higher in temperate than in alpine streams. Hydrogeological and catchment characteristics dominated groundwater storage and recession rather than vegetation. Instead, vegetation and climate were primary factors governing evapotranspiration and overland flow across the streams, while catchment properties were critical secondary factors. Seasonally, the low flows were primarily governed by local hydrogeology, catchment size, and snowmelt, whereas vegetation characteristics modulated high flows. Vegetation and catchment properties controlled the diel and seasonal flows, respectively. Low-intensity precipitation (<10 mm hr-1) facilitated highest recharge in temperate and alpine catchments. Future precipitation intensification and snowfall decline will likely generate higher runoff in temperate and reduced baseflows in alpine catchments. The study fills a critical knowledge gap in improving the accuracy of regional land-surface-atmosphere interaction models by factoring in the contrasting responses of different ecosystems in the Himalaya, where changes in vegetation and precipitation patterns with a changing climate may significantly impact the ecosystem services and regional water security.