Vegetation studies establishing direct mechanistic linkages between stand transpiration and streamflow are rare from sub-tropical and tropical montane forests (TMFs) like Himalaya. We quantified the impact of diurnal and seasonal transpiration on lean season streamflow in a broad-leaved evergreen secondary TMF in Eastern Himalaya. Whole-tree and stand transpiration were measured using Granier’s thermal dissipiation sap flow probes at one of the wettest (4500 mm yr-1) and highest elevation (2100 m) sites in the world to date. The observed daily and annual transpiration rates were double of the reported values from TMFs in relatively drier Central Himalaya, but at the lower bound of TMFs globally. Solar radiation was the key driver of transpiration in energy-limited winter under hydrated conditions. Vapour pressure deficit (D) controlled transpiration in energy-abundant summer. We also found that moderate precipitation events (10-30 mm) followed by clear skies can induce significant increase (93±110 %) in stand transpiration. In turn, transpiration was the main driver of lean season streamflow in dry winter and to a lesser extent in wet summer. Thus, in winter, the transpiration-driven abstraction induced corresponding diurnal cycles in soil moisture and streamflow with an average lag of 1.3±1.8 hours and 2.9±2.5 hours, respectively, and strong negative correlations (-0.8±0.1). Thus, changes in vegetation cover and precipitation patterns are likely to impact local and regional moisture recycling by vegetation and lean season flow, thereby affecting regional water security in the Eastern Himalaya.

We examined the effect of land cover on stream discharge in hilly catchment streams during extreme rain events. Three years of rainfall-runoff observations, between January 2014 and December 2016, were collected in eleven neighbouring catchments. Each catchment was dominated by a different land cover, namely natural shola forests, natural grasslands and wattle (Acacia mearnsii). Rain intensities between percentiles 25-90, 90-95 and over 95 were categorised as light, heavy and extreme and were used to study stream discharge responses. Land cover significantly influenced the hydrologic response to extreme rain events. During light rains (< 38 mm/day), grassland dominated catchments showed higher discharge than shola (0.01 mm/s) and wattle (0.004 mm/s). However, during extreme rain events (> 71 mm/day) discharge was significantly higher in wattle dominated catchments when compared to the natural shola (0.033 mm/s) and grasslands (0.023 mm/s). Antecedent moisture conditions played a major role in determining peak flows along with rainfall, catchment shape and drainage density.