Under a changing climate, the Sub-Himalayan ecosystems are likely to experience marked transformations in hydrological, biogeochemical and biophysical processes. To explore this, we have been observing various hydrometeorological parameters in a completely rainfed sub-Himalayan watershed (Salla Rauetella Watershed) since 1991. We noted a changing trend for some of the hydrometeorological parameters over the 22-yr period. While the annual air temperature has increased significantly, the annual rainfall also shows an increasing trend with a higher probability of increased rainfall intensity. The run-off data show a peculiar trend that the watershed has been transforming itself from a perennial to an ephemeral system, despite an increasing trend of rainfall magnitudes. This is primarily attributed to the increasing trends of rainfall intensities exceeding the infiltration capacities of the soil which trigger large but high-intensity run-off events with dry spells in other periods, which makes the river ephemeral. We infer a likely dynamic change in the run-off-generation mechanism which warrants the need for a more precise and rigorous observation-cum-measurement strategy of ecohydrological processes in Himalayan ecosystems, supported with modelling and remote sensing approaches. This will help identify the optimal headwater treatment measures for augmenting groundwater to sustain the rainfed streams of the Himalaya under a changing climate.