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Highly seasonal water supplies from the Himalayan watersheds of Jhelum, Kabul and upper Indus basin (UIB) are critical for managing the world's largest contiguous irrigation system of the Indus basin and its dependent agrarian economy of Pakistan. Here, we assess changes in the contrasting hydrological regimes of these Himalayan watersheds, and subsequent water availability under the Paris Agreement 2015 targets that aim of limiting the mean global warming to 1.5 degrees C (Plus1.5), and further, well below 2.0 degrees C (Plus2.0) relative to pre-industrial level. For this, calibrated semi-distributed hydrological model setups of the University of British Columbia are coupled with the 80-member ensemble experiments of Plus1.5 and Plus2.0 performed under the half a degree additional warming, projections, prognosis and impacts (HAPPI) framework. Our results suggest that during October to February, HAPPI ensembles project a median warming of up to 1 degrees C under Plus1.5 and of more than 1.5 degrees C under Plus2.0 relative to the historical period (2006-2015). Such warming is generally higher in daily minimum than daily maximum temperature and over high altitudes. Both scenarios project drying of the monsoon (July-September) and the main snowpack accumulation/melt seasons (March-June). Results of hydrological modeling suggest significant changes in the timings of contrasting hydrological regimes that collectively reflect on changes in the overall hydrology of the Himalayan watersheds. Median changes generally indicate strengthening of the nival and glacial regimes for all basins under Plus1.5, except for the nival regime of the Jhelum basin, whereas such changes are more pronounced under Plus2.0. Subsequently, annual water availability from three Himalayan watersheds will increase by 34% and 43% under Plus1.5 and Plus2.0, respectively. These findings provide an overall picture of the surface water availability in Pakistan under the aspirations of the Paris Agreement 2015 and can inform on policy decisions together with the country's first-ever national water policy approved recently.