Spatio-Temporal Heterogeneity in Snow and Glacier-Melt Runoff: Implications for Water Management in the Western Himalayas Transboundary Chenab River basin

The Chenab River is a major tributary of the Indus River that originates in the Lahaul range of the Western Himalayas. As a transboundary river shared by India and Pakistan, the Chenab is subjected to water management under the Indus Water Treaty and faces a variety of challenges including climate-induced changes in cryosphere and hydrology, baseline water stress, and hazard risk in the context of rapid socio-economic change, hydro-political tension, and water governance effectiveness. In this study, we analyzed the hydrological regime of the Chenab River basin having a drainage area of 22,545 km² and an elevation range of 287 to 7044 m. Towards characterizing its spatial heterogeneity, the basin is divided into 13 sub-basins. The soil moisture method in the Water Evaluation and Planning (WEAP) model was used to simulate glacier melt, snow melt and streamflow from over a decadal period. The model was calibrated using multiple datasets including glacier mass balance, satellite derived Snow Cover Area (SCA), and measured discharge at three sites in the Chenab basin to appropriately represent the historical evolution of cryosphere resources. Noteworthy preliminary findings unveil the Chenab basin’s complex hydrological dynamics. Area-weighted average mass balance of the glaciers in the Chenab basin is -0.63 meter water equivalent per annum (m.w.e.a^-1) with a cumulative mass loss of -11.7 m.w.e from 1995 to 2013. Sub-basins show variations in the average mass balance attributed to distinct temperature and precipitation regimes. The highest and the lowest mass balance observed in Bhut (-0.25 m.w.e.a^-1) and Ans (-1.12 m.w.e.a^-1) basins, respectively are negative, signifying baseline water stress. Further, we observed that the highest SCA occurred February when about 83% of the Chenab basin is covered in seasonal snow whereas the lowest SCA is observed in the August (18%) which represents the permanent snow or snow in the accumulation zone of glaciers. Deciphering runoff components reveals the interplay between snowmelt, rainfall-runoff and baseflow, constituting the Chenab’s annual streamflow. Preliminary analysis of the simulated hydrological components suggests that the annual streamflow of the Chenab at Akhnoor during 1995-2013 was mostly contributed by snowmelt (35.1%), rainfall-runoff (27.7%), and baseflow (28.2 %), and glacier melt (9%), which is low on an annual basis but represents an important seasonal component of the water budget. Interannual variation in the streamflow components suggests that dry years have increased contribution of ice-melt, highlighting the buffering effect of glacier melt with higher loss in warm years due to evapotranspiration. Snowmelt runoff starts in March and reaches its peak in July contributing from 31 to 61% of the flow in these months. Ice-melt contribution is largest in July (61%) followed by August (23%). The relative contribution of rainfall runoff ranges from 20 to 42%. The relative contribution of baseflow is largest in winter months when about 70% of the streamflow is supported by baseflow. Furthermore, high-altitude sub-basins such as Chandra receive low precipitation and have runoff regimes dominated by glacier melt (28%) and snowmelt (38%) with only 9% contribution of rainfall runoff. Middle-altitude basins such as Marusudag show a runoff regime dominated by snowmelt (44.4%) and rainfall runoff (33%). In low-altitude sub-basins such as Ans and Kalnai, rainfall runoff dominates the streamflow. Thus, we observe the relative contribution of glacier melt decreases and that of rainfall runoff increases from higher to lower altitudes. The spatio-temporal complexity in streamflow components provides a new understanding of the Chenab basin’s hydrological response. These insights hold implications for devising adaptive water management strategies amidst a changing climate.