A quantitative assessment of the genetic sources of the hydrologic flow regimes in Upper Indus Basin and its significance in a changing climate

Abstract

• Glacio-hydrometeorological conditions with four flow regimes in Upper Indus Basin. • Highest flow regime from July to September carry 53–62% annual flow volumes (m 3 ). • Three most glaciated watersheds contribute 48–54% flow to the highest flow regime. • Melt water from altitude >3500 m constitutes 41–54% of annual flow volumes. • Near the outlet, glacial and snow melt components respectively constitute 21% and 49% of annual flows. Reliable quantitative estimates of contributions melt water of different genetic sources make to river flows in Himalayan river basins are largely unknown. Here we provide such estimates for Upper Indus Basin (UIB). Analyses of historical flow records at 11 gauging stations spanning 14–48 years during a period of record from 1962 to 2010 reveal: a uniform character of annual flow distributions at all gauging stations given by a Gaussian function implying a unique glacio-hydrometeorological condition prevailing throughout the basin controlling four hydrologic flow regimes within UIB. Two low flow regimes occur during the months of October to December (L1) and January to March (L2) and two high flow regimes that occur during April–June (H1) and July–September (H2). For all stations, flow magnitudes follow, H2 > H1 > L1 > L2. In the main stem of Upper Indus River, the contributions to total annual flow volumes (m 3 ) during these flow regimes are 53–62% during H2, 24–32% during H1, 8–9% during L1, and 4–6% during L2. In the main tributaries, these ranges are 47–74% during H2, 15–38% during H1, 8–10% during L1, and 4–6% during L2. Separation of annual hydrographs by linear smoothing and recursive digital filtering technique shows that the annual contribution of melt water (M2) from an elevation band 3500–5300 m to total annual flow volume (m 3 ) varies from 41% to 54% along the main stem of Indus, upstream of the Himalayan foothills. Contribution of melt water (M1) from an elevation band 2500–3500 m varies from 16% to 29%. In the tributaries, annual contributions of M2 vary from 37% to as high as 65%. Similarly, annual contributions of M1 in the tributaries vary from as low as 12% to 34%. Thus, the relative importance of melt water originating from high-altitudes far overweighs that originating from mid-altitudes, in river runoff within UIB. The chief component of M1 is seasonal snows whereas M2 is a mixture of glacial melts, seasonal snows falling in winter and spring, and monsoonal snows falling in the summer (July–September). The M2 component contributes to base flows during L1 regime. Base flow recession occurs during L2 regime. During the H2 regime, three watersheds with greatest glaciated surfaces straddling the Karakoram Mountains contribute 48–54% of flows at Shatial Bridge, a point upstream of Tarbela reservoir up to which rainfall contributions to river discharges in UIB are inconsequential. During the H1 regime, these watersheds drained by Shyok, Shigar, and Hunza rivers contribute 20–31% of flows at this point. During L1 and L2 regimes, their contributions are 33–39% and 31–32% respectively. Contributions of glacial melt and snowmelt to annual river flows vary from 18–35% and 38–50% respectively in the major tributaries and the main stem of Upper Indus, depending on the location. Upper Indus River just upstream of Tarbela Reservoir carries annual flows constituted of 70% melt water of which 21% is contributed by glacial melts and 49% by snowmelts. Thus, changes in climatic trends will greatly control the future water availability within UIB. If glacial retreat and reduction of the perennial snow and ice covers are happening in UIB in a changing climate, then there will indeed be long-term reductions in river flows in UIB and hence sustainability of water resources in this basin will potentially be at risk.