Water balance of the Indus River Basin and moisture source in the Karakoram and western Himalayas: Implications from hydrogen and oxygen isotopes in river water

[1] Stable isotope measurements of hydrogen and oxygen for surface waters from the Indus River Basin (IRB), together with historical records for river discharge, annual precipitation, and groundwater levels, are used to assess water balance for the basin. The Indus River presently drains 53 km(3) yr(-1) or roughly one-eighth of the 398-km(3) water that annually falls on the basin in the form of rain and snow, with the remainder returned to the atmosphere by evapotranspiration. Monthly samples for the Indus River close to its mouth, for the water year March 1994 to February 1995, show a tight correlation in deltaD and delta(18)O space. The slope of the linear regression is 7.5, which is not significantly different from the slopes of the Local Meteoric Water Lines (LMWL; 7.3 and 7.1). This observation argues against significant loss of water by direct evaporation from river surfaces or from soils in hydrologic continuum with surface waters. An upper limit for evaporation from poorly drained soils is calculated to be 10 km(3) yr(-1) or only 2.5% of the annual precipitation flux. Groundwater storage in the entire Canal Command Area received a maximum of 23 km(3) yr(-1) or 5.8% of the annual precipitation during the early stages of irrigation, but modern recharge is probably balanced by discharge to rivers and well exploitation. Transpiration by natural vegetation and crops annually returns 83% of the precipitation flux and constitutes the largest pathway for the loss of water from the basin. Deuterium excess (d-excess) in the IRB ranges between 4parts per thousand and 28parts per thousand, with values for 95% of the sample population exceeding 10parts per thousand. The Indus main channel close to its mouth varies in d-excess between 12parts per thousand and 20parts per thousand during low and high water stands, respectively, with a discharge weighted average of 18parts per thousand. These values are distinctly higher than the long-term average for the Indian monsoon (similar to8parts per thousand) and reflect contributions from water vapor originating in the Mediterranean (22parts per thousand) or other inland seas. Using these end-member compositions and the discharge weighted average d-excess, isotope balance calculations require that up to 72% of the Indus discharge close to its mouth must be derived from the Mediterranean end-member and implies that bulk of the Indus discharge is the result of delayed runoff from the Karakoram and the Himalayas.