Hydrological regimes under the conjunction of westerly and monsoon climates: a case investigation in the Astore Basin, Northwestern Himalaya

In a changing climate, river basins with limited summer precipitation but abundant snow and glacier melt-water are affected severely by reductions and seasonal alterations in annual stream flows. However, high altitude glacio-hydrological observations and investigations to address the linkage between the timing of glacier changes and river runoff fluctuations remain ambiguous, particularly in the northwestern Himalayan region of Pakistan. In this context, the hydrological regime of the Astore Basin, a sub-catchment of the Upper Indus River Basin, was comprehensively investigated by employing in situ hydrometeorological observations in combination with satellite remote sensing data. Two-thirds of the annual precipitation in the Astore Basin falls in winter and spring, mainly deposited by westerly winds, whereas summer and autumn precipitation deposited by the monsoon and local jet streams accounts for only one-third. Some 14 % of the basin area is covered by glaciers and, added to the accumulated seasonal snow deposited by westerly circulations, this can reach 80-85 % of the basin area. Therefore more than 75 % of the annual basin runoff depends on melt-water produced by the predominant seasonal snow and glacier ablation. The non-parametric Mann-Kendall trend test was applied to the whole time-series of hydro-meteorological data. This indicated a lowering of annual and summer mean temperatures during the period 1980-1995 and slight warming during the 1996-2010 period. Similarly, annual, winter and summer precipitation, and annual mean discharge, increased from 1980 to 1995 but, in contrast, a slight decrease in annual and summer precipitation was observed during the 1996-2010 period; however, discharge evidence is slight making any increase in this period insignificant. In addition, the Spearman and Mann-Kendall correlation results depict annual stream flow fluctuations during the 1980-2010 decades in the Astore River that were predominantly influenced by precipitation variations, but not by any alteration in catchment temperatures, and so not governed by enhanced glacier ablation and retreat. The results of the analysis presented here were also substantiated by satellite remote sensing investigation, which points to evidence of stable conditions in the Astore Basin glaciers during this period.