Spatiotemporal heterogeneity of snow cover in the central and western Karakoram Mountains based on a refined MODIS product during 2002–2018

Climate warming has a significant impact on the water cycle, especially in regions where the water supply is dominated by snow and glacier meltwater. In this study, spatiotemporal changes of snowpack in the Yarkant River Basin (YRB) and the west of Upper Indus Basin (WUIB), associated with the Karakoram Mountains, were examined using Moderate-resolution Imaging Spectroradiometer (MODIS) daily cloud-free snow cover product. A sensitivity analysis was carried out to explore the climatic effects on snow cover changes. From 2002 to 2018, overall slight (but non-significant) decreasing and increasing trends in snow cover extent were observed in the basins, which occupy the northern and southern slopes of the central and western Karakoram Mountains, respectively. In both basins, the low-altitude areas experienced a slight decrease in snow cover extent, while the high-altitude areas experienced an increase. Significant elevation dependences were found for the snow cover percentage (SCP), snow covered days (SCD) and snow cover melting dates (SCMD), with an inverse elevation dependence for snow cover onset dates (SCOD). Most of the study area showed a decreasing trend in SCD, which was delayed in SCOD, and an advanced trend in SCMD by a rate of <2 d/a. An increased trend in SCD was observed at 5000–6000 m a.s.l. in the YRB and at 3000–4000 m a.s.l. in the WUIB, both attributed to delayed SCMD. Parts of the Karakoram and eastern Pamir experienced lengthened SCD, advanced SCOD, and delayed SCMD against a background of increased temperature. This was attributed to increased precipitation in the snow accumulation season, and might be related to the Karakoram Anomaly. Moreover, SCOD was found to be positively sensitive to air temperature in snow accumulation season, but negatively sensitive to precipitation. Similarly, SCMD had a positive sensitivity to precipitation, but a negative sensitivity to air temperature in both the snow accumulation season and snowmelt season.