Changes in seasonal snow water equivalent distribution in High Mountain Asia (1987 to 2009)

Abstract

Snow meltwaters account for most of the yearly water budgets of many catchments in High Mountain Asia (HMA). We examine trends in snow water equivalent (SWE) using passive microwave data (1987 to 2009). We find an overall decrease in SWE in HMA, despite regions of increased SWE in the Pamir, Kunlun Shan, Eastern Himalaya, and Eastern Tien Shan. Although the average decline in annual SWE across HMA (contributing area, 2641 × 10³ km²) is low (average, -0.3%), annual SWE losses conceal distinct seasonal and spatial heterogeneities across the study region. For example, the Tien Shan has seen both strong increases in winter SWE and sharp declines in spring and summer SWE. In the majority of catchments, the most negative SWE trends are found in mid-elevation zones, which often correspond to the regions of highest snow-water storage and are somewhat distinct from glaciated areas. Negative changes in SWE storage in these mid-elevation zones have strong implications for downstream water availability.

Fig. 1. Study area.

(A) Topographic map of HMA with major catchment boundaries (black) derived from SRTM data (68) and names of major mountain ranges. Inset map shows political boundaries, as well as wind direction of major weather systems (WWD, ISM, and EASM). (B) Twenty-two–year average DJF daily SWE volume across the study area, as derived from SSMI data. (C) DJF SWE standard deviation. Each point represents a 0.25 × 0.25 dd grid cell.

Fig. 2. Seasonality in SWE trends.

Significant (P < 0.05) (A) DJF, (B) MAM, (C) JJA, and (D) SON trends in SWE volume (1987 to 2009), as derived from SSMI data, with major catchments (black outlines, see Fig. 1A). We limit our analysis to regions where the seasonal average SWE is greater than 5 mm to remove spurious results in areas with shallow or infrequent snow cover. MAM and JJA trends across HMA are overwhelmingly negative, except a few isolated regions. DJF trends are more widely positive and are also present in SON in the western Himalaya, the Tien Shan, and the Kunlun Shan. Yearly aggregated SWE trends available in fig. S2.

Fig. 3. SWE contribution and SWE trend synthesis.

(A) Elevation distribution of SWE in each catchment, where each point shows the percentage of total catchment SWE at each five-percentile elevation bin. (B) Mean SWE trend at each five-percentile elevation bin. In the majority of catchments, maximum SWE occurs below the maximum catchment elevation, despite differences in catchment hypsometry. Each catchment is characterized by a unique elevation-trend relationship. The Indus, Amu Darya, and Tibetan Plateau catchments see the most negative SWE trends at their mid-elevations. The Ganges in the central Himalaya sees the most negative trends at the highest elevations.

Fig. 4. Differences between the snow distribution of the Ganges and Indus catchments.

(A) The Ganges and (B) Indus catchments showing catchment hypsometry (gray) (68), percentage glaciated area (red) (69), and SWE elevation distribution (blue). Dashed lines indicate catchment elevation percentiles. Both catchments show SWE maxima below their elevation peaks, despite differences in their SWE distributions. The altitude of SWE maxima are also minimally overlapping with glacier areas, indicating that snow and glacier meltwaters are often distinct and are affected by different climatic processes.