Taking a balanced approach to estimating snow and ice melt contributions to High Asia runoff

Abstract:

We assess the magnitude and variability of glacier and snow melt input to rivers draining the major glacierized and snow covered river basins of High Asia. The water budget provides a basic framework for hydrological analysis. Measurements of all components of the water budget allow the accuracy of estimates of each component to be checked. In glacierized catchments, the water budget is the balance between runoff, precipitation inputs (as rain or snow), loss by evaporation, and the release or uptake of water by soil moisture and ground water stores, snow cover, glaciers and permafrost, as well as by engineered reservoirs and lakes.

In this paper, we estimate these components of the water budgets of the Syr Darya, Amu Darya, Indus, Ganges and Brahmaputra rivers using a combination of station observations, satellite remote sensing and reanalysis output, mass balance measurements, and simple snow cover and glacier models. We estimate the errors for each of these terms with a view to answering the question; how well can we assess glacier and snow melt contributions. Monthly changes in land surface moisture storage are estimated from the difference between runoff and net precipitation (P-E) and also from GRACE. Net precipitation (P-E) is estimated from upper air fields from the NASA MERRA reanalysis using the aerological method and from observations and reanalysis fields of precipitation and evaporation. The challenge is to separate land surface moisture storage into the component reservoirs. Estimates of glacier mass balance in each basin are based on in situ measurements, satellite altimetry, and DEM differencing. Changes in snow storage and contributions from ice melt are estimated using simple accumulation and melt models. Estimates of soil moisture, groundwater and permafrost storage are taken from the literature.