Seasonal snow cover in the Qilian Mountains of Northwest China: Its dependence on oasis seasonal evolution and lowland production of water vapour

Back and forth exchange of water vapour and liquid water from oases at the base of the Qilian Mountains (NW China) and from the Qilian Mountains to oases as surface and shallow subsurface flow has been previously shown by model simulation to be a potentially important mechanism in the long-term stabilisation of oases in westcentral Gansu (Bourque and Hassan, 2009). In a subsequent re-examination of oasis self-support, we use monthly snow-cover patterns in the Qilian Mountains to determine the extent oasis vegetation and evapotranspiration in the low-lying portions of the upper and middle Shiyang and Hei River watersheds control snowfall dynamics in the Qilian Mountains. Monthly snow-cover area (SCA) in the watersheds is simulated with a spatially-distributed model designed to address differences in (i) topography along the prevailing wind direction, (ii) water-vapour production and transport, (iii) in-mountain production of precipitation, and (iv) precipitation phase changes. Seasonal variations in oasis vegetation, surface temperature (for model input), and SCA (for model validation) are described as separate timeseries of monthly composites of enhanced vegetation index, land surface temperature, and normalised difference snow index generated from MODIS optical reflectance and thermal emission data. Comparisons of modelled and snow-index-based estimates of SCA in the Shiyang and Hei River watersheds for the hydrological year, from August 2004 to July 2005, provide nearly similar spatiotemporal patterns; overlap between SCA’s exceeds 60% for most months. An exception to this is in mid-summer of 2004, where overlap between SCA’s is <30%. Agreement between monthly SCA’s reinforces the importance of oasis-vegetation dynamics and mass transfer of water vapour to the atmosphere in guiding seasonal formation of precipitation and snow-cover dynamics in the Qilian Mountains.