| Summary: | Groundwater withdrawals are growing in most developing countries, including Mongolia, where freshwater resources are limited and unevenly distributed, and most surface waters are frozen during winter. Groundwater represents some 80% of the water supply in the country. Computation of recharge is important, but is complicated in cold regions, because of phase change and permafrost, which is found on 63 percent of the country, and causes conventional physically-based land surface models to be inaccurate. We have developed a two-compartment water and energy balance model that accounts for freezing and melting and includes vapor diffusion as a water and energy transfer mechanism. It also accounts for the effect of slope orientation on radiation, which may be important for mountain areas. We applied this model to the Upper Tuul River Basin to evaluate recharge under different soil and vegetation types. The basin is divided into 12 zones (models) based on elevation ranges, orientation and slope. Due to the limited number of observation data in this area, precipitation, air temperature and relative humidity were corrected as a function of elevation by means of lapse rates. Results show that recharge is relatively high and delayed with respect to snowmelt during spring, because it is mainly associated to thawing at depth, which may occur much later. Most importantly, we find that vapor diffusion plays an important quantitative role in the energy balance and a relevant qualitative role in the water balance. Except for a few large precipitation events, most of the continuous recharge is driven by vapor diffusion fluxes. Large vapor fluxes occur during spring and early summer, when surface temperatures are moderate, but the subsoil remains cold, creating large downwards vapor pressure gradients. Temperature gradients reverse in fall and early winter, but the vapor diffusion fluxes do not, because of the exponential shape of the saturated vapor pressure as a function of temperature giving smaller vapor pressure differences ...
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