Water Resources Assessment in Cold Regions: the Upper Tuul River Basin, Mongolia PDF Download

Author: Enkhbayar Dandar
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Languages : en
Pages : 105

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Book Description
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 at lower temperature. The computed sensible heat flux is higher than the latent heat flux, which reflects the dry climate of the region. The downward latent heat flux associated to vapor diffusion is largely compensated by an upward heat conduction, which is much larger than in temperate regions. The alluvial aquifer around Ulaanbaatar supplies water to the city and is under pressure because of the growing water demand. To address this concern, we built a numerical model, which is challenging, not only because of the lack of data, but also because the river freezes during winter. River flow under the ice is sustained by groundwater, which provides the energy to prevent full freezing of the whole river thickness, but which may not occur where groundwater levels are depleted by pumping. At present, the river still flows under the ice during winter at both ends of the Ulaanbaatar alluvial aquifer. The downstream end, to the West, receives aquifer discharge, whereas the river is fed by discharge from adjacent alluvial aquifers upstream of the east end. But, in the central portion, the river is fully frozen. In fact, the river bed in this portion becomes dry in April most years, probably because of sublimation and because melted water immediately infiltrates into the aquifer. If groundwater pumping increases, either at the Ulaanbaatar alluvial aquifer or at the alluvial aquifer near Gachuurt village, it is likely that the currently winter flowing portion of the river will also dry or, rather, become fully frozen during winter. This will not be a major problem from a quantitative point of view because aquifer storage is sufficient to support winter pumping, even if pumping is increased. However, it may have other environmental and cultural implications. Therefore, further study is needed to monitor at both the upper and downstream stream parts of the aquifer.