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Watersheds in the arid and semi-arid regions of Mongolia are especially vulnerable to the effects of climate and land cover change, which will likely exacerbate the disparity between water availability and demand. This study was conducted to analyse land cover changes between 1972 and 2013 and to characterize the currently prevailing livelihood strategies and farming practices in the river oasis Bulgan sum, Western Mongolia. The Soil and Water Assessment Tool (SWAT) was used to simulate the water-use efficiency for the concerned Bulgan River watershed.

Comparison of the two land cover maps (1972 and 2013) of the Bulgan sum river oasis showed a major change from natural rangeland to mostly built-up areas (+288%) and river water receiving sites (+594%). The Bulgan River watershed covers an area of 7409 km(2) and the river itself has a long-term annual average discharge of 9.53 m(3) s(-1). Overall, an area of 769 ha was actively irrigated (2013). A total withdrawal of 555460 m3 irrigation water was estimated, 86% of which was applied to hay fields (2013). The irrigation water productivity ranged from 5.38 (fruits of fruit trees) to 0.43 (hay) m(3) kg(-1) fresh matter yield and a related average income of 0.58 (sic) per kg dry matter per m3. The SWAT model simulated an average evapotranspiration of 100 nun and potential evapotranspiration of 993 mm per year. Water-use efficiency varied from 11 to 52 kg dry matter ha(-1) mm(-1) for rangeland and wetland. Auto-irrigation (whenever plant growth drops below this threshold, the model will automatically apply water until field capacity is reached) and fertilization management simulated by the SWAT model increased yields of irrigated crops, vegetable, hay and fruit trees by about 46 and 77% on average. To enhance biomass productivity per unit water transpired and income per unit water applied, improved irrigation technologies and management practices would be needed.