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Determination of the Ecological Groundwater Depth Considering Ecological Integrity over Oasis Irrigation Areas in the Yanqi Basin

WANG Shuixian1, WU Bin2, YANG Pengnian2, LI Xia3, DONG Xinguang2 (1. College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China; 2. College of Hydraulic and Civil Eengineering, Xinjiang Agriculture University, Urumqi 830052, China; 3. College of Forestry and Horticulture, Xinjiang Agriculture University, Urumqi 830052, China)  
In arid areas, the buried depth of groundwater and salt content jointly affect the growth of natural vegetation. Salts dissolved in groundwater can be carried upward by capillary water and accumulated in top soil if the groundwater buried depth is too shallow under violent evaporation conditions. As such, soil salinization and salt stress often occur. Upward capillary water cannot reach the rhizosphere of plants if the groundwater buried depth is too deep. Therefore, the growth of vegetation is in part determined by the groundwater buried depth, and land desertification occurs. The Yanqi basin, located in northwestern China, has been used for agriculture for a long time. Extensive agricultural activities in the basin have started since the 1950’s with flood irrigation techniques. The groundwater table has been raised since then as a consequence of the absence of an effective drainage system. In terms of the field data in 2007, the groundwater depths in the basin ranged generally between 0.5 m and 3 m. As a result of the shallow groundwater table and high evaporation, soil salinization has become one of the most serious environmental problems in this region due to the groundwater being severely contaminated by salinization. Therefore, agriculture is only feasible with full irrigation. The overall environment of the Yanqi basin has been incrementally degrading because of unreasonable water resources development during the past 60 years, especially irrigation with large amount of surface water and with little groundwater. This has raised the groundwater level and led increasingly serious soil salinization. Based on field investigations and synthetic analysis, the paper discusses the relationship between the buried depth of groundwater table and soil salinization, vegetation growth, and evaporation. Subsequently, a concept of the buried depth of eco-security on groundwater table was put forth from the perspective of ecological safety across oasis irrigation areas. The buried depths of eco-safety on groundwater table range between 3~4 m in oasis irrigation areas of the Yanqi basin. This range is considered suitable for forest growth and limiting evaporation. The groundwater ecological depth was determined for the estimation of ecological water use, and preventing land desertification and salinization. This study provides a basis for reasonable groundwater exploitation and effective salinity control in the Yanqi basin.
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