Vegetation carrying capacity of soil water is the key of the ecological environmental construction and sustainable development in the Loess Plateau. Based on local monitoring data of rainfall, runoff, soil moisture dynamics and Biomass under condition of 4 different sites in loess region of Northern Shaanxi from 2008 to 2010, relationships between natural precipitation and soil water supply, soil water supply and biomass, biomass and soil water consumption were analysed and established in this paper, and Soil Water Carrying Capacity of alfalfa had been separately calculated by FAO method and water balance method. The results showed that: in alfalfa land replenishment of soil moisture in alfalfa land and biomass showed a linear relationship; biomass and soil moisture consumption was a quadratic function. By using FAO method, it could be estimated that the maximum alfalfa yield of soil moisture can bear was 3 992.2 to 4 173.7 kg/hm2 in Northern Shaanxi's Loess Plateau; however, the biomass was estimated by water balance method was 2 600 to 3 500 kg/hm2, it was 16.07% to 33.52% less than the result estimated with FAO method. With the same aspect and position, capacity reduced while slope was increasing. With the same aspect and position, capacity downhill was larger than that uphill. With the same slope and position, capacity in southern slope was smaller than that in northern slope.%土壤水分植被承载力是黄土高原生态环境建设和可持续发展的核心.该文根据陕北黄土区4种不同立地条件下苜蓿地(Medicago sativa L.)连续3a的降雨、径流、土壤水分动态和生物产量的小区定位观测结果,研究分析了自然降水与土壤水分补给、土壤水分补给与地上部生物量、地上部生物量与土壤水分消耗的关系；并采用FAO法和水量平衡法分别计算出了苜蓿地土壤水分的承载力.结果表明:苜蓿地土壤水分补给量与地上部生物量呈线性关系,地上部生物量与土壤水分消耗量呈二次函数关系.用FAO法估算可得陕北黄土区土壤水分可承载的苜蓿最大产量为3 992.2～4 173.7 kg/hm2；而根据水量平衡原理计算可得陕北黄土区苜蓿地可承载的地上部生物量为2 600～3 500 kg/hm2,比FAO法低16.07％～33.52％.坡向、坡位相同时,坡度增大,承载力降低；坡向、坡度相同时,下坡承载力大于上坡；坡度、坡位相同时,南坡承载力小于北坡.
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