中文摘要：

在模型验证和数据库组建基础上，用WinEPIC模型定量模拟研究了黄土高原半湿润区长武、半干旱区固原和半干旱偏旱区海原20～30年内苜蓿草地水分生产潜力、10m土层土壤有效含水量和土壤湿度剖面分布特征的动态变化．结果表明：长武、固原和海原苜蓿草地水分生产潜力模拟值随降水量变化而呈现波动性降低趋势，其平均值分别为8．81、3．83和2．48t·hm^-2；长武、固原和海原苜蓿草地10m土层逐月土壤有效含水量模拟值均呈现明显的波动性降低趋势，模拟初期，4～8年生苜蓿草地土壤干燥化趋势十分强烈，此后，随降水量变化长期在较低水平上波动；随着苜蓿生长年限的延长，苜蓿草地土壤干层逐年加深、加厚，长武、固原和海原土壤干层分布深度达到10m所需时间依次为6、6和4年，此后苜蓿草地降水渗深以下土层长期维持较为稳定的干燥化状态；苜蓿草地水分持续利用的合理年限为半湿润区8～10年，半干旱区6～8年，半干旱偏旱区4～6年．

英文摘要：

Based on model validation and database establishment, the dynamic changes of water potential productivity, available water amount in 0-10 m soil layer, and moisture distribution in soil profiles on the alfalfa grasslands at Changwu of semi-humid area, Guyuan of semi-arid area, and Haiyuan of semi-arid prone drought area on Loess Plateau were simulated with WinEPIC model in a 20-30 years period. The results showed that the simulated water potential productivity of alfalfa grasslands at Changwu, Guyuan and Haiyuan fluctuated but declined with precipitation, and the mean value was 8.81 , 3.83 and 2.48 t · hm^-2, respectively. The simulated monthly available water amount in 0 - 10 m soil layer also declined obviously but with fluctuation. On the 4-8 years old alfalfa grasslands, soil desiccation was quite serious, and soil available water amount fluctuated at low level with precipitation for a long period. With the increasing year of alfalfa growth, the thickness and distribution depth of desiccated soil layer increased gradually, and the duration for the desiccated soil depth over 10 m was 6 years at Changwu, 6 years at Guyuan, and 4 years at Haiyuan. After then, the soil layer beneath rainfall infiltration depth lasted permanently in a desiccated status. The reasonable duration for water sustainable use on alfalfa grasslands was 8-10 years on semi-humid area, 6-8 years on semi-arid years, and 4-6 years on semi-arid prone drought area.