| With the global trend of drought and water shortage exacerbated , along with enhancement of soil fertility and grain yield on farmland simultaneously,soil water use efficiency enhanced remarkably,available soil water amount of high-yield grain field of dryland on loess plateau dropped continually,soil water depletion in deep soil layers led to desiccated soil layer has already became hidden barrier of sustainable development of high and stable yield grain production on the region. Based on the extensively investigation of crop yield and deep soil moisture on various crop fields, WinEPIC model was used to simulate major grain crop yields and the effects of deep soil desiccation under different fertilization treatments on semi-humid region of the loess plateau during 1960 to 2000. Suitable soil water use intensity and soil water carrying capacity on grain crop fields of dryland on loess plateau were discussed. The results provided scientific foundation for sustainable and high-efficiency soil water use and stable development of grain production of dryland on loess plateau. Progresses of the study were as following:(1) The investigation result of grain crop yield and soil moisture in semi-humid region of loess plateau indicated,along with precipitation reduced and fertilizer application enhanced, soil moisture in deep soil layers of grain crop fields decreased gradually,desiccated soil layers was deepening and thickening gradually. Soil desiccation phenomenon was obvious in 1-3 m soil layers,and soil moisture reduction also occurred in deep soil layers below 3m.(2) The databases of daily weather variable sequences,physical and chemical properties of soil profile,crop growth parameters of WinEPIC model at Baoji and Luochuan were built for the simulation. The model verification results show that the simulation result of crop yield and dynamic change of soil water for continuous winter wheat and continuous spring maize was precise. The WinEPIC model could be used to simulate continuous winter wheat and continuous spring maize production on loess plateau.(3) During 1960-2000 simulation period,continuous winter wheat yield at Baoji and Luochuan under five fertilization treatments(N0,N90,N120,N150 and N180) decreased with the precipitation reduction. Its average yield were 1.588,2.306,3.327,3.981 and 4.764 t/hm~2 at Baoji, and 1.444,1.939,2.454,2.866 and 3.241 t/hm~2 at Luochuan, respectively. wheat fields at Baoji was more vulnerable to nitrogen stress,while wheat fields at Luochuan was more vulnerable to drought stress. Water use efficiency (WUE) and the rate of increased yield by fertilizer of continuous winter wheat appeared as Baoji was higher than Luochuan. Monthly available soil water in 0～7 m soil layers of Baoji was higher than Luochuan,and which at Baoji was more steady than Luochuan. Average monthly available soil water decreased as fertilizer amount increased. The annual changes of simulated soil moisture distribution in 0-7 m profile on wheat fields at Baoji and Luochuan showed that a steady desiccation layer shaped after annual soil moisture reduction and desiccated soil layer thickened gradually. Thickness of desiccated soil layer of wheat fields under various fertilization treatments of Baoji and Luochuan were same as 3,3,4,4 and 5 m respectively. Soil desiccated rate increased along with fertilizer increased on wheat fields. Considering yield increase and sustainable soil water use,soil water carrying capacity of wheat field was 4.500 t/hm~2 at Baoji and 2.700 t/hm~2 at Luochuan, respectively. The corresponding fertilizer application amount for soil water carrying capacity on winter wheat field was N165 kg/hm~2, P80 kg/hm~2 at Baoji, and N 135 kg/hm~2, P65 kg/hm~2 separately.(4) During 1960-2000 simulation period,continuous spring maize yield at Baoji and Luochuan under five fertilization treatments(N0,N60,N120,N180 and N240) decreased with precipitation reduction. and its average yield was 2.681, 3.949, 5.403, 6.110 and 6.358 kg/hm~2 at Baoji, 3.086, 3.574, 4.133, 4.828 and 5.253 t/hm~2 at Luochuan, respectively. Maize fields at Baoji was more vulnerable to nitrogen stress while at Luochuan was more vulnerable to drought stress. Water use efficiency (WUE) and the rate of increased yield by fertilizer of continuous spring maize at Baoji were higher than Luochuan. Monthly available soil water in 0~7 m soil layers of spring maize field at Baoji was higher than Luochuan,and which at Baoji was more steady than Luochuan. Average monthly available soil water decreased as fertilizer amount increased. The annual changes of simulated soil moisture distribution in 0-7 m profile on maize fields at Baoji and Luochuan showed that a steady desiccation layer shaped after annual soil moisture reduction and desiccated soil layer thickened gradually. Thickness of desiccated soil layer of maize fields under various fertilization treatments of Baoji and Luochuan were same as 2,3,4,4 and 5 m respectively. Soil desiccated rate increased along with fertilizer increased on maize fields. Considering yield increase and sustainable soil water use,soil water carrying capacity of maize field was 6.000 t/hm~2 at Baoji and 4.000 t/hm~2 at Luochuan, respectively. The corresponding fertilizer application amount for soil water carrying capacity on maize field was N160 kg/hm~2, P80 kg/hm~2 at Baoji, and N 100 kg/hm~2, P50 kg/hm~2 separately.
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