| Based on the long-term continuous experiment and a large number of data of Changwu, by U.S. environmental policy integrated climate model-WinEPIC model, quantitative simulation of the period 1985-1996 in Loess Plateau under different grain and grass rotations of water productivity and soil water consumption and restoration of the rule were determined, to analyze the influencing factors and variation of different grain and grass rotation and explore soil water utilization laws, dry layer of soil formation and distribution of rainfall for different model years of the next rotation for the local model in the area, to evaluate various food crops depletion of soil water utilization rules in order to provide rational reference to use of precipitation for the region ecological environment construction, grain and grass rotation field of sustainable and efficient use of soil water resources, as well as to increase the crop yield. The results are as follows:(1) The soil moisture survey in Changwu showed: the water consumption of corn, wheat and alfalfa was on the order of increasing, regardless of crop, soil moisture was lower in the vigorous growth period. Dry field of corn is relatively low in the degree of desiccation, shallow-level (1~ 2 m), it can be fully restored in the year of common precipitation. Dry field of wheat was in higher degree of desiccation, deep-level (1~ 3 m), it can be restored in the humid year or months of heavy rainfall in the 0 ~ 3 m, but it hardly could restore to the initial soil moisture. Dry fiepd of alfalfa was in the highest level of the desiccation and the deepest level (1~ 10 m), the recovery is difficult, the soil of 0 ~ 3 m can be recovered to some degree in the year of high precipitation, the dry layer of 6 ~ 10 m was difficult to restore.(2) Making use of the continous and long-term data of Changwu, the simulation accuracy verification of winter wheat, spring corn, alfalfa and clover - potato - wheat yield showed: after the adjustment of Win EPIC model, the different precipitation years under wheat, corn, alfalfa cropping and rotation system effects on yield and soil moisture simulation is more accurate, and it was better for alfalfa rotation system simulation in the Loess Plateau.(3) In the 30 years of the research of 11 kinds of different alfalfa - food crops of the rotation simulation in Loess Plateau, by the analyze of soil moisture content in 0-10m layer monthly, the better result was PPPP> PPCC> PCCC, the moisture was respectively 145.49mm, 107.47mm, 96.18mm, variation coefficient was PPWW> PPCC> WWCC, respectively 95.23%, 93.09%, 92.79%, to compare the integrated performance, PPCC's rotation was the best to restore the soil moisture levels and potato was better for the recovery of soil moisture. To compare and analysis the different grain and grass rotation in the soil moisture profile distribution of dynamic change, the PPPP rotations could restore soil moisture under the maximum depth of 4m, while other rotation could restore generally within 1-3m, indicating potato moisture was conducive to recovery deep soil; the rotation mode in the PPCC soil moisture can restore to over 0.4 m / m, the other is about the 0.3 m / m, which indicated that the rotation mode is more suitable to local soil moisture recovery. In comparing the average yields of different crop rotation patterns, the significantly higher pattern than the other rotation system were PPPP> PPCC> PPWW, the average value was 6.30 t/hm2, 5.15 t/hm2, 5.14 t/hm2, because the potato is not suitable for many years'rotation, therefore, the rotation model PPCC was corresponding to local reality. Above all, suitable rotation model for the area is "alfalfa - alfalfa - alfalfa - alfalfa - alfalfa - alfalfa - potato - potato - maize - corn," which could achieve environmental and economic benefits of coordinated and sustainable development.
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