The reasonable allocation of a farm land water project ， is an important method in improving the water resources use efficiency and fighting down natural disaster in lower hill regions. Grasping the temporal and spatial distribution of regional water resources, and identifying the regional water balance between supply and demand are the key point of the development of regional water project allocation scientifically and effectively, and the enhancement of regional water resources utilization. Considering the Zhongxian County ' s representative and sensitivity of location, its vulnerability of ecological environment, its necessity of water conservancy facilities, and the superiority of study prospect in Xiaziling watershed, the spatial and temporal variation of Xiaziling watershed in Zhongxian County is studied with the SWAT model to confirm the regions for urgent water resource allocation, and then it will provide theoretical foundation in allocation of regional runoff control system. It is found that the earth ' s surface runoff in the eastern region is much more than that in the western region. Since there are a greater number of water storage projects in eastern regions, the western region should be the focus of the next tune to consider the regional water storage project layout. But, the selection of water storage time can not simply depend on the amount of surface runoff, it should be further combined with the calculation of crop water demand, so that a reasonable and sciontific impounding time may be ultimately determined.