| Irrigation district is the basic guard for the safety of national crop, important support of regional economic development and also the vital of environmental protection. With the sharp increase of population, rural economic development, and quickening pace of construction of small town, the shortage of agricultural water resources has been become a constraint for the development of irrigation districts. Henceforth, making research of the contradiction of the water supply and need, dispatching and utilizing the limited water resources rationally has vital importance for the sustainable development and economical development for irrigation districts.Based on the following idea, this paper took Baojixia irrigation district, which is the largest one, as the research project. According to the natural terrain and projects lay out, Yuanshang and Yuanxia irrigation districts were viewed as the two large sub irrigation system, for both which, the calculation and analysis of water supply and need under the present condition, different precipitation typical years, different guarantee ratio of water income and different combination of irrigation ways were made. Besides, on the basis of irrigation time and laws of water need for the crop, the disposition and simulation model was constructed based on the balance of water supply and need, and according to the different dispatching principles of reservoirs and water need, many schemes were made to make research of rational disposition of water resources. The main conclusions were as follows:(1)Taking 2003 as the datum year, according to the scale of water supply projects, planted crop types, planted areas under the present condition, different frequency of income water and precipitation together with different irrigation ways were combined to calculate the water balance for both the sub areas respectively. The analysis showed that the income was less in the dry year, and the water shortage was 4.37×108m3 under the sufficient irrigation, with the water shortage ratio of 37.38%, which showed that the water shortage was serious and the contradiction between water supply and need was conspicuous.(2) The disposition and simulation model was constructed based on water balance for the irrigation districts. The model could simulate and adjust the schemes under different irrigation ways, different plant structure and different storage ways to obtain the water balance results of different crop of both Yuanshang and Yuanxia irrigation sub system. Different schemes were decided according to the dialogue between people and computer.(3)The irrigation water need was divided to the twelve months under the limited irrigation and sufficient irrigation and according to the different principles of reservoir adjusting and water demand, many schemes were produced and for which, the water balance was simulated and calculated. The calculation results showed that under the normal yeas with sufficient irrigation, compared with scheme 1, scheme 2 can increase water supply by 2.790×107m3, 4.221×107m3 and 8.73×106m3 was increased under the dry year with sufficient irrigation and limited irrigation respectively. It can be seen that scheme 2 can supply more water supply than that of scheme 1, which is the superior water dispatching way. Under the limited irrigation, the water shortage was only 8.73×106m3 when scheme 2 was adopted, and decreased the water supply of the irrigation districts apparently to loose the water supply pressure and groundwater exploitation compared with other schemes.(4)The water-saving irrigation and water reservoir adjusting measures can the water shortage situation, however, except for the measures of taping new resources, reducing water supply, polluted water controlling, the water storage engineering projects should be built to expand water supply capacity and actualize the harmony of sustainable utilization of water resources and eco-environmental protection.
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