| Drought and water scarcity is seriously constricting the continuing development of agriculture, there are amplitude areas of arid and semi-arid in our country, and that limited irrigation can guide the way of water allocating during crop growing stages and different crops in those areas. This paper will study on this & forecasting of irrigation under limited water supply.Considering of different meteorologic data in different regions, five methods (namely Penman-Monteith method, Radiation method, Blaney-Criddle method, Pan method and Hargraves method ) of calculating reference Evapotranspiration (ETo) proposed by FAO are used, and FAO Penman-Monteith method is taked as the criterion to revise other four methods, thus educing the revised formulae in studied region.The results show that among four revised formulae, three have linear relation with Penman-Monteith ETo result except Blaney-Criddle method has exponential relation with Penman-Monteith method. The revised Hargraves formula has the best result compared with Penman-Monteith formula, the average relative error is 6%, moreover, Hargaves method requiring the least data in calculating ETo, so this method has the high actual application value; The following are Radiation method and Blaney-Criddle method ; The worst is the Pan method, even after revised, the highest and average relative error is 36% and 10%, respectively. So, in this study, Penman-Monteith method, Radiation method, Blaney-Criddle method and Hargraves method are used except Pan method.Based on the different methods of calculating reference evapotranspiration, three kinds of main functions of forecasting system are accomplished, namely Calculating reference evapotranspiration, Allocating limited water to crops and crop grow stages and Irrigation forecasting function. In designing stage, VC++6.0 integrated design environment is used. The whole system has the characteristic of friendly interface, reasoned structure and simple way of operation.
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