One example of a irrigation model is the free software CROPWAT 8.0 which was developed by the Land and Water Development Division of the FAO. The model is a computer program for the calculation of crop water requirements and irrigation requirements based on soil, climate and crop data. In addition, the program allows the development of irrigation schedules for different management conditions and the calculation of scheme water supply for varying crop patterns. CROPWAT can also be used to evaluate farmers’ irrigation practices and to estimate crop performance under both rainfed and irrigated conditions.

All calculation procedures used in CROPWAT are based on the two FAO publications of the Irrigation and Drainage Series, namely, No. 56 "Crop Evapotranspiration - Guidelines for computing crop water requirements” and No. 33 titled "Yield response to water".

As a starting point, and only to be used when local data are not available, CROPWAT 8.0 includes standard crop and soil data.   When local data are available, these data files can be easily modified or new ones can be created. Likewise, if local climatic data are not available, these can be obtained for over 5,000 stations worldwide from CLIMWAT, the associated climatic database. The development of irrigation schedules in CROPWAT 8.0 is based on a daily soil-water balance using various user-defined options for water supply and irrigation management conditions. Scheme water supply is calculated according to the cropping pattern defined by the user, which can include up to 20 crops.

(FAO 2015)

On the FAO hompage it is also possible to get a tutorial based on an example in India (http://www.fao.org/nr/water/docs/CROPWAT8.0Example.pdf)