Quoted from: http://www.fao.org/land-water/databases-and-software/cropwat/en/ 

CROPWAT is a decision support tool developed by the Land and Water Development Division of FAO.

CROPWAT 8.0 for Windows 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 8.0 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 8.0 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.

CROPWAT 8.0 is a Windows program based on the previous DOS versions. Apart from a completely redesigned user interface, CROPWAT 8.0 for Windows includes a host of updated and new features, including:

• monthly, decade and daily input of climatic data for calculation of reference evapotranspiration (ETo)
• backward compatibility to allow use of data from CLIMWAT database
• possibility to estimate climatic data in the absence of measured values
• decade and daily calculation of crop water requirements based on updated calculation algorithms including adjustment of crop-coefficient values
• calculation of crop water requirements and irrigation scheduling for paddy & upland rice, using a newly developed procedure to calculate water requirements including the land preparation period
• interactive user adjustable irrigation schedules
• daily soil water balance output tables
• easy saving and retrieval of sessions and of user-defined irrigation schedules
• graphical presentations of input data, crop water requirements and irrigation schedules
• easy import/export of data and graphics through clipboard or ASCII text files 
• extensive printing routines, supporting all windows-based printers
• context-sensitive help system
• Multilingual interface and help system: English, Spanish, French and Russian

CROPWAT 8.0 for Windows was developed using Visual Delphi 4.0 and runs on the following Windows platforms: 95/98/ME/2000/NT/XP and Vista.