Improvement of SAPWAT as an irrigation planning tool

English: In a world with a continuous reduction in per capita availability of fresh water, the increase in the efficiency of irrigation water use becomes more important as a means to postpone the time when water shortages will restrict crop production. Irrigation uses 62% of South Africa’s fresh water resources; therefore a saving in irrigation water through an increase in efficiency could have a large impact on total water use. Informed irrigation requirement planning is one way in which irrigation water use efficiency could be increased. Associated to efficient irrigation water use, is the effective use of irrigation soil as a resource. Problems such as waterlogging and salinity are found on 19% of irrigated soil in South Africa. Increasing the efficiency of irrigation water use could reduce the rate of increase of these problems and it might even decrease the occurrence. With the eye on the efficient planning of irrigation areas, research in crop irrigation water requirements has been done over time. Various approaches and planning aids have been developed for the estimation of irrigation requirements. During the second half of the 20th century products like the FAO’s CROPWAT and the South African SAPWAT were developed. Both these programs had shortcomings which made their use somewhat difficult. Development of SAPWAT3 followed with the objective to develop a user-friendly program that could be used as widely as possible. The estimation of irrigation water requirements by SAPWAT3 is based on the internationally accepted Penman-Monteith approach. The former links the climate data of a specific weather station with crop characteristics to determine a water requirement for a specified place and time. The growth and development of crops are influenced by temperature; therefore the crop growth characteristics have been linked to the Köppen climate system as a means of growth and development periods for warm and cool areas. About 5 100 weather stations in 144 countries with either daily or monthly values are included in SAPWAT3. A large number of crops are also included in the data files. If enough daily climate data are included, SAPWAT3 does consecutive year-on-year irrigation requirement calculations, which are then used to determine different levels of nonexceedance of the irrigation requirement. This enables the designer of systems or the water use planners to plan for different levels of risk. A linkage between enterprise budgets and estimated irrigation requirements is also built into SAPWAT3. This enables the user to plan crop combinations which will provide a potential income while also considering water supply constraints. The crop growth characteristics included in SAPWAT3 and in similar programs, are the weak point of such programs because they are based on calendar time and not on thermal time. A computerised methodology has been developed that uses measured crop water requirements and temperature data to link crop growth and development to thermal time. This methodology will be included as a module in the next version of SAPWAT3. SAPWAT was accepted by the South African irrigation fraternity. To determine why this was so, and to determine future upgrade approaches that need to be considered, the level of adoption of SAPWAT was investigated. Good and bad points about SAPWAT which had been identified through verbal feed-back from users were kept in mind and confirmed during the development of SAPWAT3. The feedback on SAPWAT indicated the need to improve the functionality of SAPWAT as an irrigation planning tool, to evaluate and to verify its output and to test its potential for adoption by users. The feedback also indicated that SAPWAT3 is easy to use and that it gives credible results, two aspects that enhance adoption. Therefore it can be expected that future improved versions will also be well received, acceptable and used by the irrigation planning community.