Simulating water uptake of irrigated field crops from non-saline water table soils: Validation and application of the model SWAMP

• Model adequately simulated crop water uptake from water table soils.
• Water balance in water table soils was solved satisfactorily with the model.
• Amount of over- or under-irrigation by the farmer was quantified accurately.
• Model can be used to assess on-farm practices affecting efficient water use.

Mathematical models are invaluable tools for irrigation farmers in semi-arid regions to assess their water management practices under water table conditions. The application of complex models remains a challenge, because determination of input variables and model parameters is tedious and complicated. Thus, the credibility of the Soil WAter Management Program, SWAMP, to assess on-farm management practices, as an alternative to the more complicated numerical models, was investigated. This was accomplished by determining how accurately SWAMP simulates water use of field crops from shallow water tables and how can the model be applied to assess current water management practices by farmers. To accomplish the objectives two data sets were used, i.e. data from a lysimeter experiment and from a case study conducted on an irrigated field located within the Orange-Riet Irrigation Scheme, South Africa. According to the aggregated accuracy, correlation and pattern analysis (ISWAMP), SWAMP performed well in simulating weekly evapotranspiration (ISWAMP = 70%) and water tale uptake (ISWAMP = 90%) of wheat, peas and maize grown on sand to sandy loam soils. SWAMP was also successful in solving the soil water balance under water table conditions at field level. This was done with easily obtainable inputs, while maximizing in situ field observations, which are vital considering that farmers cannot adopt alternative management practices if their current practices cannot be measured. Due to these strengths SWAMP should therefore be easily adopted by irrigation farmers and agricultural advisers to ensure efficient water use.