On-farm management of salinity associated with irrigation for the Orange-Riet and Vaalharts schemes
Barnard, Johannes Hendrikus
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Salinity associated with irrigation is and will remain a major obstacle for farmers in most semi-arid regions throughout the world, like the Orange-Riet and Vaalharts Irrigation Schemes in South Africa. On-farm water and salt management should, therefore, be continually evaluated and/or improved. Especially in water table soils where the saturated zone within or just below the potential root zone is not stagnant and lateral flow occurs to lower lying areas and/or artificial drainage systems, which present unique management complexities. Hence, the aim of this study was to evaluate and/or improve on-farm water and salt management of irrigated field crops grown under these conditions. To accomplish this aim the following best water and salt management practices were formulated from literature, i.e. i) use of efficient irrigation systems, ii) introduce scheduling practices that optimize water and salt applications and reduce drainage losses, iii) utilize shallow water tables as a source of water for crop water requirements and iv) monitor root zone salinity to decide when to apply controlled, irrigation-induced leaching for salt removal. Some of these practices were evaluated on a case study basis on two farms within the Orange-Riet and Vaalharts Irrigation Schemes by comparing them to current water and salt management practices. Some aspects of this comparison are difficult to accomplish under field conditions. Supplementing field measurements with mathematical modeling was, therefore, critical to the successful completion of the study. This, however, presented some difficulties because most models require extensive effort to determine input variables and unambiguous numerical model parameters. From the multitude of available models, the Soil WAter Management Program, SWAMP, was selected. According to the aggregated accuracy, correlation and pattern analysis (ISWAMP) of SWAMP, it was found that water uptake of wheat, peas and maize from non-saline water table soils was simulated well (>70%). Consequently it was shown that the soil water balance under fluctuating water table conditions at field level can be solved successfully by SWAMP with limited easily obtainable input variables. This was accomplished by optimizing simply measured in situ field observations, which is vital towards the successful evaluation of water and salt management by irrigation farmers in the region. However, in order to truly revise on-farm water and salt management practices, mathematical models that can simulate the dynamic response of crops to both water (matric) and salt (osmotic) stress are required. A salinity subroutine for SWAMP was, therefore, developed and validated, i.e. mathematical algorithms that can simulate upward and downward salt movement in water table soils according to the cascading principle, and the effect of osmotic stress on water uptake and yield according to the layer water supply rate approach. It was found that SWAMP was able to simulate the accumulation of salt within the root zone above the water table due to irrigation and capillary rise well, and consequently simulate the effect on crop yield. This was possible because SWAMP was able to successfully (ISWAMP > 70%) simulate a reduction in water uptake during the growing season of field crops due to osmotic stress. Consequently SWAMP was used in the case study to solve the water and salt balances of two irrigated fields over four growing seasons and investigate whether the farmers employed best water and salt management practices, using different scheduling approaches. It was concluded that with both centre pivots, crop water stress was prevented, therefore, apparently detracting from the merits of irrigation scheduling. However, it was possible to conserve 20% of irrigation water using scientific based objective, compared to intuitive subjective scheduling, while at the same time also reducing salt additions considerably. Despite less irrigation due to objective scheduling, almost all of the applied salt was still leached into the water table. This was because the presence of a water table within or just below the potential root zone limits storage for rainfall and/or irrigation above the capillary fringe, hence presenting favorable leaching conditions. Since the water below the water table, at both fields, was not stagnant, lateral flow of water through the saturated zone was responsible for removal of the salts. This continual removal of salt is generally not considered good practice because ideally salt must be allowed to accumulate and only periodically leached during high rainfall events and/or fallow periods. Although both scheduling approaches resulted in similar yields, better on-farm water and salt management was achieved with scientific objective scheduling. In doing so farmers can address the environmental problems associated with irrigation, i.e. degradation of water resources due to uncontrolled leaching while achieving similar yields using less water.