Evaluation of salinity and irrigation guidelines for lucerne
Fourie, Kevin Louis
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Evaluation of salinity and irrigation guidelines for lucerne remains important to improve current management practices under irrigation. Internationally, well-established yield response curves, set over 30 years ago, serves as a general guide for salinity management. However, more specific guidelines for lucerne production under South African conditions are needed. The aim of this study was to determine the effect of increasing irrigation and soil water salinity on the water uptake and yield of lucerne and evaluate simulations of these results, with the model SWAMP, under osmotic stress conditions. An experiment was conducted in a non-weighing lysimeter facility. Lucerne (cv. SA Standard) was grown under controlled conditions using irrigation water with salinities that ranged from a control treatment up to 1200 mS m-1. Irrigation water of the different treatments consisted of various amounts of salts to achieve the desired concentrations. The soil water balance was used to reflect on water gains and losses during the growing season. The mean daily transpiration rate as well as the seasonal transpiration of the cuttings decreased with increasing irrigation water salinity. Similarly, water table depletion and yield decreased with increasing water and soil salinity. The relationship between relative mean above-ground biomass and water salinity was curve linear, which differs from the well-established relationship reported in literature. A calculated critical level divided water and soil salinity into two management classes each with different rates of a reduction in yield. A linear decrease in the crop productivity with an increase in water salinity was obtained. The cultivar SA Standard is more salt tolerant than those used in literature. Results from the lysimeter trail was used to validate water uptake and yield simulation under osmotic stress conditions with SWAMP. Most of the soil parameters e.g. evaporation, transpiration, root density, infiltration and redistribution of rainfall and/or irrigation water, drainage and water table uptake have been calibrated for the two soils. Data from the control treatment was used to calibrate the parameters used in simulating the transpiration requirement. Default values were used for the remaining parameters. Various indices and test statistics were aggregated into a single indicator module (ISWAMP when 0 = good and when 1 = poor) with a fuzzy-logic based expert system, which represent the model’s aggregated accuracy, correlation and pattern performance. SWAMP was able to reasonably simulate a yield decline due to an in increase in water salinity (ISWAMP = 0.0903), which was also true for seasonal transpiration (ISWAMP = 0.0305). Weekly simulations of transpiration were not good. A high pattern value indicated the presence of some macro-patterns. This was attributed to the fact that the residuals were not evenly distributed during the growing season, which was not the case with an increase in water salinity. Hence, the crop growth algorithm for simulating the daily transpiration requirement needs to be improved.