Soil, Crop and Climate Sciences
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Browsing Soil, Crop and Climate Sciences by Author "Barnard, Johannes Hendrikus"
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Item Open Access Leaching of excess salts from the root zone of apedal soils(University of the Free State, 2006-05) Barnard, Johannes Hendrikus; Van Rensburg, L. D.; Bennie, A. T. P.English: In South Africa a huge amount of energy was spend on irrigation research over the past two decades, mainly to optimise water application in order to prevent crop water stress. In the quest to conserve water for transpiration, researchers tended to neglect the importance of drainage or percolation, which eventually results in the accumulation of salts in the root zone. Salts also accumulate in the root zone where shallow water tables are present. Farmers along the Lower Vaal River expressed their concern about yield losses induced by build-up of salts in the root zone. The detrimental affect of salinity on field crops are extensively reported in the literature and the only way to address the problem is through leaching. Sustainable utilization of these saline or potential saline soils depends on adequate natural drainage or artificial drainage systems, which ensures a net downward flux of water and salts below the root zone for optimum development and functioning of roots. This dissertation focuses mainly on the management of salts in the root zone of apedal soils. The research was conducted on two soil types (Clovelly and Bainsvlei) reconstructed in 5000 litre lysimeters on the experimental farm, near Bloemfontein, of the Department of Soil, Crop and Climate Sciences (University of the Free State). A total of 30 lysimeters, 15 per soil type arranged in two parallel rows under a moveable rain shelter were used. It was assumed that the artificially prepared soil profiles are stable because more than 10 cropping cycles were completed before the commencement of this experiment. The first aim of Chapter 3 was to address the effect of irrigation water salinity on the accumulation of salt in the root zone under shallow water table conditions. A total of 612 mm was irrigated with irrigation water salinity treatments that varied between 15 and 600 mS m-1. Results showed that in the absence of drainage, salts will accumulate in the root zone at an alarming rate. In fact, salinity of the soil water almost doubled with respect to that of the irrigation water during only one growing season. These various saline profiles were used to characterise the impact of soil water salinity on the hydraulic characteristics of the two soils under investigation. After saturation of the profiles, drainage curves were in situ determined by allowing water to drain freely from the profiles for approximately a month. These drainage curves revealed that the initial soil water salinity did not significantly influence the hydraulic characteristics of both soils. It was possible to quantify the amount of salt removed during a drainage cycle. Although both soils are apedal, the two soils differed markedly in their discharge rates and amounts. Chapter 4 had focused on quantifying the pore volume of water required to leach excess salts from the profiles. It was found that piston flow can describe the leaching process, because one pore volume of drainage was sufficient to remove 100% of the excess salts, irrespective irrigation water salinity or soil water salinity. The results also showed that it is more efficient to remove 80% of excess salts in stead of 100%. On freely drained soils it is therefore possible to effectively and efficiently manage the salinity level of the root zone through controlled irrigation in excess of crop water demand, when necessary. Complex dynamic models are helpful in understanding the nature and complexity of solute movement in soils, but unfortunately they are not widely used by irrigators and managers. The final objective (Chapter 5) was to derive a simple model capable of estimating the depth of water required to remove excess salts from the root zone. The non-linear exponential association (y = a {1- exp –b x}) of the in situ determined leaching curves provided the best mathematical description of the fraction of excess salts removed in relation to the depth of leaching water required per unit depth of soil. Verification of the proposed model showed that it is possible to accurately estimate the leaching requirement for effective and efficient management of root zone salinity in apedal soils. It was recommended that the proposed model should be expanded to include more soil types.Item Open Access On-farm management of salinity associated with irrigation for the Orange-Riet and Vaalharts schemes(University of the Free State, 2013-07) Barnard, Johannes Hendrikus; van Rensburg, L. D.; du Preez, C. C.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.