Doctoral Degrees (Soil, Crop and Climate Sciences)

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Browsing Doctoral Degrees (Soil, Crop and Climate Sciences) by Author "Bennie, A. T. P."

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    Estimating water retention for major soils in the Hararghe region, Eastern Ethiopia
    (University of the Free State, 2003-02) Tsehai, Kibeebw Kibret; Bennie, A. T. P.
    English: Soil water retention IS a fundamental property controlling water storage and movement in the solurn. To determine the water retention characteristic curve is time consuming and expensive. Several attempts have been made to establish relationships between easily measurable soil properties, like particle size distribution, organic carbon content, and the water retention characteristic curve. Those relationships are referred to as pedotransfer functions (PTFs). More conveniently, it is described by analytical functions that are suitable in the solution of numerical flow equations as well as in implementation of closed-form methods for predicting other hydraulic properties, such as unsaturated hydraulic conductivity. The objectives of this study were to describe the water retention characteristics of soils from the Hararghe Region, eastern Ethiopia, in relation to certain soil properties; to identify water retention functions for describing the water retention characteristic curves of these soils and to develop a procedure for estimating water content either at certain matric potentials or the complete curve from readily available soil properties. Two approaches, point estimation and parametric estimation techniques, were used for estimating the water content at certain matric potentials and at any matric potential, respectively. To establish relationships between water retention and relevant soil properties, regression analyses were carried out. From the regression analyses, point PTFs that can be used to estimate the water content at certain matric potentials were developed. This was done firstly by using the complete data set consisting of 216 retention curves and secondly by dividing the complete data set into topsoil and subsoil samples. Due to observed differences in water retention characteristics, the subsoil samples were divided into two groups based on their silt (Si) to clay (C) ratio. The dividing line between these two groups was 0.75. The topsoil and the two subsoil groups were divided into classes based on their silt plus clay content. This resulted in 7 classes for topsoils and subsoils with Si:C ratios < 0.75 and 6 classes for the subsoils with Si:C ratios> 0.75. For all the point estimation PTFs, the silt plus clay content functions described the variability in water content best. The relationship between water content and silt plus clay content was curvilinear. In order to quantify the prediction accuracy of these equations, the mean of the mean absolute error (mMAE), the mean of the root mean square error (mRMSE), the mean of the mean bias error (mMBE), d-index of agreement and coefficient of determination (R2) were used. In some instances, the slopes and intercepts of the 1:1 lines, between measured and predicted values, were used. The silt plus clay content functions for the complete data set explained 78 to 87 % of the variability in water content at specific matric potentials. The mMBE ranged from -0.001 to -0.003 cm' cm", the mMAE 0.022 to 0.034 crrr' cm", the mRMSE 0.027 to 0.042 crrr' ern". The d-values ranged from 0.838 to 0.867. The silt plus clay content functions for the topsails explained 88 to 94 % of the variability in water retention with the mMBE ranging from 0 to -0.001 crrr' cm", mMAE 0.018 to 0.031 crrr' cm", mRMSE 0.024 to 0.036 cm3 ern" and the d-values 0.765 to 0.886. The silt plus clay content functions for the subsoils with Si:C ratios < 0.75 were able to explain 78 to 87 % of the variability in water retention with the mMBE ranging from -0.001 to -0.004 crrr' ern", mMAE 0.019 to 0.036 crrr' ern", mRMSE 0.023 to 0.045 cnr' cm" and d-values 0.793 to 0.884. The silt plus clay content function for the subsoils with Si:C ratios> 0.75 explained 86 to 98 % of the variability in water content with mMBE ranging from -0.001 to 0.004 cm' ern", mMAE 0.013 to 0.031 ern' cm", mRMSE 0.015 to 0.038 crrr' cm" and d-values 0.737 to 0.99l. Of the three groups, the mean values of the classes were used to develop PTFs with higher R2-values and lower errors compared with the PTFs developed from the complete data set in each respective group. From the six water retention functions tested, the Van Genuchten (1980) function, with the restriction m = 1 - lIn, gave the best description of the water retention curves, followed by the Smith (1992) and the ordinary power functions. Over all, the Brooks-Corey (1964) function gave the poorest description of the water retention curves studied. The parameters of the Smith (1992) and Hutson & Cass (1987) functions correlated better with relevant soil properties compared to the parameters of the Van Genuchten function. With the parametric approach the Smith (1992) function estimated water content for topsails and subsoils with Si:C ratios> 0.75 with a higher accuracy compared with the Van Genuchten and Hutson & Cass functions whereas the Hutson & Cass function was better for the subsoils with Si:C ratios < 0.75. Testing the functions derived from the point estimation and parameterization techniques on an independent data set indicated that both approaches estimated water content with a reasonable degree of accuracy, although the point estimation techniques gave slightly better results for the subsoils with Si:C ratios> 0.75.
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    ItemOpen Access
    Response of crops on shallow water table soils irrigated with deteriorating water qualities
    (University of the Free State, 2007-11) Ehlers, Louis; Bennie, A. T. P.; Van Rensburg, L. D.
    This study was undertaken to investigate a number of issues regarding the effect of using saline irrigation water for crop production on soils with shallow water tables. The experiments were conducted in large drainage lysimeters, filled with a yellow sandy soil and a red sandy loam soil in which shallow saline water tables were maintained at a constant depth of 1.2 m. Wheat, beans, peas and maize were grown under controlled conditions using irrigation water with salinities that ranged from 15 to 600 mS m-1. This facility was used to determine the effect of irrigation water and water table salinity on crop yield and water uptake, as well as salt accumulation in the root zone during growing seasons. The field experiments simulated conditions of adequate water supply to the crops through irrigation in the presence of a shallow saline water table. Except for wheat that gave better yields in the more clayey soil, the growth of the other three crops was similar on both soils for comparative irrigation water salinity treatments. The above-ground biomass of wheat, maize, peas and beans started to decline when irrigated with water of 600, 450, 300 and 150 mS m-1, respectively. The water use of all four crops, as indicated by evapotranspiration, declined with deteriorating irrigation water salinity. On a relative basis the evapotranspiration of peas, beans, maize and wheat decreased at rates of 0.0007, 0.0005, 0.0004 and 0.0001 mm per unit increase of soil water salinity measured in mS m-1. A decrease in the osmotic potential of the soil water to -300 kPa, which is equivalent to an electrical conductivity of 750 mS m-1, reduced evapotranspiration in comparison to the control by 7, 30, 38 and 53% for wheat, maize, beans and peas, respectively. The water use efficiency of the crops, expressed in above-ground biomass produced per unit mass water used, started to decline only when the threshold ECe-values were exceeded. Water uptake from the shallow water tables decreased with an increase in irrigation water salinity for all four crops on both soils. The relative water uptake from the capillary zones above the water tables declined linearly when the soil water salinity in these zones exceeded certain threshold values. These values varied between 57 mS m-1 for beans to 279 mS m-1 for maize, with an average value of 136 mS m-1. The crops less affected by the increase in salinity, were wheat followed by maize, beans and peas. Salts accumulated at or just below the capillary fringe in both soils, with maximum accumulation at 700 mm from the soil surface or 500 mm above the water table. Equations were derived from the accumulation of salts in the root zone to calculate the salt accumulation in soils with restricted drainage during a crop growing season. These equations were incorporated in proposed procedures for salinity management on irrigated soils. The procedures made provision for five different conditions: i) where added salts to the root zone accumulate without any possibility for leaching and the mean root zone salinity is lower than the crop ECe-threshold value; ii) where added salts to the root zone accumulate without any possibility for leaching and the mean root zone salinity is higher than the crop ECe-threshold value; iii) where added salts can leach naturally from the root zone, but with not enough irrigation water to supply in the crop water demand; iv) where the natural leaching of added salts can be accelerated by irrigating more than the required crop water demand; and v) to irrigate according to the crop water demand in order to utilize rainfall for leaching. The different salinity management procedures were compared on the two soil types by means of computer simulations for a range of irrigation water qualities and long-term climatic conditions. The simulated results indicated that under conditions with zero drainage, sustainable production could be maintained for only 25 to 40 years if good quality water was used for irrigation. Irrigation water with an ECi > 50 mS m-1 resulted in severe soil salinisation and crop losses within 5 to 10 years. On freely drained soils additional leaching was required within 5 years, even with the use of good quality irrigation water. It was clear from the simulated results that an increase in root zone salinity in soils with shallow water tables, necessitate adaptations in the normal approaches to irrigation scheduling and irrigation water management.