Evaporation from aeolian soils with shallow water tables
Mengistu, Achamyeleh Girma
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Soil water evaporation under water table conditions was identified as a very important water loss in the soil water balance in aeolian soils. Globally it is estimated that 7.2 × 1013 m3 of water lost through evapo-transpiration per year. In South Africa, an average of 65% of the precipitation will be lost through evapotranspiration. The main aim of this study was to determine the effect of water table depth on daily evaporation (Es) for two aeolian soils that cover large areas in Africa below the equator. The study was divided in to three content chapters: The first chapter concentrated on characterization of the basic soil physical properties, the second content chapter investigated the effect of soil water content and temperature on the thermal properties, while the last content chapter described the effect of soil type and water table depth on the daily rates of evaporation as well as the temperature distribution in the profiles. In the first content chapter, soil physical properties were determined with application of laboratory, field or indirect estimation methods and summarized as follows. (i) Particle size analysis was determined in laboratory by the pipette method. The two soils had similar textural class (loamy-sand) in the Ap horizon, but Clovelly categorized as sandy and Hutton as sandy-loam in the B-horizon. (ii) Bulk density was determined by the core sampling method and varied between 1.3 to 1.7 g cm-3 with Clovelly the lowest. (iii) Soil water retention curves (SWRCs) were determined in laboratory by the Hanging Water Column and the Pressure Plate Apparatus (assisted by the RETC computer program and the van Genuchten model). The water content at lower suctions was higher in Clovelly than Hutton and the opposite was true for higher suctions to the dry end. Hence, Clovelly had a well-developed S-shape SWRC than Hutton. (iv) Saturated hydraulic conductivity (Ks) was determined in situ by the constant head permeameter and was higher in the Clovelly soil. (v) Unsaturated hydraulic conductivity (KL) and hydraulic diffusivity (Df) were estimated from the SWRCs by the Mualem-van Genuchten conductivity model. The Clovelly soil horizons had higher KL and Df. (vi) The pore volume-pore size response curve was inferred from the SWRC by applying the capillary theory. The classification of the soil pore by their function proposed in the literature was also modified based on the concepts of field water capacity of soils. In the second content chapter, the influence of soil water content and temperature on thermal properties (Kt, C and D) was analysed in laboratory using the KD2 Pro Thermal Analyser. Five water and temperature levels were investigated in a two factor factorial experiment. The results showed an interaction effect between water and temperature on all thermal properties. From the analysis, three important water content and temperature combinations were identified, i.e. wetting of a dry soil with a rising temperature (up to 60oC), the effect of freezing (0oC) and thawing (10oC) with increasing water content, and the excessive wetting of soils beyond 0.16 mm3 mm-3 with increasing temperature. The relationship between thermal properties with the combined effect of soil water content and temperature was non-linear. A mathematical model was also developed with an average R2 of 0.8 that enables to estimate the three thermal properties by using the soil water content and temperature data. The validation procedure showed that the model can predict thermal properties within the temperature range of 0-60oC. In the last content chapter, soil water evaporation and temperature distribution on the different water table depths was characterised. Time series soil water and temperature data was collected by using DFM probes. The soil water evaporation was determined by the water balance technique. Whereas the effect of water table depth on the diurnal temperature distribution was analysed by using the daily experimental mean, water table mean and the daytime amplitude temperatures as indicators of temperature variation. The study showed that the daily rate of evaporation (Es) and the cumulative evaporation (ΣEs) was highly influenced by potential evaporation and the soil’s hydraulic properties. In shallow water table depths of 0-500 mm, Es was shown to be dependent on the potential evaporation. However, as the water table depth increased beyond 500 mm, the soil hydraulic properties were the determinant factors. The results showed that as the water table depth increased, daily and cumulative evaporation also increased linearly. The relative loss of water by Es was compared among the water table depths and the average contributions from two measurement cycles varied from a minimum of 5.55% from the No WT treatment to a maximum of 100% from the 0 mm depth. The influence of soil type was also significant with higher Es in Hutton soil.The distribution of temperature was affected by the water table depth. As the water table depth increased, the temperature of the profiles increased and vice versa. But there was no significant difference in temperature distribution between the two soils. A significant amount of water is lost through Es from shallow water tables especially from 0-500 mm depth. This unproductive water loss can be converted into transpiration if water table depth under irrigated crop fields are maintained within 500 to 1000 mm depth. In addition, integration of conservation tillage practices such as mulches and zero tillage could reduce evaporation. A further study on the contributions of shallow water table depths on evaporation under different conservation tillage are recommended.