Doctoral Degrees (Soil, Crop and Climate Sciences)
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Browsing Doctoral Degrees (Soil, Crop and Climate Sciences) by Author "Botha, John Jacobus"
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Item Open Access Evaluation of maize and sunflower production in a semi-arid area using in-field rainwater harvesting(University of the Free State, 2006-11) Botha, John Jacobus; Van Rensburg, L. D.; Hensley, M.English: Subsistence farmers occupy a large area east of Bloemfontein around Thaba Nchu in the Free State Province of South Africa. They do not enjoy food security because the area is marginal for crop production. There are three reasons for this: (a) low and erratic rainfall that amounts to a mean of 543 mm per annum; (b) a corresponding high evaporative demand of 2198 mm per annum; (c) dominantly duplex and clay soils on which rainwater productivity (RWP) is low due to high runoff (R) and evaporation (Es) losses. It was hypothesised that the in-field rainwater harvesting (IRWH) technique could improve crop yields compared to conventional tillage (CON), and thereby serve to improve food security. Field experiments were conducted on the Glen/Bonheim; Glen/Swartland (dark brown A horizon); Khumo/Swartland and Vlakspruit/ Arcadia ecotopes to study the benefits of the IR WH technique on maize and sunflower yields. This thesis distinguishes between ex-field (REx) and in-field runoff (RIn). RIn is transportation of water over the 2 m runoff strip in the IR WH technique. REx occurs on CON and represents a loss of water and soil. Runoff and sedimentation results indicate that IR WH stops REx completely and has the ability to harvest extra rainwater in the basins through RIn and minimize sedimentation. The results also indicate that mulch on the runoff area decreases RIn and sedimentation. The Es process with different surface coverings was studied on two ecotopes viz. Glen/Bonheim and Glen/Swartland (red-brown A horizon). The soil coverings were as follows: bare soil; stone and organic mulch covering 50% of the surface; and organic mulch covering 100% of the surface. The studies were conducted during summer (69 days) and winter (52 days). Results indicated that the % cover affected Es more than mulch type, and that the influence of mulch on Es was more efficient when the drying-out period did not exceed 16 days. New terminology for the various Es stages was introduced. The role of Eo, water content and hydraulic conductivity during the Es process were clarified. Es measurements shallower than 300 mm were shown to be unreliable. Field experiments were conducted on four ecotopes over two to seven growing seasons during the period 1996/1997 to 2002/2003 with maize and sunflower. The treatments were CON; IRWH with a bare basin and bare runoff area (BbBr); IRWH with organic mulch in the basins and a bare runoff area (ObBr); IR WH technique with organic mulch in the basins, stones on the runoff area (ObSr); IRWH technique with organic mulch in the basins, organic mulch on the runoff area (ObOr); IRWH technique with stones in the basins, organic mulch on the runoff area (SbOr). Results showed that IR WH significantly increased maize and sunflower yields compared to CON. This was shown to be due to the ability of IR WH to stop REx completely; enhance RIn and its resulting beneficial redistribution of water in the soil profile; minimize Es/ET, and contribute towards higher transpiration. Both yield and RWP results showed that IRWH stabilises crop production on these ecotopes, compared to CON. Comparing the IR WH techniques revealed that there was a consistent trend in yield and RWP viz. ObSr> ObOr ~ SbOr > ObBr> BbBr. All the IR WH treatments with mulch on the runoff area produced higher RWP values and yield increased between 7 and 16% compared to ObBr. Although Es/ET results indicated that the IR WH treatments with mulch on the runoff area lost smaller portions of ET to Es than ObBr, mulch type on the runoff area and basins did not significantly affect Es in any of the years. The most reliable way to describe the effectiveness with which rainwater was converted into grain by various techniques was by using the parameter RWPn. It was computed by using long-term experimental and simulated yield data, which included rainfall during the fallow and growing seasons. An empirical crop water stress model "Crop Yield Prediction for Semi-Arid Areas" (CYP-SA) was developed. Model composition and validation results with maize and sunflower are described. CYP-SA was used to make long-term maize and sunflower yield predictions with long-term climate data (8I-year period). Cumulative probability functions of simulated long-term maize and sunflower yields have shown that IR WH is significantly superior to CON. The ObSr treatment was shown to be the best. It was also shown that it is advisable to plant maize or sunflower early in January, especially when the soil water profile is between ¾ full and full. The JRWH technique was introduced to rural communities In the target area to improve household food production. The thesis reports on the rapid spread of the application of JRWH amongst homesteads, and on its ability to eradicate poverty at household level. Selected case studies were reported. Very promising results were obtained showing that households can reduce poverty by selling the produce. The five pillars of sustainability, as defined by Smyth & Dumanski (1993) viz. agronomic productivity; crop production risk; conservation of natural resources; economic viability and social acceptability, were investigated in relation to JRWH. Results indicate that in the agro-ecological and socio-economic environment present in the rural communities around Thaba Nchu CON was non-sustainable and that JRWH was sustainable.