Development of updated design norms for soil and water conservation structures in the sugar industry of South Africa.

Abstract

Sugarcane in South Africa is grown on wide-ranging soils, sometimes in non-ideal climates and on steep topographies where soils are vulnerable to erosion. A consequence of unsustainable soil loss is reduction in field production capacity. Sugarcane fields are protected against erosion through, inter alia, the use of engineered contour banks, waterways and spill-over roads. A comparison of design norms in the National Soil Conservation Manual and norms used in the sugar industry clearly shows discrepancies (e.g. maximum slope and cover factor of sugarcane) that need to be investigated. Furthermore, the sugar industry design nomograph was developed based on an unsustainable soil loss limit, does not include any regional variations of climate and the impact on soil erosion and runoff and does not include vulnerability during break cropping. The aim of this research was to develop updated design norms for soil and water conservation structures in the sugar industry of South Africa. Many soil loss models exist, of which empirical models are the most robust and provide stable performances. The Modified Universal Soil Loss Equation (MUSLE) which is embedded in the Agricultural Catchments Research Unit (ACRU) model, estimates event-based soil erosion and, given that the majority of soil erosion occurs during a few extreme events annually, the design norms were updated using the MUSLE. The ACRU model is a daily time step, physical- conceptual agrohydrological model. Runoff volume, peak discharge and sediment yield were simulated with the ACRU model and verified against the respective observed data. The results showed good correlations and the ACRU model can be confidently applied in the development of updated design norms for soil and water conservation structures in the sugar industry of South Africa. The ACRU model was used to conduct simulations for the different practices in the sugar industry and the results used to build the updated tool for the design of soil and water conservation structures in the sugar industry of South Africa, using MS Access with a background database and a graphical user interface. The updated tool is robust, based on sustainable soil loss limits, includes regional variations of climate and their impact on soil erosion and runoff and also includes vulnerability during break cropping. It is more representative of conditions in the sugar industry of South Africa and therefore recommended for use in place of the current sugar industry design norms. The results also indicate that soil and water conservation structures result in insignificant reductions in stream flow and would not likely necessitate their declaration as Stream Flow Reduction (SFR) activities as contained in the National Water Act of South Africa. Consequently, a 20 year return period is recommended for the design of soil and water conservation structures and the cost implication of varying design return periods from the minimum 10 year return period to the 20 year return period ranges from 16% to 35% across the four homogenous regions in the sugar industry of South Africa.