Modeling Subsurface Drainage to Control Water-Tables in Selected Agricultural Lands in South Africa

Abstract

Like many other arid parts of the world, South Africa is experiencing irrigation-induced drainage problems in the form of waterlogging and soil salinization, like other agricultural parts of the world. Poor drainage in the plant root zone results in reduced land productivity, stunted plant growth and reduced yields. Consequentially, this hinders production of essential food and fiber. Meanwhile, conventional approaches to design of subsurface drainage systems involves costly and time-consuming in-situ physical monitoring and iterative optimization. Although drainage simulation models have indicated potential applicability after numerous studies around the world, little work has been done on testing reliability of such models in designing subsurface drainage systems in South Africa’s agricultural lands. This study investigated the occurrences of shallow water-tables South Africa’s agricultural lands as well as the applicability of drainage simulation models in optimized designing and evaluating performance of subsurface drainage systems as means for controlling water-tables in cropped fields in South Africa. The study also used tried and tested models to assess the effectiveness of existing drainage systems that were designed by conventional means. Three models (DrainMod, EnDrain and WaSim) were tested and compared to recommend the most appropriate model considering local conditions. It was observed that Makhathini irrigation scheme is suffering from shallow water-tables, although the trend is decreasing in some blocks due to a drainage system that was introduced in them. EnDrain model performed very well in simulating subsurface drainage system dynamics, as shown by the good agreement between observed and simulated drain discharges (R2 = 0.81), comparably better than DrainMod and WaSim especially during validation. The existing drainage systems at Breede proved to be effective in lowering water-table to levels that maintain optimum crop yields. Therefore, shallow water-tables are a reality in agricultural areas where they exist, and subsurface drainage systems can solve this problem, while simulation models can be simple and cost-effective means of designing and evaluating their performance of subsurface drainage systems. EnDrain, which is userfriendly, basic and requires the least input data, is a better drainage simulation model. Further study should focus on impacts of climate change on subsurface drainage dynamics and crop yields, as well as testing drainage simulation models at other alternative study sites with different soil and climatic conditions.