ABSTRACT
Drainage is defined as ‘the removal of excess surface and subsurface water from the land to enhance crop growth, including the removal of soluble salts from the soil’ (Bos and Boers, 2006). To know how much water has to be removed the water and salt balances have to be calculated to determine the design criteria. Before the water balance of the area can be made, a number of surveys must be undertaken, resulting in adequate hydrogeological, hydropedological and topographic maps. Further, all (sub-)surface water inflows and outflows must be measured or estimated. Precipitation and the relevant evapotranspiration data from the area must be analysed. In addition, all relevant data on the hydraulic properties of the soil should be collected. In some cases, the proper
identification of the source of ‘excess water’ will avoid the construction of a costly drainage system. For example:
• If irrigation water causes waterlogging, the efficiency of water use in the water supply system and at field level should be studied in detail and improved;
• If surface water inflow from surrounding hills is the major cause of excess water in the area, this water could be intercepted by a hillside drain which diverts the water around the agricultural area;
• If the problem is caused by the inflow of (saline) groundwater, this subsurface inflow could be intercepted by a row of tube wells, which dispose of their effluent into a drain that bypasses the agricultural area;
• If the area is partially inundated because a natural stream has insufficient discharge capacity to drain the area, a reconstruction of the stream channel may solve the drainage problem.
