WC/WDM
Reducing water losses in irrigation canals As the biggest water user in South Africa, the agricultural sector presents significant opportunities to save water, and the possibility to make the biggest impact on overall water loss control in South Africa. By Kobus Du Plessis*
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t present, around a million hectares of land is under irrigation, accounting for approximately 62% of total national water use. Large volumes of water are transported through thousands of kilometres of irrigation canal systems across the country, all of which experience a level of water loss. It is estimated that a minimum average of 20% to 30% of the water supplied to the irrigation sector is lost due to leakages out of conveyance structures, evaporation, evapotranspiration, spillage and flooding, among others. Some older schemes record
periodic losses of more than 50% on certain canals. Based on current agricultural tariffs, a mere 10% saving in the current water loss (taken as 25% of usage by the agricultural sector), can result in savings of at least R120 million per annum and release more than 550 million m3/annum of water for use by other sectors or extensions of irrigation land.
The vegetation challenge
Many of these water conveyance structures (both lined and earthen structures) are plagued by aquatic vegetation, such as Algae build-up in a canal
Kobus Du Plessis, managing director, Envirokonsult Scientific Services
filamentous algae and submersed aquatic macrophytes. During the summer season, optimal environmental conditions, along with the presence of plant nutrients such as phosphates and nitrates, lead to an explosion in aquatic vegetation biomass on affected irrigation canal systems. Aquatic vegetation proliferates in a relatively short period to form dense masses of organic material that result in substantial operational problems with economic implications in water supply systems and waste significant volumes of water.
Resulting problems
A canal being treated for aquatic vegetation build-up
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SE PT /O CT 2020
Aquatic vegetation causes a host of operational problems in water conveyance structures, such as reducing hydrological capacity and flow speed in affected canals – some to such a degree that the supplied water cannot reach the terminal point of the canal system. Vegetation can also impede sluice gates, water log long-weirs, block irrigation systems and filters at water purification works, and cause structural failures of concrete-lined irrigation canals due to flooding. The summer season often sees an explosion in aquatic Furthermore, submersed vegetation biomass aquatic vegetation can replace significant volumes of the capacity of a canal, leading to spillage and water loss from the system, and extends to crop losses due to under-supply of water to irrigators. As aquatic vegetation biomass replaces water in a canal, a higher volume of water is recorded at
