Verdantly vile!
By managing the aquatic vegetation in irrigation canals, large volumes of water can be saved.
By Kobus du Plessis
Irrigation agriculture is the largest consumer of water in South Africa, as it uses approximately 62 % of the total national water supply. At present, a million hectares from a potential 1.4 million hectares of land is under irrigation. Large volumes of water are transported through irrigation canal systems to water crops, to supply food to the nation. All water conveyance structures experience water loss, and irrigation canals are definitely no exception. As agriculture is the main water user and the sector with the highest potential to save water, it makes sense that savings in water loss in the agricultural sector will make the biggest impact on overall water loss control in South Africa. South Africa has thousands of kilometres of canals criss-crossing the countryside, from which most of the irrigation schemes receive their water supply. These canals are both lined and earthen structures. Many of these water conveyance structures are plagued by the proliferation of nuisance aquatic vegetation, such as filamentous algae and submersed aquatic macrophytes.
Following the winter months, the increasing daylight hours and consequent rise in ambient temperatures result in increased water temperature and light penetration into dams and canals. During the summer season, optimum environmental conditions and the presence of sufficient plant nutrients( phosphates and nitrates) lead to an explosion in aquatic vegetation biomass on affected irrigation canal systems. As Envirokonsult Scientific Services’ managing director, I have studied this phenomenon over the years, and can say with confidence that aquatic vegetation plays a significant role in water loss from canal infrastructure. Aquatic vegetation proliferates in a relatively short period to form dense masses of organic material that result in substantial operational problems, while wasting significant volumes of water. Large volumes of the design capacity of canals and other water conveyance structures are displaced by the exponential increase in plant biomass, leading to spillage, water loss, and failure of civil structures. It is estimated that a minimum average of 20 – 30 % of the water supplied to the irrigation sector is lost through leakages out of conveyance structures, evaporation, evapo-transpiration, spillage, and flooding, among others. Some older schemes continue to record periodic losses of more than 50 % on certain canals. Where water is released into rivers for irrigation downstream, water loss can easily reach 50 % of the volume released( part of this water is essential, however, for provision for the reserve— aquatic ecosystem). As aquatic vegetation contributes significantly to water loss from water conveyance structures, the problem is also addressed by the Department of Water and Sanitation’ s Water Conservation and Water Demand Management Strategy( WC / WDS). 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( savings on crop losses not included) and release more than 550 million m 3 / a of water for use by other sectors.
Operational problems
Aquatic vegetation causes a host of operational problems in water
Images by Envirokonsult Scientific Services
Algae problems in Hartbeespoort irrigation canals.
20 Water Sewage & Effluent March / April 2018