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