Evaluation of river losses from the Orange River using hydraulic modelling

Releases into the Orange River from Vanderkloof Dam flow some 1400 km through extremely hot and arid areas, without further control structures. Accurate estimates of evaporation losses are therefore important to manage the riverʹs resources efficiently. A study was initiated to improve estimates of river losses downstream of Vanderkloof Dam, and to develop a methodology for estimating evaporation losses from South African rivers. Theoretical losses were estimated from a measured evaporation rate multiplied by the water surface area and an appropriate riparian vegetation area. In addition, hydraulic modelling was used to perform a dynamic water balance to verify the initial loss estimate. Evaporation rates from a flowing river (in an arid region) were found to be in the same order of magnitude as A pan evaporation data. The evaporation losses from the Orange River derived from this research range from 575 million m3/a at an annual low flow release rate of 50 m3/s to 989 million m3/a at an annual release rate of 400 m3/s. The variation in evaporation losses is due to the change in surface area with flow.