Evaluation of river losses from the Orange River using hydraulic modelling

Releases into the Orange River from Vanderkloof Dam Row some 1400 km throug h extremely hot and arid areas, without further control structures. Accurat e estimates of evaporation losses are therefore: important to manage the ri ver's resources efficiently. A study was initiated to improve estimates of river losses downstream of Vanderkloof Darn, and to develop a methodology f or estimating evaporation losses from South African rivers. Theoretical losses were estimated from a measured evaporation rate multipli ed by the water surface area and an appropriate riparian vegetation area. I n 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 mill ion m(3)/a at an annual low Row release rate of 50 m(3)/s to 989 million m3 /a at an annual release rate of 400 m(3)/s. The variation in evaporation lo sses is due to the change in surface area with flow. (C) 2001 Published by Elsevier Science B.V.