Low flow characteristics of river Notwane at Gaborone Dam

Abstract

Low flow studies for the Notwane River at Gaborone dam has been undertaken using daily flow records between 1979 and 1999 to determine the magnitude of annual maximum deficit volumes and deficit durations at a threshold level equivalent to 75 % dependable flow. Statistical modeling of these annual maximum values, separately, using a PWM/L-moment procedure, showed that a GEV distribution best describes these series. Among others, it has been estimated that deficit volumes of 3000 and 4700 m3 can be experienced on an average every 2 and 5 years respectively. Analysis even show that these deficit volumes are likely to be spread over a period of 318 and 390 days respectively, meaning that while deficit duration at T=2 years may spread over the whole year, deficit duration at T=5 years may spread into the next year thus posing challenges to the supply position over two subsequent years and hence requiring development of sound strategy in water supply.

Keywords: Low flows, Percentile flow, L- Moments, Probability Weighted Moments