Simulation and evaluation study of the three Water Supply Reservoirs in the Notwane
Catchment was undertaken using a hybrid modelling approach linking the reservoir
simulation model (HEC-ResSim) model and a reservoir reliability analysis
(RRA) model. It was used to understand the management challenges and operation
aspects facing the recent failure and declining water supply from three reservoirs in
Gaborone and the surrounding areas, a typical arid and urbanized environment
where current and future water supply reliability is challenged by both climate and
anthropologic factors. The model was analysed for a calibration period of ten years
(1993-2002), and verification period of eight years (2003-2010) and then simulation
period of 40 years (2011-2050). The simulation period up to the year 2050 was
considered to include the year 2035, which is the planning horizon of the National
Water Master Plan. The model calibration and verification results are satisfactorily
accepted for the fit of the daily water levels. The values of R2 and the Nash-Sutcliffe
model efficiency criteria for the calibration period, are 0.81/60%, 0.62/27% and
0.54/39% for the Bokaa dam, Gaborone dam and Nnywane dam, respectively. Various
scenarios were considered to determine the plausible sources of uncertainty
and challenge for operation and management of the water supply reservoirs considering:
population and urbanization, sedimentation, seepage, climate change, operational
aspects, among others. From the RRA model, it was found that Gaborone
dam, which is the largest of the three dams has lower resilience, lower reliability
and higher vulnerability associated with increasing population pressures, urbanisation
and climatic factors. Climate change, sedimentation, seepage, operational
rules, contributing to the operation and management of the dams could have accelerated
the drying up of the reservoirs and the prevailing water supply situation,
which might continue to be the future possible challenges of water supply in the
area.