ABSTRACT
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.