Simulation and evaluation of water supply reservoirs using HEC-ressim model and RRV indices in the Notwane catchment

Abstract

Botswana has gradually developed its water resources at great expense but it is estimated that as much as half of it is wasted through leakage, lack of water demand management, knowledge gap, inefficient management and operation practices and policies. Reservoirs are one of the most valuable tools for integrated water resource development and management. However, their operation and management are still a challenge for integrated water resource development and management. With the increasing population and rapid development of the economies, the function and operation of reservoirs has become more and more important to help meet society’s energy and water requirements. Models are usually developed and used in order to facilitate decision-making. Simulation models strive to fill the knowledge gap, cross check analyses, support and address water resource development and management decisions. HEC-ResSim Model is one such model used to simulate the water resource system in the Notwane River Basin. It is a public domain model that enables easy sharing of knowledge and data among researchers. The base line scenario is configured in such a way to represent the current infrastructure and known management practices and calibrated with historical hydrological regime. The model is calibrated and its performance is checked using Nash-Sutcliffe efficiency and Coefficient of Determination, (R2), at key stations. The model calibration is satisfactorily accepted as the values for R2 and for NS coefficient for the fit of the daily water levels for the calibration period are 0.81 and 0.60 respectively and 0.62 and 0.27 and 0.54 and 0.39 respectively for the Bokaa, Gaborone and Nnywane dams in that order. Different scenarios including climate change, population growth, sedimentation and operation schemes for each dam have been simulated for the period of 2015 to 2050. These scenarios are analyzed and evaluated by comparing the reliability, resilience and vulnerability indices (RRVs) on the dam’s release capabilities. As introduced by Hashimoto et al. (1982), RRV metrics measure different aspects of a water resources system performance. Together, RRV metrics provide one of the most comprehensive approaches for analysing the probability of success or failure of a system, the rate of recovery of a system from unsatisfactory states, as well as quantifying the expected consequence of being in unsatisfactory states for extended periods. Assessing these comprehensive metrics at current (baseline) and future scenarios provide insight into system performance in changing or varying conditions. Outputs from the HEC-ResSim model were then used to define criteria over which the RRV and other metrics were evaluated. Results from the simulation indicates that the scenario of Population Growth greatly lessens the reliability of the Gaborone, Bokaa and Nnywane reservoirs by 66, 2 and 49 % respectively on average while sedimentation on the dams negatively affect the reliability of water supply by 30, 1 and 4 % respectively. The resilience of all scenarios for all the dams is very low expressing the low speed of recovery of the dams from failure states to satisfactory states. This is evidenced by a large number of days of maximum of unsatisfactory duration (demand not met) of about 284, 107, and 133 days for Gaborone, Bokaa and Nnywane dams. Lastly, the results indicate that the Bokaa dam is the least vulnerable followed by Nnywane and Gaborone dams. The significance of the study results will help in coming up with policies and rules which, if implemented, will promote efficient and sustainable planning, management and operation of reservoirs in the Notwane catchment.