Dam operation for environmental water releases; the case of Osborne dam, Save catchment, Zimbabwe

Abstract

There is limited capacity in terms of knowledge and experience on how to calculate the environmental water requirements (EWR) in Zimbabwe. In this paper the EWR were assessed using the desktop model developed by [A Desktop Model used to provide an initial estimate of the ecological instream flow requirements of rivers in South Africa. Institute for Water Research, Rhodes University, Grahamstown, 2001] and a spreadsheet model (Waflex) was developed to incorporate a component of EWR in reservoir simulation. The paper assesses whether EWR as established by Hughes method can be incorporated into a reservoir simulation and water allocation model, and if it is possible to derive EWR directly from naturalised flow series. The paper further considers the possibility of using the concept of capacity sharing for allocating water rights to the environment. The results show that at present use levels the EWR in the Odzi river can easily be met. However when in future water abstractions will increase, the effective water releases for the environmental will increase significantly. Also a very simple method is proposed to establish a first approximation of EWR. The paper shows that the capacity sharing model concept is a transparent institutional arrangement, which can be used to allocate water rights to the environment. It can be concluded that the Waflex model can provide practical guidelines to catchment managers and dam operators to implement EWR.