Assessment of environmental flow requirements for river basin planning in Zimbabwe

There is a growing awareness and understanding of the need to allocate water along a river to maintain ecological processes that provide goods and services. Legislation in Zimbabwe requires water resources management plans to include the amount of water to be reserved for environmental purposes in each river basin. This paper aims to estimate the amount of water that should be reserved for environmental purposes in each of the 151 sub-basins or water management units of Zimbabwe. A desktop hydrological method is used to estimate the environmental flow requirement (EFR). The estimated EFRs decrease with increasing flow variability, and increase with the increasing contribution of base flows to total flows. The study has established that in order to maintain slightly modified to natural habitats along rivers, the EFR should be 30–60% of mean annual runoff (MAR) in regions with perennial rivers, while this is 20–30% in the dry parts of the country with rivers, which only flow during the wet season. The inclusion of EFRs in water resources management plans will not drastically change the proportion of the available water allocated to water permits, since the amount of water allocated to water permit holders is less than 50% of the MAR on 77% of the sub-basins in the country.