Application of a quantitative method to evaluate flow regulating functions of ecosystems in the Zambezi Basin. Project report submitted to UNEP under the SADC project "Dam Synchronization and Flood Releases in the Zambezi River Basin".

By affecting transpiration and evaporation and influencing how water is routed and stored in a basin, forests, wetlands and floodplains play a crucial role in the hydrological cycle. Although they are widely attributed a major role in regulating flows (i.e. both attenuating floods and maintaining flow during dry periods) these services are seldom, if ever, explicitly factored into the planning and management of water resources. One reason for the failure to include them is lack of understanding of the hydrological functions occurring, their dynamic nature, and the interaction of those functions with the catchments in which the ecosystems are located. Very often it is unclear exactly which functions are performed and how those functions change over time (i.e. between seasons and between years). Furthermore, both the lack of quantitative information and a recognized method to incorporate them into decision-making processes, make it very difficult to integrate natural hydrological functions into the planning and management of water resources. This report describes a pragmatic approach for quantifying the flow regulating functions of floodplains, headwater wetlands and forests in the Zambezi Basin. The method utilizes observed stream flow records and flow duration curves to derive a simulated time series of flow in the absence of the ecosystem. This can then be compared with an observed time series to evaluate the impact of the ecosystem on the flow regime. The method, which is easy to utilize and could easily be incorporated into a decision support system for the synchronization of dam operation, has been applied and results obtained for 16 locations in the basin. Results indicate that the different ecosystems affect flows in different and complex ways. Broadly: i) floodplains decrease flood flows and increase low flows; ii) headwater wetlands increase flood flows and decrease low flows; iii) miombo forest, when covering more than 70% of the catchment, decreases flood flows and decreases low flows. However, in all cases there are examples which produce contrary results and simple relationships between the extent of an ecosystem type within a catchment and the impact on the flow regime were not found