Impacts of wetland loss and fragmentation on the hydrology of Zimbabwe's highveld

Abstract

Wetland loss and fragmentation are among the greatest threats to water resources in developed and developing countries. While several studies on wetland fragmentation have been done, a few have looked at impacts of wetland fragmentation on hydrology, with none having been done on Zimbabwe's highveld headwater catchments. There is a critical need to investigate the influence of wetlands on flow regimes of highveld headwater catchments, to understand the hydrological role that wetlands play in sustaining water resources. Endowed with dambos, marshes and riverine wetlands, Zimbabwe's highveld play a significant role in sustaining Zimbabwe's water resources, with major river systems originating from the highveld plateau, having wetlands as their source regions. Hydrologic impacts of wetland fragmentation on flow regimes of highveld headwater catchments i.e., Upper Manyame (Manyame catchment), Nyagui (Mazowe catchment) and Macheke (Save catchment) were analyzed for the period from 1984–2021. Analysis of landcover and wetland change as well as streamflow characteristics was done for 1984, 1994, 2004, 2014 and 2021 periods. Simulation of streamflow under wetland fragmentation was done using the topographically driven rainfall-runoff model (TOPMODEL), which was set up, calibrated and validated for the most sensitive parameters, which include scaling parameter (m), transmissivity (To) and root zone available water capacity (SRmax). Results from landuse/cover analysis for the period between 1984 and 2021 showed a decrease in wetland area, followed by an increase in built up area and bare land for the same period, owing to expansion of urban areas and cultivation into wetland areas. Hydrological simulation by TOPMODEL and flow duration curve analysis show that wetland fragmentation has resulted in increased peak flows, while low flows have declined for the three catchments. The findings of this research would be helpful in understanding the hydrological functions of highveld wetlands, providing the reference for protection and sustainable utilization of wetland resources in the highveld catchments. HIGHLIGHTS Hydrological impacts of wetland loss/fragmentation in Zimbabwe's headwater catchments.; Simulation of streamflow for wetland fragmentation under the TOPMODEL approach.; Expansion of urban areas and cultivation into wetland areas.; Decrease in wetland area, increase in built up area, increased peak flows, decrease in low flows.; Reference for protection and sustainable utilization of wetlands in headwater catchments.;

Keywords

• calibration
• flow duration curve
• hydrologic signature
• model parameter
• topmodel
• wetland fragmentation