Techniques for assessing the impacts of wetlands on hydrological responses under varying climatic conditions.
Gray, Ryan Paul.
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Wetlands are considered sensitive eco-tones that provide numerous goods and services, not only to the communities which are immediately dependent upon them, but also to the many downstream stakeholders who benefit from the hydrological influences that wetlands have on a catchment. The three main objectives of this study, the foci of which included an assessment of impacts of wetlands on catchment hydrological responses (viz. flood attenuation and streamflow regulation) in the Thukela catchment under varying geographical and climatic conditions, are: · A modification and validation of the ACRU Model’s Wetland Routine; · Assessing impacts of wetlands on hydrological responses from catchments in varying climatic regions under historical climatic conditions; and · Assessing impacts of wetlands on catchment hydrological responses for climate change scenarios by using outputs from a Regional Climate Model (RCM). The ACRU Model was selected to undertake the daily hydrological simulations, while historical climate data and climate information derived from the C-CAM Regional Climate Model were used as inputs into the model. These varying climatic inputs, as well as the changes in water fluxes between simulations with and without the wetlands routine switched on, enabled the author to assess the impacts of wetlands on catchment hydrological responses under varying climatic conditions. The ACRU wetland routine initially did not produce output in line with conceptualisation of wetlands processes. As a result of this, certain modifications had to be made to the model to ensure that the results obtained mimicked wetlands hydrological processes realistically. A validation was performed on the re-configured ACRU wetlands routine to show that the simulated results of impacts of wetlands on catchment hydrological responses were realistic when compared to findings from the literature review (e.g. in regard to streamflow regulation and flood attenuation). These validation results also show that the impacts of wetlands on catchment hydrological responses are dependent on the level of soil water saturation of the wetland at the start of a streamflow event and the volume of the streamflow event in relation to the relative size of the wetland. The results further illustrate that wetlands have a relatively small flood attenuation and streamflow regulation impact on mean annual catchment hydrology at the outlet of the 29 136 km2 Thukela catchment. However, mean monthly results show pronounced effects (20 – 30%) of flood attenuation in the summer months and streamflow regulation throughout the year, especially in the drier winter months. The climate change scenario results illustrate that the impact of wetlands on hydrological responses are virtually entirely masked by the impact of climate change, with only minor changes shown on outflows of the Thukela between climate change scenarios without and with wetlands.