Understanding the hydrological functioning of wetlands (CB)
Early work on the Craigieburn wetlands (see Pollard et al. 2005) suggested that the gulley erosion and loss of fines at the toe of the wetlands steepens the hydraulic gradient and hence increases groundwater flow within the soil, so that there is a gradual drawdown of the water table. The concomitant desiccation of the landscape creates conditions which are unfavorable for the production of organic carbon, and hence fertility declines. Indeed, a comparison of the historical hydrological characteristics (indicated by soil characteristics) and the contemporary vegetation distribution suggested that there had been widespread desiccation of the landscape, and a 50% reduction in wetland areas, probably over the last two decades. Hydrological simulations for a number of sub-catchments in the upper reaches indicate that wetlands contribute to hydrological functioning of the micro-catchment through the attenuation of peak discharges and increased baseflows. These results suggest that wetland degradation impacts through a moderate reduction of base flows.
However, it was clear that more detailed hydrological research was needed particularly given the rehabilitation interventions by Working for Wetlands. This has become the focus of a hydrological monitoring program designed to determine the hydrodynamic response of these wetlands to rehabilitation interventions. Initial work as part of a PhD study examines the behaviour of a headwater wetlands groundwater phreatic surface prior to rehabilitation. The findings of hydrometric observations include the delineation of a stratified water table system and the behaviour of this suggests the occurrence of hydro-dynamically distinct regions within the wetland, and loss of groundwater through head-cut erosion. Two-Dimensional Electrical Resistivity Tomography (ERT) surveys have identified a zone of finer sediment which is thought to act as a sub-surface flow buffer within this otherwise sandy wetland substrate. These findings suggest that these impacted wetlands need to be ‘plugged’ in order to restore their hydrological regime. (see article Riddell, E.R., 2007 'Water Table Dynamics of a Severely Eroded Wetland System, Prior to Rehabilitation, Sand River Catchment, South Africa')