Wetland hydrology of the Elmley marshes
Despite the importance of the hydrological regime for the functioning of wetland environments, the understanding of hydrological processes, particularly evaporative dynamics and clay soil moisture fluxes, is limited and the original research outlined in this thesis constitutes a real contribution to further the scientific understanding of wetland systems. Two lines of investigation are followed based upon field experiments and monitoring of groundwater and ditch water levels together with soil moisture over time and space. The first investigation assesses the effectiveness of the Environmentally Sensitive Area (ESA) scheme of the North Kent Marshes to achieve its objective of soft moist ground conditions through the manipulation of ditch water levels. Results demonstrate that due to the clay soil texture and low hydraulic conductivity of the marsh substrate, little relationship exists between the position of ditch water levels and groundwater. Flooding the marsh surface with brackish ditch water does promote high surface soil moistures and optimal habitat for waterfowl. However the deleterious effects upon soil structure and floristic diversity by flooding large areas need to be taken in consideration for the sustainable management of the wetland. Results from this investigation prove that precipitation and evaporation are the dominant fluxes of water, and so the second investigation focused upon the loss of water from the wet grassland by evaporation. There are two foci of research. Firstly the relationship between surface resistance and soil moisture content is examined, so that 'correct' surface resistance values could be input to the Penman Monteith method to compute actual evaporation according to the wetness of the marsh soil. Research results show a complex relationship with decreasing rates of actual evaporation below potential rates with soil moisture loss, and a functional relationship has been quantified. Secondly the actual evaporative water loss of the wetland was determined taking into account the small-scale heterogeneity of surface wetness conditions. The Weighted Penman Monteith (WPM) approach was followed referencing results against data by the Bowen Ratio Energy Balance method. Research results show that the WPM method could be used to compute the actual evaporation loss, however the adoption of the Priestly-Taylor approach with suitable values of a is a simpler method and of equal accuracy.