Transient modelling of water-table variation in a floodplain wetland, Narborough bog, Leicestershire

This paper describes the application of a transient three-dimensional groundwater model to simulate water flux through a floodplain wetland, Narborough Bog, in Central England. The results of a programme of water-table monitoring are summarised to indicate the extent of water-table fluctuation, and experiments to determine hydraulic flow parameters are detailed. Hydraulic conductivities of deposits are highly variable; observations were in the range 1.16 x 10(-3)-4.63 x 10(-2) cm s(-1) for herbaceous peat, 1.16 10(-4) cm s(-1) for wood peat, and 9.14 x 10(-6)-3.36 x 10(-8) cm s(-1) for silty clay. A three-layer groundwater model of the wetland is developed using MODFLOW from these initial values, and with stratigraphic data obtained by hand augering. The model is calibrated using data for two periods of recharge from precipitation by adjusting specific yield and vertical hydraulic conductivity within prescribed limits. The accuracy of the model is then assessed by comparing daily model predictions of water-table response to potential evapotranspiration at specific monitoring points. Model results demonstrate the importance of variation in hydraulic characteristics with depth in wetlands, thus emphasizing the dependence of water-table response upon water-table position, and preceding moisture conditions. The resulting water-table configuration reflects the balance between horizontal and lateral water flow through the wetland, and its relationship to river stage. The success of the stratigraphic representation is considered and its general applicability evaluated. Finally, the use of the calibrated model for local hydrological management is considered, and the effects on the wetland water budget of varying river stage by specified amounts are examined.