Recharge and flow processes in a till aquitard

Eastern Denmark is primarily covered by clay till. The transformation of the excess rainfall into laterally diverted groundwater flow, drain flow, stream flow, and recharge to the underlying aquifer is governed by complicatedinterrelated processes. Distributed hydrological models provide a framework for assessing the individual flow components and forestablishing the overall water balance. Traditionally such models are calibrated against measurements of stream flow, head in the aquiferand perhaps drainage flow. The head in the near surface clay till deposits have generally not been measured and therefore not consideredin the calibration procedure.In a 16 km2 rural catchment, 15 shallow wells were installed in the upstream end for continuous measurements of the fluctuations in hydraulic head. In addition data were obtained from two wells penetrating to the deeper artesian aquifer, one located near the shallow wells and one in the valley adjacent to the stream. Precipitation and stream flow gauging along with potential evaporation estimates from a nearby weather station provide the basic data for the overall water balance assessment. The geological composition was determined from geoelectrical surveys along three transects, supported by geophysical logs in deepwells, lowflow records at the outlet of the catchment and three tributaries, and soil maps. Slug tests were carried to obtain data forhydraulic conductivity.The time series of hydraulic head depth in the shallow wells were analyzed using linear transfer noise functions on driving input timeseries and kriging techniques in order to identify correlation structures in time and space among the wells.The distributed and physically based hydrological model code MIKE SHE was applied to the catchment. The model considers one-dimensional flow in the unsaturated zone and three-dimensional below. Drainage flow isempirically modelled as a linear reservoir using a time constant related to drain pipe capacity, spacing and soil hydraulic conductivity.Key parameters are calibrated against records of precipitation, potential evaporation and stream flow. Simulation based on historicalrecords prior to the installation of subsurface drainage in 1/3 of the catchment was carried out in order to investigate the impact ofdrainage on streamflow and access the use of the linear reservoir assumption. Subsequently, data from the shallow wells wereconsidered in order to analyse the value of such data in the calibration procedure and particularly in estimating the areal variation inrecharge.