Hydrological processes in the Chalk unsaturated zone – Insights from an intensive field monitoring programme

As part of the NERC lowland catchment research programme (LOCAR), the Pang-Lambourn catchments (Berkshire, UK) have been extensively instrumented to provide an improved understanding of the hydrological processes in the unsaturated zone of the Chalk – a subject of much debate over the past 30 years. To quantify the movement of water through the unsaturated zone it is necessary to measure matric potential and water content (as well as input and output fluxes from precipitation, evapotranspiration and groundwater recharge). Previous studies have indicated the need for fine temporal resolution data to monitor this system effectively. Due to the challenges faced in meeting this objective, a monitoring scheme was designed that combines a variety of instrumentation to provide the required range, accuracy and spatial and temporal resolution of data. This paper firstly presents an overview of the monitoring programme and a critical discussion of the instrumentation used. Subsequently, the field data are used to characterise the hydrological properties of the unsaturated zone and to gain insight into recharge processes. The interpretation of the data presented in this paper indicate that, in the Chalk unsaturated zone at West Ilsley, matrix flow is the dominant recharge mechanism.