Use of geological weighing lysimeters to calibrate a distributed hydrological model for the simulation of land-atmosphere moisture exchange

Confined aquifers can act as large-scale “geological weighing lysimeters” that can be used to monitor changes of total water balance on a scale of kilometers. Observation well data for such aquifers can be used to evaluate vertical moisture balance and the lateral and vertical fluxes that are simulated with hydrologic models. This can be especially useful for the accurate modeling of vertical moisture fluxes when coupling atmospheric and hydrologic models. The concept was tested using 40 years of records from a deep observation well in Saskatchewan, Canada. These data were compared with the modeled water balance for one grid cell of a distributed hydrological model that had been set up for a large river drainage basin and calibrated on the basis of stream flow data only. The observation well data indicate that the model captures the water balance for the grid cell quite well, but underestimates evapotranspiration in dry years because it does not allow for transpiration losses from groundwater. The study shows how geological weighing lysimeters offer a promising potential for testing and calibrating distributed hydrological models at the scale of typical model grid cells.