Using Geolysimeters for an Independent Validation of Automated Precipitation Gauge Measurements

It has long been recognized that changes of mechanical load acting on the ground surface lead to changes of hydraulic head in underlying aquifers. Barometric effects on groundwater levels are familiar to every hydrogeologist dealing with observation well data. Changes of total moisture above an aquifer (canopy interception, snow, surface water, soil moisture, water table storage, evaporation) also represent changes of mechanical load and therefore are reflected in the water-level records of all observation wells in confined aquifers. Under favourable hydrogeology conditions, observation wells can be used as large-scale “geolysimeters”. Such geolysimeters involve minimal site disturbance and can provide direct and reliable measures of the total precipitation that has accumulated on a site. If there is no surface or subsurface runoff, the geolysimeter records can serve to validate the precipitation data collected by means of standard precipitation gauges. This possibility may be especially useful for evaluating records of solid precipitation where wind undercatch and other effects can result in data that do not represent the actual amount of precipitation that reached the ground. Numerous rainfall and snowfall events observed by geolysimeter sites in Saskatchewan, Canada have been compared with precipitation measured with single Alter shielded Geonor accumulating precipitation gauges as well as with periodic SWE measurements obtained from representative snow courses. The agreement between standard precipitation measurements and the geolysimeter is excellent but the gauge undercatch due to wind is evident even at relatively low wind conditions. This provides an opportunity for independent validation of not only the accumulating gauge measurements, but also the various wind adjustment techniques that are emerging from the WMO Solid Precipitation Intercomparison Experiment (SPICE). >