MOISTURE LOADING EFFECTS IN GROUNDWATER PRESSURE RECORDS

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) also represent changes of mechanical load and therefore are reflected in groundwater pressure records for all confined formations. Identification and analysis of such moisture loading effects can provide valuable information on the hydrogeology and hydrology of the well site. However, such moisture loading effects are generally obscured by groundwater pressure fluctuations due to other causes, primarily atmospheric pressure changes. As a result moisture loading effects are generally not recognized or are misinterpreted as being due to other effects such as groundwater recharge or discharge.

Under favourable hydrogeology conditions groundwater observation wells can be used as large-scale geological weighing lysimeters or “geolysimeters”. Unlike conventional weighing lysimeters that are used to monitor soil moisture changes, geolysimeters involve minimal site disturbance and can provide a high-resolution measure of the changes of total water storage over areas of hectares to km². The geolysimeters may be in the form of normal observation wells in confined aquifers, some of which have many years of records. They can also be in the form of shut-in pressure sensors positioned in the interior of thick aquitards, in which case the sensing area is well defined. Geolysimeters can thus provide a unique measure of storage changes on a scale commensurate with that of the “pixels” of regional hydrogeological and hydrological models. Such storage changes are increasingly recognized as a critical link between precipitation and streamflow in watershed hydrology. Conversely, incorporation of moisture loading theory in numerical models of groundwater-soil moisture-vegetation interactions would open the way for use of detailed records of groundwater levels to verify and improve model performance.