Monitoring the moisture balance of a boreal aspen forest using a deep groundwater piezometer

This study evaluates the piezometric weighing lysimeter method of van der Kamp and Schmidt, based on two annual cycles of measurements in a mature boreal aspen forest in western Canada. The method is used to infer components of the moisture balance on a scale of hectares. The method is based on accurate measurement of groundwater pore pressure in thick clay formations, and depends on the principle that changes of mass load result in changes of groundwater pressure. The resulting data are similar to those obtained by conventional weighing lysimeters, but on a much larger scale and with no significant disturbance of the site. The groundwater pressure was measured at the 34.6 m depth, effectively integrating the mass loading over an area of about 10 ha. Cumulative evapotranspiration and precipitation, as inferred from the groundwater pressure time series, showed good agreement with direct measurements. The piezometric and gauged estimates for cumulative precipitation agreed to within 5%. The piezometer also captured the main features of the diurnal and seasonal cycles of evapotranspiration. However, the estimation of evapotranspiration was complicated by the presence of a seasonal background cycle in the groundwater pressure time series, the result of a higher-than-ideal permeability at this site. An initial attempt to estimate the seasonal background cycle from the observed pressure changes during quiescent periods met with moderate success. Work is in progress to estimate the background cycle using a more physically-based approach.