Comparing direct measurements of root-zone soil moisture in a forest with piezometric weighing lysimeter records

In deep clay-rich aquitards, the groundwater pressure responds instantly and proportionately to changes of mass load by precipitation, evapotranspiration and runoff. The groundwater pressure, measured with piezometers, can therefore be used to infer the surface moisture balance, including snow accumulation and canopy storage. The data obtained from such piezometric weighing lysimeters are similar to those obtained by conventional weighing lysimeters but on a scale of hectares. In this study, we validate estimates of soil moisture inferred from the time series of deep groundwater pore pressure, by comparison with direct measurements of the soil moisture balance.

We present data from a boreal deciduous forest (the BERMS (Boreal Ecosystems Research and Monitoring Sites) Old Aspen site, with trembling aspen overstorey and hazel understorey). Groundwater pore pressure was monitored at 35 m depth, effectively integrating the mass loading over an area of 10 ha. Soil volumetric moisture content was measured by time domain reflectometry to a depth of 1.2m, with five profiles at the Old Aspen flux tower and three profiles at the piezometer installation, 1.3 km from the flux tower. Snow accumulation was measured monthly during the winters. We compare the measured soil moisture balance and snow accumulation for 1998 and 1999 with estimates inferred from the time series of deep groundwater pore pressure.