A method has been developed for continuous measurement of the soil moisture balance on a scale of hectares. The method is based on accurate measurement of groundwater pressure in thick clay formations and depends on the principle that changes of mechanical 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 hydrologic disturbance of the site. In principle, soil moisture recharge by precipitation and actual evapotranspiration can be measured to within a millimeter and with little cumulative error. Surface runoff can be measured in real time if independent measurements of precipitation are available.
We present data from a boreal deciduous forest (the BOReal Ecosystem Atmosphere Study’s Old Aspen site). The groundwater pore pressure is measured at 35 m depth, effectively integrating the mass loading over a radius of about 175 m. Changes in soil moisture by evapotranspiration and precipitation, as inferred from the pore pressure time series, show good agreement with direct measurements.
The method meets a need in hydrology for water balance measurements at scales intermediate between point measurements and river basins. These scales are well suited to studies of forest hydrology. They are also commensurate with the "pixel" scales of remote sensing and approach the scale of the elements in distributed hydrological models.