Field Soil Water Retention and Its Variability with Space and Time.

A multiyear test of the evapotranspiration (ET) barrier at the U.S. Department of Energy Hanford Site in southeastern Washington State has yielded in situ soil water content and pressure data from multiple locations for a 9-year period, offering the opportunity to estimate soil water retention properties at different locations and times. The upper 2 m layer of the ET barrier is a silt loam. Within this layer, valid monitoring data from 1995 to 2003 for four depths at 12 monitoring stations were used to determine the field water retention of the silt loam. The data covered a wide range of wetness, from near saturation to the permanent wilt point, and each retention curve contained 51 to 96 data points. The data were described well with the commonly used van Genuchten water retention model. It was found that the spatial variation of the saturated and residual water content and the pore size distribution parameter was relatively small, while that of the van Genuchten alpha was relatively large. Neither the primary wetting process in the winter season nor drying process in the summer season nor time had a detectable effect on the water retention of the silt loam barrier.