Spatially variable soil hydraulic properties for simulation of field-scale solute transport in the unsaturated zone

Abstract

Spatial variability of the soil hydraulic properties affects unsaturated field-scale solute transport. We obtained the hydraulic properties of a sandy loam and a loam soil through pressure outflow experiments and assessed their variability by a scaling procedure. We used the soil hydraulic properties to quantify solute transport in a fallow field over a 214-day period as a function of water table depth with the one-dimensional, vertical, transient SWAP93 model. For the sandy loam soil, simulated cumulative soil evaporation and cumulative bottom flux showed large spatial variability (standard deviation of 100 to 150 mm). For the loam soil, this variability was limited (standard deviation of 4 to 16 mm). Scaled mean soil hydraulic properties could describe the water balance and depth-wise soil salinity of the loam soil accurately. Scaled mean soil hydraulic properties did not perform satisfactory for the sandy loam soil. Use of mean soil hydraulic properties with a field-scale dispersivity value (often done in model applications) yielded deviating results for both soils, and is not recommended. Solute transport for both soils proved insensitive to spatial variability of the porosity.