Predicted drainage for a sandy loam soil: Sensitivity to hydraulic property description

Prediction of unsaturated flow phenomena at field-scale requires a set of hydraulic functions that capture local-scale variability which is present in all natural soils. Several sets of hydraulic functions measured on different core sizes collected across the field were used to predict drainage from a saturated soil profile. Simulations were carried out with a one-dimensional numerical model, based on the Richards equation. Four methods were considered: (1) Monte-Carlo simulation using 500 unimodal retention and conductivity functions representing θ(ψ) and K(s) data measured on 0.05 m diameter 0.051 m long core samples; (2) multiple simulations with a set of 60 multimodal retention and conductivity functions which better represented the measured θ(ψ) data of method 1; (3) one single simulation with a set of hydraulic functions obtained from a gravity-drainage experiment on 15 1 m long 0.3 m diameter soil columns collected from the same field; (4) one single simulation with a set of hydraulic functions obtained by scaling the retention and conductivity data from method 3. A perfect match of the final mean outflow was obtained when scaled retention data was used in combination with scaled Ks values (method 4). All other cases underestimated the total outflow to a varying degree: 30% for method 1, 29% for method 2, and 21% for method 3. The results further revealed that none of the four methods was able to completely describe the mean observed drainage from the start until the equilibrium condition. This was further demonstrated by the disparities between the mean observed soil water content profile and the simulated values using hydraulic functions from method 4. Especially after the first day, differences were large, presumably because macropore flow could not be described using the Richards flow equation. Despite the introduction of multimodal retention and conductivity functions which better described the retention behaviour of small soil cores (method 2) in comparison with unimodal retention functions (method 1), mean predicted outflow for both methods was nearly identical.