A one-parameter relationship between unsaturated hydraulic conductivity and water retention

Estimating unsaturated hydraulic conductivity often relies on using water r etention characteristics. Because the water retention curves do not provide information about the pore connectivity, an empirical correction is used i n capillary bundle models that are fitted to unsaturated hydraulic conducti vity data. The majority of authors have applied the macroscopic correction expressed as a function of water content, A microscopic correction term exp ressed as a power function of a pore radius was proposed in the literature but was not tested with a large representative soil data set. The purpose o f this work was to apply the "hydraulic conductivity-water retention" model with the microscopic connectivity correction to a large data set to see wh at accuracy can be achieved and whether it is possible to relate the connec tivity parameters to some readily available soil properties. Data for 147 s oil horizons were extracted from the unsaturated soil hydraulic database UN SODA. Water retention and hydraulic conductivity data were in the range of capillary pressures >5 kPa and from 5 to 200 kPa, respectively. The model p rovided an accurate approximation, and root mean square error (RMSE) in est imated log(10)k was 0.21. Two parameters of the model appeared to be correl ated closely so that using only one connectivity parameter was sufficient. Reducing the number of parameters from two to one and refitting the one-par ametric model to data decreased the accuracy of the estimates, The RMSE inc reased from 0.21 to 0.31. That only one empirical parameter was needed to d escribe the unsaturated hydraulic conductivity helps to reduce the number o f measurements of this hydraulic property because a single parameter can be estimated from a limited number of observations.