Scale Dependence of Soil Hydraulic Properties Obtained from Evaporation Experiments: Effect of Sample Height

Climate-induced droughts and increasingly erratic precipitation patterns are stressing water resources and underscore the need for a better understanding of soil water flow and storage. Soil hydraulic properties, in particular the water retention curve and the hydraulic conductivity curve, are fundamental inputs for predicting soil water dynamics and for simulating variably-saturated flow with the Richards equation. The simplified evaporation method is a common laboratory technique for estimating SHP. It relies on linearization assumptions that introduce only negligible errors when sample heights are small. While a handful of theoretical studies have addressed how sample height affects SHP estimates, a systematic experimental assessment of this scale-dependence is still lacking.

We performed evaporation experiments on packed soil columns (5, 10 and 15 cm high) using both a sandy and a silty soil. Throughout each run, we recorded column mass to track water content and evaporation rate, and we measured matric potential with mini-tensiometers.  Applying the simplified evaporation method, we derived point data for the water retention curve and hydraulic conductivity curve. A flexible model which accounts for capillary and non-capillary storage and flow was fitted to the data using the program SHYPFIT. Inverse simulations with Hydrus-1D were then applied to assess the influence of sample height without relying on the assumptions of the simplified evaporation method. This allowed to discriminate between an actual scale-dependence of soil hydraulic properties and differences which are caused by the assumptions of the simplified evaporation method.

Our findings reveal that column height has a minimal impact on the water retention curve, with a tendency of a slight broadening of the pore size distribution and a modest increase in residual water content. The effect on hydraulic conductivity was even less pronounced. The results of inverse simulations substantially attenuate these height-related discrepancies in soil hydraulic properties, leaving only marginal differences.