Determination of soil hydraulic functions across the full moisture range by extending the simplified evaporation method using humidity sensors

The Simplified Evaporation Method (SEM) is widely used to simultaneously determine the water retention curve (WRC) and hydraulic conductivity curve (HCC) of soils. However, its application is traditionally restricted to the suction range measurable by tensiometers. To overcome this limitation, we incorporated humidity sensors into the setup of the SEM, enabling measurements of soil water potential in the hygroscopic range. This advancement allows for the measurement of a quasi-continuous time series of soil water suction from full saturation to air dryness, which allows to determine the WRC across this range and the HCC from field capacity to air dryness. We term this approach the eXtended Simplified Evaporation Method (XSEM).

We tested the XSEM on three soil types—silt loam, sandy loam, and sand—and compared its results with those from the dew point method (DPM) and inverse modeling, observing strong agreement among the methods. Key advantages of the XSEM include (i) simultaneous determination of both hydraulic functions using a single experimental setup and straightforward calculations, (ii) reduced effort for WRC determination at suctions above 10⁴ cm compared to the DPM, (iii) high-resolution outputs, and (iv) a fully automated protocol. In particular, XSEM provides a realistic assessment of film and vapor flow contributions to the HCC, which dominate water flux in porous media at low water content. These advancements improve the modeling of soil water dynamics and actual evaporation rates in dry soil.