Extending the measurement range of the simplified evaporation method using humidity sensors

The simplified evaporation method (SEM) is widely used to determine soil hydraulic properties (SHP), specifically the water retention curve (WRC) and hydraulic conductivity curve (HCC), but its application is limited to the suction range measurable by tensiometers. While advances in tensiometer design have extended this range to ∼10^3.5 cm, methods for accurately measuring the HCC beyond this range remain scarce. This study introduces the extended simplified evaporation method (XSEM), which incorporates humidity sensors into the SEM setup to measure soil water potential, enabling a quasi-continuous suction time series from full saturation to air dryness. We tested the XSEM on three soils (silt loam, sandy loam, and sand) and compared the results with the dew point method (DPM) and inverse modeling. Results show that the XSEM accurately determines the WRC from saturation to air dryness and the HCC from ∼10² cm to air dryness in a single experiment, albeit with minor biases for sandy soils in specific suction ranges. Key advantages of the XSEM include (i) straightforward calculations, (ii) reduced effort for WRC determination above 10⁴ cm compared to the DPM, (iii) high-resolution outputs, and (iv) a fully automated protocol. Extending the determination of SHP to the dry moisture range allows to assess the contribution of film and vapor flow to the HCC, which can become the dominant flow mechanism at low soil water contents. This will benefit applications such as modeling of soil water fluxes under diverse climatic conditions, assessing salinization risks, and characterizing moisture properties of porous building materials.