The evaporation method: Extending the measurement range of soil hydraulic properties using the air‐entry pressure of the ceramic cup

Knowledge of hydraulic functions is required for various hydrological and plant-physiologicalstudies. The evaporation method is frequently used for the simultaneous determination ofhydraulic functions of unsaturated soil samples, i.e. , the water-retention curve and hydraulic-conductivity function. All methodic variants of the evaporation method suffer from the limitationthat the hydraulic functions can only be determined to a mean tension of ≈ 60 kPa. This iscaused by the limited measurement range of the tensiometers of typically 80 kPa on the dryend. We present a new, cost- and time-saving approach which overcomes this restriction. Usingthe air-entry pressure of the tensiometer’s porous ceramic cup as additional defined tensionvalue allows the quantification of hydraulic functions up to close to the wilting point. The proce-dure is described, uncertainties are discussed, and measured as well as simulated test resultsare presented for soil samples of various origins, different textures (sand, loam, silt, clay, andpeat) and variable dry bulk density. The experimental setup followed the system HYPROP whichis a commercial device with vertically aligned tensiometers that is optimized to perform evapora-tion measurements. During the experiment leaked water from the tensiometer interior wets thesurrounding soil of the tensiometer cup and can lead to a tension retardation as shown by simu-lation results. This effect is negligible when the tensiometers are embedded vertically. For coar-sely textured soils and horizontal tensiometer alignment, however, the retardation must be con-sidered for data evaluation.