Evaporation Method for Measuring Unsaturated Hydraulic Properties of Soils: Extending the Measurement Range

The evaporation method is frequently used for simultaneous determination of soil water retention and hydraulic conductivity relationships. Tension is measured at two depths within a short soil column as water evaporates from its surface. Water content and flux are determined by weighing the column. Tensions, water contents, and fluxes are used to derive the water retention curve and the unsaturated hydraulic conductivity function. The measurement range of the conventional procedure is limited on the wet end by the inability of pressure transducers to accurately register very small tension differences. Hence, the resulting calculated hydraulic gradient in the vertical direction is associated with large uncertainties. On the dry end, water cavitation in the tensiometer, which typically occurs around 70 to 90 kPa, is the limitation. We present here a new design based on improved tensiometers that resist cavitation to much higher tensions, some reaching values as high as 435 kPa. On the wet end, data from a simple steady-state method were used to supplement the evaporation method. On the dry end, applying the new tensiometers enabled the quantification of hydraulic functions up to 293 kPa average tension. Experimental results and soil water simulation affirmed the validity of the linearization assumption, even on the dry end when nonlinear tension depth profiles emerge. The application of evaporation functions as a supplement for frequent weighing reduces costs for the equipment and increases the effectiveness of the method. Their validity for deriving fluxes was confirmed for the extended range, too. Results are presented for soil samples of different textures (sand, loam, silt, clay, and peat), various origins, and various dry bulk densities.