Near Wilting Soil Hydraulic Properties Measured in Laboratory Evaporation Experiments.

Measurement of the functional relationship between soil water pressure head and water content (retention) and conductivity is important for forecasting and modeling in agro-ecology, hydrology and meteorology. A widespread method for the laboratory determination of both water retention and unsaturated hydraulic conductivity is the evaporation method formalized by Wind in 1968. Application of this method is limited to the measurement range of the tensiometers employed in the experiment, usually corresponding to pressure heads between 0 and ‑8 m. Polymer tensiometers are now a promising alternative and expand the range of measurement down to ‑150 m, the permanent wilting point. We used polymer tensiometers in laboratory evaporation experiments to determine hydraulic properties of eight tropical soils from Brazil, two horizons per soil, from near-saturation down to the wilting range. Some methodological problems are discussed, and how to overcome them. Among these are the time to establishment of an upward flow in the entire sample, sometimes up to a week; low resolution of the polymer tensiometers especially for the wet range; and low balance resolution which reveals specifically under low evaporation rates corresponding to the dry range. Experimental results are used to obtain fitting parameters of the Van Genuchten-Mualem and Van Genuchten-Burdine equation systems.