Soil Hydraulic Properties Determined in Evaporation Experiments With Polymer Tensiometers.

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. Among methods for measuring soil water retention, laboratory methods with a hanging water column under a porous plate funnel or porous plates in a pressure chamber are the most common. For the determination of unsaturated hydraulic conductivity, field and laboratory methods are available and in this case the methodological difficulties increase for the drier soil due to the slowness of the process. An alternative method for the laboratory determination of both water retention and unsaturated hydraulic conductivity is the evaporation method proposed by Wind in 1968. Its application is usually limited to the measurement range of water filled tensiometers, the relatively wet range with pressure heads between 0 and ‑8 m. A recently developed equipment, the polymer tensiometer, permits direct measurement of soil water pressure head down to values of ‑150 m, the permanent wilting point. This equipment allows using the evaporation method to determine water retention and unsaturated hydraulic conductivity over the entire range of pressure heads of interest in agronomic or ecological studies. We describe the application of the evaporation method with polymer tensiometers in some Brazilian soils. Methodological issues are discussed and obtained results are compared to Van Genuchten-Mualem and Brooks & Corey - Mualem prediction methods.