Measurement of Soil Water Potential

Knowing soil water potential values in soil is of primary importance in studies of transport processes within porous media as well as in evaluating the energy status of water in the crops.

The most widely known direct method for measuring the pressure potential in soil is that associated with use of a tensiometer [2]. The matric potential then can be calculated from measurements of the gas-phase pressure, if different from the atmospheric pressure, and of the overburden potentials in the case of swelling soils. Schematically, a tensiometer consists of a porous cup (or disk), mostly made of a ceramic material, connected to a pressure sensor (e.g., mercury manometer, vacuum gage, pressure transducer) by means of a water-filled tube. The porous cup in inserted in the soil at the selected measurement depth. In spite of specific limitations, chiefly related to a good contact between the porous cup and the surrounding soil and to the measurement range because of vaporization of liquid water, and related failure of liquid continuity, when pressure potential reaches about -85 kPa, this device is widely used in both laboratory and field experiments.

To monitor matric potential at numerous locations within a field, as necessary for automatic irrigation scheduling or environmental monitoring studies, various methods have been proposed employing sensors that measure a certain variable (e.g., electric resistance, heat dissipation) strongly affected by soil water content. An empirical calibration curve then is required for evaluating matric potential in soil. However, uncertainties when using this methodology can be relatively large and chiefly associated with the hysteretic behavior of the porous sensor and with the validity of the selected empirical calibration curve for all or most of the sensors installed in the area of interest. All of these methods are indirect methods for matric potential measurements.

Measurements of water potential (sum of matric potential and osmotic potential) are particularly useful for evaluating crop water availability. These measurements usually are carried out employing thermocouple psychrometers [9], which actually measure the relative humidity of the vapor phase in equilibrium with the liquid phase of the soil.

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