Application of thermocouple psychrometers to field measurements of soil moisture potential.

Abstract

The recent development of the peltier-type thermocouple psychrometer has made possible the measurement of soil moisture potentials to values as low as -80 bars. The applicability of this type of psychrometer to "in situ" measurements of moisture potential at a Sonoran desert field site is investigated. An evaluation is made of the effect of variations in soil temperature, moisture content, and solute concentrations on the psychrometric measurements. Moisture potential measurements with the psychrometer are shown to be limited to a moisture content range composing approximately 50 per cent of the total variation in soil moisture observed during the study. A significant quantity of moisture is transferred across the soilatmosphere interface at moisture contents both above and below the measurement range of the psychrometer. Psychrometric measurements cannot be used to determine the total moisture flux into or out of the soil horizon. The temperature component of the total soil moisture potential is not measurable with the thermocouple psychrometer. Under conditions occurring frequently within the study period, this component is shown to be of equivalent or greater magnitude than the components measured with psychrometric techniques. Laboratory measurements of the sorption-desorption isotherms for the field soils were made using thermocouple psychrometers. The isotherms vary significantly among samples, as a function of soil composition. This variation prevents the determination of absolute values of soil moisture content from measurements of soil moisture potential. However, for the soils at the field site, the slope of the moisture isotherms at a given potential does not vary significantly among samples. Psychrometric measurements can be used to determine moisture content changes at the study site, within the measurement range of the psychrometers. The variation in moisture isotherms, the significance of temperature induced moisture flux, and the limited moisture range of psychrometric measurements prevents the construction of a quantitative model of soil moisture movement from potential measurements made with thermocouple psychrometers. The effect of Celtis pallida (desert hackberry), a native plant species, on the soil moisture regime is described. Moisture uptake by the plant, and precipitation input to the soil near the plant are described in terms of the potential variations they produce. The particular hackberry plant studied is shown to be removing moisture from the soil at potential values as low as -30 atmospheres. Moisture potentials in the root zone were within the measurement range of the psychrometers throughout most of the year. The measurement of soil moisture potentials with the thermocouple psychrometer is shown to be an effective means of studying moisture content variations in the root zone of desert plant species.