Field Performance of the Dew-Point Hygrometer in Studies of Soil–root Water Relations

In situ measurements of soil water matric potential and root xylem water potential in a Douglas fir forest using a Wescor HR-33T dew-point microvoltmeter and PT51-10 hygrometers are described. Root xylem water potential measurement required tangential insertion of the hygrometer into the root xylem and sensor protection from plant resins using gypsum powder. Soil water matric potential measurements were compared with matric potentials computed using gravimetric and neutron meter measurements of soil water content and laboratory soil water retention data, while root water potential measurements were compared with measurements made using the pressure chamber method. The comparisons showed agreement to within ± 0.3 bar over an 8-bar range. There was good agreement between soil water matric potentials determined using the dew-point and psychrometric modes of operation of the instrument. The root xylem water potential showed a marked diurnal response to atmospheric demand.