Temperature Gradient Effects on in situ Hygrometer Measurements of Soil Water Potential. II. Water Movement1

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Abstract
Temperature gradients can cause serious errors in soil water potential determinations by in situ thermocouple hygrometers. One source of error in thermally unstable soil could be condensation or soil drying at the hygrometer caused by temperature induced water movement. In a laboratory study we imbedded various types of hygrometers in sieved loam soil (skeletals, mixed, frigid Lithic Calciorthid Aredisol) and measured temperature gardients and water movement near the hygrometers while the surrounding soil was subjected to temperature gradients. Greatest condensation or drying occurred at the sample surface of end window units while cylindrical sample surface units permitted through diffusion with minimum condensation. Greater heat conduction within hygrometers was correlated with steeper thermal gradients in adjacent soil and with greater water movement. Construction of metals, with large lead wires, and large hygrometer size contributed to greater thermal gradients and water movement in adjacent soil. Hygrometers are relatively insensitive to temperature gradient errors if they are constructed with cylindrical sample surface geometry, with concentric placement of the measuring junction, of materials having low thermal conductivity, with small guage lead wires, and small size.

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