Validation of Capacitance Based Soil-water Probes for Coastal Plain Soils

Abstract

Soil-water dramatically influences infiltration and runoff and consequently water quality. In-situ measurements of soil-water are critical for understanding hydrologic and water quality processes. Precise in-situ measurements of soil-water are sparse. However, many advances have recently been made in soil-water measurement techniques. In particular, instruments for estimating soil-water from measurements of soil electrical properties have become very popular. While the instruments have been shown to be good indicators of relative changes in soil-water, questions remain regarding their ability to yield quantitative estimates. Most of these techniques rely upon a limited set of calibration equations obtained through laboratory analysis of homogeneous soil materials (i.e. sand, silt, and clay). This paper describes a validation study of two capacitance based soil-water probes for a range of Coastal Plain soils. The probes measure the capacitive and conductive properties of the soil. These measurements are then related to soil-water through calibration equations. Laboratory and field tests indicate that the probes track changes in soilwater and yield adequate relative estimates of soil-water. In most cases, the factory calibration was within +/- 50% of the observed soil-water. Results indicate improved relationships could be developed through laboratory tests. These probes should prove to be a useful tool for estimating soil-water in many Coastal Plain soils. However, additional work is required to evaluate better equations for absolute prediction of soil-water.