Continuous water content measurements with time-domain reflectometry for sandy soils

Time-domain reflectometry (TDR) is a technique that indirectly measures soil volumetric water content based on the effect of changing dielectric properties of the medium on applied electromagnetic waves. A calibration equation that has been developed and widely used, based on the relationship between fast-rise electromagnetic properties and water content of a porous material, remains to be tested for soils from additional geographical areas including deep sands from the southeastern Coastal Plain. An experiment was conducted for the following purposes: to demonstrate that TDR allows fully automated, continuous, soil water content measurements; to test calibration curves for soil volumetric water content (θ_r) determinations by TDR using an automated system; and to obtain soil moisture characteristic curves for two sandy soils (Astatula and Ellzey fine sands) from Florida. Estimation of soil water content by TDR was highly correlated with gravimetric analysis of soil cores, with correlation coefficients (r² values) of 0.98 for both the Astatula and Ellzey soils. Soil moisture characteristic curves were obtained for the two soils that should be useful for irrigation-scheduling purposes. Results verified that TDR was an effective method for monitoring soil water content in sandy soils, and that a wide range of water content, to near saturation, could be accurately measured. Time domain reflectometry provides reliable, nondestructive, soil water content measurements. It is easy to install, safe to use, relatively non time-consuming, and allows fully automated and continuous measurements.