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

Time-domain reflectometry (TDR) is a technique that indirectly measures soi l volumetric water content based on the effect of changing dielectric prope rties of the medium on applied electromagnetic waves. A calibration equatio n that has been developed and widely used, based on the relationship betwee n fast-rise electromagnetic properties and water content of a porous materi al, remains to be tested for soils from additional geographical areas inclu ding deep sands from the southeastern Coastal Plain. An experiment was cond ucted for the following purposes: to demonstrate that TDR allows fully auto mated, continuous, soil water content measurements; to test calibration cur ves for soil volumetric water content (7 theta(v)) determinations by TDR us ing an automated system; and to obtain soil moisture characteristic curves for two sandy soils (Astatula and Ellzey fine sands) from Florida. Estimati on of soil water content by TDR was highly correlated with gravimetric anal ysis of soil cores, with correlation coefficients (r(2) values) of 0.98 for both the Astatula and Ellzey soils. Soil moisture characteristic curves we re obtained for the two soils that should be useful for irrigation-scheduli ng purposes. Results verified that TDR was an effective method for monitori ng soil water content in sandy soils, and that a wide range of water conten t, to near saturation, could be accurately measured. Time domain reflectome try 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.