A critique of the time domain reflectometry technique for determining field soil-water content

Time domain reflectometry (TDR) has been used to monitor soil-water content for the past decade. During this time there have been marked advances in both the theoretical understanding and practical methodology and technology of the TDR technique. In this chapter we examine critically some of the key issues. We look at empirical and theoretical relationships between volumetric water content, Θ, and apparent dielectric constant, K_a, factors influencing the accuracy and resolution of measurements, the volume of soil sampled by TDR probes and their spatial weighting functions, the effect of probe geometry and orientation, and the impact of soil electrical conductivity on TDR measurements. In addition, we give comparative tests of the use of TDR in the field under simulated rainfall, and under prolonged wetting and drying by evapotranspiration on a daily and hourly basis. Finally we discuss applications to other porous materials. We conclude that TDR is best suited for use in lighter textured soils. Measurements are extremely sensitive to the soil closest to the probe wires, the breadth of soil sampled being proportional to wire diameter. This means that probe insertion must be carried out carefully, and imposes limitations on the minimum diameter of probe wires. Soil electrical conductivity, through saline soilwater or surface conduction in clay soils, attenuates the TDR signal, limiting the technique to low conductivity soils. The use of a “universal” empirical Θ(K_a) relation gives water balances in the field to within ± 100/0 of that found using weighing lysimeters on a daily basis. Finally we conclude that individual calibration curves are required when TDR is used to monitor water content in other porous materials such as coal.