TIME-DOMAIN REFLECTOMETRY - A SEMINAL TECHNIQUE FOR MEASURING MASS AND ENERGY IN SOIL

Soil water exerts a strong influence on the transfer and storage of so lutes, heat, air, and even water itself, within the soil profile. Soil water also dominates the mass and energy balance of the soil-atmosphe re interface. Over the last decade or so, the development and continui ng refinement of the time-domain reflectometry (TDR) technique for in situ, nondestructive measurement of water, ionic solutes and air has r evolutionized the study and management of the transfer and storage of mass and energy within the soil profile. TDR-measured water content ha s been applied successfully to water balance studies ranging from the km scales of small watersheds to the mm scale of the root-soil interfa ce. TDR-measured ionic solute status, which applies to the same sample volume as the water content measurement, has been used successfully o n soil column, held plot and whole field scales for in situ determinat ion of solute transport parameters, such as pore water velocity and di spersivity. TDR-measurement of air-filled porosity in space and time h as given new insights into the mechanisms controlling aeration and gas eous exchange in the crop root zone. The combined water content - solu te mass measurement capability of TDR has made this technique a very p owerful tool for characterizing solute leaching characteristics, as we ll as for evaluating solute transport theories and solute transport mo dels. The portability of TDR instrumentation coupled with the simplici ty and flexibility of TDR soil probes has allowed the separation of wa ter and solute content measurement error from soil variability, result ing in the capability for determining the mechanisms behind the spatia l and temporal variability in held-based soil water content distributi ons and solute leaching patterns. The usefulness and power of the TDR technique for characterizing mass and energy in soil is increasing rap idly through continuing improvements in operating range, probe design, multiplexing and automated data collection. (C) 1998 Elsevier Science B,V. All rights reserved.