Point specific measurement and monitoring of soil water content with an emphasis on TDR

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Abstract
During the past decade, a number of developments have improved the possibilities for real-time, in situ monitoring of soil water content, providing potential for much improved testing and validation of soil water models Brief reviews of the principles and procedures are presented for the gravimetric and gamma ray attenuation methods. A summary of the developments of time-domain reflectometry (TDR) demonstrates its high capability for continuous monitoring of soil water content. Portable dielectric probes (PDP) and capacitance instruments also use the high dielectric constant of water as an indicator of soil water content. Commercially available TDR instruments offer a choice of options that allow this method to be fitted to a variety of specific measurement requirements. The ease of use and portability of TDR instruments and the flexibility of design for the TDR probes has made possible the rapid measurement of surface and near surface water contents on a field scale. Multiplexers and data loggers now allow the efficient monitoring of water content to record both spatial and temporal patterns not previously detectable. Fine resolution and analyses within the electric field of a TDR probe has allowed us to measure and record the diurnal patterns of water uptake from, and its release to, relatively dry sod adjacent to growing corn roots Water distribution calculations show that the water measured as moving in and out of roots daily is sufficient to hydrate and stabilize the sheaths of soil that surround the root growing in dry soil. Key words: Soil water content, TDR, root uptake, root exudation, field measurement, water balance

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