POINT SPECIFIC MEASUREMENT AND MONITORING OF SOIL-WATER CONTENT WITH AN EMPHASIS ON TDR

During the past decade, a number of developments have improved the pos sibilities for real-time, in situ monitoring of soil water content, pr oviding potential for much improved resting and validation of soil wat er models. Brief reviews of the principles and procedures are presente d for the gravimetric and gamma ray attenuation methods. A summary of the developments of time-domain reflectometry (TDR) demonstrates its h igh capability for continuous monitoring of soil water content. Portab le dielectric probes (PDP) and capacitance instruments also use the hi gh 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 r equirements. The ease of use and portability of TDR instruments and th e flexibility of design for the TDR probes has made possible the rapid measurement of surface and near surface water contents on a field sca le. Multiplexers and data loggers now allow the efficient monitoring o f water content to record both spatial and temporal patterns not previ ously detectable. Fine resolution and analyses within the electric fie ld of a TDR probe has allowed us to measure and record the diurnal pat terns of water uptake From, and its release to, relatively dry soil ad jacent to growing corn roots. Water distribution calculations show tha t the water measured as moving in and out of roots daily is sufficient lo hydrate and stabilize the sheaths of soil that surround the root g rowing in dry soil.