USE OF TIME-DOMAIN REFLECTOMETRY (TDR) TO MEASURE THE WATER-CONTENT OF SANDY SOILS

We investigated the potential sources of error when using time domain reflectometry (TDR) to measure the water content of sandy soils and ev aluated the technique as a means of measuring evaporation from columns of soil and changes in soil water storage beneath crops. Inaccurate d epth location of the transmission lines or the development of a hole a t the tip of the transmission lines introduced an error about 10 times larger than the errors associated with hardware and software. Calibra tion in two sandy soils gave a curve of similar shape to that found by others except for values of dielectric constant < 6 when measured val ues of water content were less than those expected. Daily evaporation from soil columns measured by weighing and with TDR showed large diffe rences between the two techniques (up to 32%) but compensating errors over time allowed cumulative evaporation to be estimated with TDR to w ithin 6.6% of that determined by weighing over a 162 h period. Under f ield conditions, the agreement between TDR and neutron probe measures of changes in soil water storage in the upper 0.3 m was good and gener ally within 10% over both 14 day and longer periods.