No abstract

An experiment was conducted to determine if time-domain reflectometry (TDR) could be used to measure the water content at different depths in the O-to-75 cm soil layer. Probes of three wires (1/8 inch diameter and 30 cm exposed length) were installed in field plots differing in current crop-fertilization history. Measurements of volumetric water content using bulk density and gravimetric water content were made to calibrate the TDR method. Comparison of water contents determined by TDR with those from gravimetric samples showed that there is a linear relationship (small offset but same slope) of water content with depth, indicating that there is little difference in volumetric water content from the 0 to 75 depth. However, the TDR method gives consistently lower water content values as compared with values obtained by gravimetric determination. Continuous measurements of profile soil water content with TDR in wet and dry periods during the year indicated that the mayor differences in volumetric water content correspond to the first 30 cm depth.