CALIBRATION OF THE TIME-DOMAIN REFLECTOMETER AND DETERMINATION OF THEVOLUMETRIC WATER-CONTENT OF THE SOIL-PROFILE IN AN ULTISOL OF COSTA-RICA

An experiment was conducted to determine if time-domain reflectometry (TDR) could be used to measure the water content at different depths i n the 0-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 c urrent crop-fertilization history. Measurements of volumetric water co ntent using bulk density and gravimetric water content were made to ca librate the TDR method. Comparison of water contents determined by TDR with those from gravimetric samples showed that there is a linear rel ationship (small offset but same slope) of water content with depth, i ndicating that there is little difference in volumetric water content from the 0 to 75 depth. However, the TDR method gives consistently low er water content values as compared with values obtained by gravimetri c determination. Continuous measurements of profile soil water content with TDR in wet and dry periods during the year indicated that the ma yor differences in volumetric water content correspond to the first 30 cm depth.