Calibration of the time‐domain reflectometer and determination of the volumetric water content of the soil profile in an ultisol of Costa Rica

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.

Comparison of soil water-contents as measured with a neutron probe and time domain reflectometry in a Mediterranean forest (?Sierra de Gata?, Central Western Spain)

“…Indeed, on applying two-way ANOVA, a significant difference between both data sets was found (degrees of freedom = 1; 201; P = 8.2´10 -6 ; table III); TDR afforded slightly underestimated values of q as compared to NP (0.218 versus 0.222 cm 3 cm -3 ). Salas et al [25] also found that TDR underestimated q.…”

“…Also, the increase in soil bulk-density with depth, which leads to greater soil homogeneity [11,15], could contribute to the improvement of the results with depth. Salas et al [25] also observed greater differences between the gravimetric and TDR techniques for the first 30 cm of soil depth (greater heterogeneity).…”

“…Jacobsen and Schjonning [9], using different calibrations methods for TDR, also obtained a higher bias with higher q values. Finally, Salas et al [25] obtained TDR data that underestimated q for any range of soil water contents.…”

“…Most comparative studies addressing the techniques used for measuring soil water contents refer to comparisons of an indirect method with another direct one: traditionally gravimetry [18,19,25,28]. The literature contains very few references to works that compare different indirect methods in the field [8,13,33].…”