Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2008
Publication Date: 8/1/2008
Citation: Mazahrih, N., Nedal, K., Evett, S.R., Ayars, J.E., Trout, T.J. 2008. Field calibration accuracy and utility for four down-hole water content sensors. Vadose Zone Journal. 7:992-1000.
Interpretive Summary: Soil water content measurements are made within and below crop rooting zones in order to schedule irrigations. They are also made to determine crop water use and water use efficiency in studies aimed at finding agronomic and irrigation practices that conserve and efficiently use our increasingly scarce water resources. ARS scientists led an international team to compare the accuracy and usefulness of several soil water sensors for this work in an important agricultural soil of the San Joaquin Valley of California. They found that the neutron moisture meter (NMM) was the only sensor accurate enough to do this work. The capacitance sensors studied were both less accurate and much more influenced by soil properties that vary with depth. This means that soil water content values obtained from the capacitance sensors were too inaccurate, and too dependent on soil properties other than water content, for them to be useful for irrigation scheduling or crop water use determination. Since the NMM faces increasing regulatory burdens, this study points up the need for development of more accurate soil water sensors that can replace the NMM.
Technical Abstract: Soil water balance studies aimed at determining crop water use, spatial variability of water use, profile water content, and changes in stored water demand accurate soil water determinations that are representative across at least field sized areas. Several capacitance and other electromagnetic (EM) sensors for use in access tubes to determine profile water content have appeared commercially. Scientists and practitioners need to know if they are suitable replacements for the neutron moisture meter (NMM) in terms of accuracy and utility. We conducted a field calibration of the NMM and three EM sensors in a Panoche clay loam soil in the San Joaquin Valley of California, USA. For each sensor, three access tubes were installed in a site dried by plant water extraction and three were installed in an adjacent plot wetted to saturation and allowed to drain. Sensors were read and volumetric water content samples taken at several depths at each access tube; and calibrations of water content versus sensor reading were calculated for each depth and for appropriate combinations of depths by regression analysis. Calibrations for the EM sensors changed rapidly with depth, often requiring separate calibrations for every 10 or 20 cm depth range, and were relatively inaccurate (RMSE values ranging from 0.015 to 0.063 m**3 m**-3). The relative inaccuracy of the EM sensor calibrations, compared with those of the NMM, and EM sensor changes in calibration with depth made the NMM the preferred choice for accurate profile water content and change in storage determination. In general, the EM sensors cannot be recommended for profile water content or change in storage determinations due to their relatively less accurate (larger RMSE values) calibrations, strong dependence of calibration slopes and exponents on depth, probable dependence of the calibrations on soil bulk electrical conductivity (BEC), and the likelihood of BEC changes in the field over the irrigation season.