PRECISION OF NEUTRON SCATTERING AND CAPACITANCE TYPE SOIL WATER CONTENT GAUGES FROM FIELD CALIBRATION

Soil water content gauges based on neutron scattering (NS) have been a valuable tool for soil water investigations for some 40 yr. However, licensing, training, and safet y regulations pertaining to the radioactive source in th ese devices makes their use expensive and prevents use in some situations such as unattended monitoring. A capacitance probe (CP ) gauge has characteristics that would make it seem to be an ideal replacement for NS gauges. We determined the relative precision of two brands of NS gauges (three gauges of each) and a brand of CP gauge (four gauges) in a field calibration exercise. Both brands of NS gauges were calibrated vs. volumetric soil water content with coefficients o f determination (r ) ranging 2 from 0.97 to 0.99 and root mean squared errors (RMSE) <0.012 m m water content. 3 -3 Calibrations for the CP gauges resulted in r ranging from 0.68 to 0.71 and RMSE of 0.036 m 2 3 m water content. Average 95% confidence intervals on predictions were three to five times -3