Calibration of a neutron moisture meter for catchment hydrology

A comprehensive method is described for obtaining calibration curves for a neutron meter. As part of a study of the hydrology of a catchment with swelling clay soils, 30 access tubes were each monitored at six depths, giving a total of about 200 measuring positions, with each position possibly needing its own calibration curve. Three sources of data were used to specify the calibration for each measuring position: (1) Calibration points were derived for each position from data obtained at the installation of each tube. (2) Detailed calibrations were derived for several access tubes in undisturbed soils, using a 'cylinder calibration method' developed for this project; this method enables two or more calibration points to be determined for each measuring position. (3) Neutron soil properties were obtained by direct measurement with simple equipment developed for this project, and used in a modified theoretical model to predict the calibration curves for undisturbed tubes. The technique of individual calibration removes possible errors in the derived soil moisture data over the catchment area.