C. H. M. van Bavel

Test and analysis of a model of water use by sorghum

We carried out a field experiment with a well-developed sorghum crop to validate a water use and uptake model. The calculated evapotranspiration over a 50-d period was not significantly different from the experimental values. Soil water content profiles calculated with the model showed some systematic deviation from the measured values, possibly because the physical nature of the profile was assumed to be homogeneous. The spread of the simulated results reflects the field variation of hydraulic soil properties.-

FIELD CALIBRATION OF NEUTRON METERS USING A TWO-PROBE, GAMMA-DENSITY GAUGE1

An improved method to calibrate neutron meters in the field is proposed and evaluated. The method consists of using a two-probe, gamma-gauge densitometer to measure dry bulk density and the volumetric water content. The latter is related to the corresponding neutron count ratio. We applied the procedure repeatedly over time to obtain the calibration for two neutron probes. Field tests of the calibration equations indicated that they were accurate within 1% of the total profile water content.

Calibrating two-probe, gamma-gauge densitometers

Using current, improved commercial equipment, the authors reexamined the calibration procedure of the dual-probe gamma soil densitometer, using exact density standards. When each instrument is individually calibrated, the procedure, as originally designed, gives consistent and accurate results.