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The Use of Portable Neutron Equipment to Measure the Quantity of Water in Large Soil Columns¹

ABSTRACT

The total water content (surface cm. of water) in 4 soil columns 91 cm. in diameter and from 80 to 110 cm. in depth was estimated periodically from readings taken with portable neutron equipment. The readings were taken with the detector tube placed at various depths in a centrally located "well" formed by an aluminum tube. The relationship between meter readings and percent moisture by volume for several mineral soil types had been previously determined by a field calibration procedure. Total water content values calculated by analytical and graphical methods were comparable.

Reproducible water-depletion curves for several irrigation cycles were shown as water was removed from the soil by tomato plants. Satisfactory agreement between the calculated changes of total water content and the measured amounts of added water were obtained for two soils, a sandy loam and a silt loam.

From core samples taken at the conclusion of the study the ranges of total water content values calculated at the 95% confidence interval were as follows: 12.1 to 10.7 cm. and 20.6 to 18.4 cm. for the sandy loam, and 15.1 to 12.0 cm. and 17.4 to 9.0 cm. for the silt loam. Corresponding values calculated from rate-meter readings were 11.4, 19.2, 13.2 and 13.9 cm., respectively.

¹ Paper No. 997, University of California Citrus Exp. Sta., Riverside, Calif.

² Principal Laboratory Technician and Assistant Irrigation Engineer, respectively, in the Experiment Station (Department of Irrigation and Soils).

Received for publication August 12, 1957. Accepted for publication January 28, 1958.