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Soil Water Measurement with Gamma Attenuation¹

ABSTRACT

Gamma-ray attenuation measurements of wet density were made with a 5-mc. Cs¹³⁷ source over a pathlength of about 30 cm. Data were obtained for glass-and-water systems as well as for soil materials of varying water content. The equipment was designed for use in the measurement of water content of soil in situ in 1-cm. increments, without any collimation of the beam.

Experimental results verify theoretical considerations on the use of gamma attenuation for soil water measurement. For equipment calibration, the unattenuated beam intensity and the attenuation coefficients for oven-dry soil and water need be determined to make use of the exponential inverse square law for calculating the water content, given the dry density. The data demonstrate that the difference in the attenuation coefficients of dry soil and water must be taken into account.

Accuracy of the method, for a total count of 20,000, is 0.010 g. cm.^-3 for water contents from 0.050 to 0.400 g. cm.^-3 at a dry density of 1.300 g. cm.^-3. For the same total count, a precision of 0.006 g. cm.^-3 is demonstrated.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA.

² Soil Scientist and Chief Physicist, respectively, U. S. Water Conservation Laboratory, Tempe, Ariz.

Received for publication January 16, 1964. Accepted for publication July 27, 1964.