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Soil Moisture Interface Effects upon Readings of Neutron Moisture Probes¹

ABSTRACT

Measurements of soil moisture by the neutron scattering method are inaccurate when made in the presence of abrupt moisture changes. The effects of air-soil interfaces and of wet-dry soil interfaces were investigated using neutron probes of four different designs. Three of the probes were used to study the effects of wet soil layers within dry soil masses.

Readings taken at shallow depths were found to have the greatest errors. These errors were always negative. Absolute error and depth within the soil to which this effect occurs were related to soil moisture content. Some means of correcting such errors should be used. Errors in determining moisture content of soil zones containing wet-dry interfaces or wet layers were not as serious, and could be neglected in many practical applications. This was especially true when depth reading increments of 6 inches or less were used. These errors were usually negative. Neutron probes tend to underestimate soil moisture content at interfaces or in stratified profiles.

Probe depth reading increments of 1, 3, and 6 inches all gave about the same accuracy of moisture determination. Twelve-inch reading increments gave errors of as much as 6 times greater than those of the smaller increments.

The influence of probe geometry upon neutron meter readings is also discussed. When compared on a basis of equivalent source strength, in uniform soils, the probe with end placement of the source was found to be considerably more efficient than the probes with center placement of the source.

¹ Joint contribution from Soil and Water Conservation Research Division, ARS, USDA, and Department of Water Resources, State of California.

² Soil Scientist, Southwest Branch, Soil and Water Conservation Research Division, ARS, USDA, Lompoc, Calif., Land and Water Analyst, Department of Water Resources, State of California, Bakersfield, Calif., and Agricultural Engineer, Southwest Branch, Soil and Water Conservation Research Division, ARS, USDA, Lompoc, Calif.

Received for publication October 29, 1962. Accepted for publication January 3, 1963.