The estimation of moisture content in the top 10 cm of soil using a neutron probe

Abstract

The neutron method of estimating soil moisture content in the field has the advantage that once access tubes are installed no further disturbance to the site is necessary and readings can be taken repeatedly throughout the soil profile at a fraction of the time necessary for the gravitational method. However, measurement of moisture content in the surface layer of soil, the top 10 cm, has proved difficult since the source and detector must be at a depth less than the sphere of influence allowing some neutrons to escape to the atmosphere. This air-soil interface effect can lead to an underestimation of moisture content which is greatest in wet soil (Van Bavel, Hood & Underwood, 1954).

A model for estimating soil moisture deficits under cereal crops in Britain:2. Performance

“…McGowan & Williams's (1980) review suggests errors of the order of + 5 mm, and the additional possibility of small errors arising in very dry conditions is inferred from the work of Hanna & Siam (1980). The FP model gives excellent fits (RMSE < 10 mm) to measured soil moisture deficit data throughout six test crop seasons on soils ranging from Newport series (a sandy loam) to Denchworth series (a clay), and with growing-season rainfall ranging from 99 to 336 mm.…”

“…Further experimental details may be obtained by consulting the sources of data cited in Table 1. Apart from separate calibrations for surface horizons, no special measures such as surface shields of appropriate soil were used at any of the sites, a procedure subsequently suggested by Hanna & Siam (1980) as being preferable to separate calibrations to correct for differing soil-air interface effeots as the soil dries. Their work suggests that in very dry conditions, soil moisture deficit in the surface horizon may be overestimated, perhaps by up to 5 mm.…”

Water in the environment

“…Fadl, 1978;Turk and Hall, 1980;Greacen, 1981;Devitt et al, 1983;Hall and Jones, 1983;Hanna et al, 1983;Hodnett, 1986), but natural ecosystems such as forest and grassland as well (e.g. Hanna and Siam, 1980;Kachanoski and Dejong, 1982;Dodd et al, 1984). This technique is best suited for deep finer-textured soils where holes and tubes are easily installed.…”

“…This radius makes it difficult to accurately measure soil moisture content in the surface, with erroneously low counts being obtained (Hanna and Siam, 1980). To overcome this problem, special calibration curves can be developed for readings near the surface.…”

“…To overcome this problem, special calibration curves can be developed for readings near the surface. Surface samples may also be monitored differently, such as through gravimetric sampling (Hanna and Siam, 1980). Also, fiberglass or aluminum shields have been set in place at the soil surface and filled with surface soil, to keep neutrons from escaping from the soil.…”