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United States Department of Agriculture

Agricultural Research Service

Title: Unsaturated Soil Hydraulic Properties in the Region Near Saturation

Author
item Logsdon, Sally

Submitted to: Agronomy Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: October 24, 1997
Publication Date: N/A

Technical Abstract: Several techniques can be used to obtain unsaturated soil hydraulic properties in the wet end of the curve. The objectives of this paper are to review use of tension infiltrometers and other associated techniques for obtaining soil hydraulic properties, and to examine some limitations. Tension infiltrometers determine hydraulic conductivity (K) as a function of head (h). Examining 71 sets of data each with K at six h's, the Gardner approach fit the whole curve for 16 sets, but at least two connecting lines were needed for 41 sets. The other sets either did not have a satisfactory measurement of K(O) or the two lines did not connect. Measurements of K(h) have a hysteresis effect, with ascending measurements having lower K's than descending measurements. To overcome these difficulties K could be measured as a function of water content (theta). These can be measured from multiple tension infiltrometer measurements at each h combined with sampling for final theta. Alternatively, multiple field measurements of K(h) with tension infiltrometers can be combined with equilibrium theta(h) determined with the rotated core procedure Also K(h) can be determined by multiple steady-state measurements in the laboratory with corresponding theta determined by weighing after steady state. These measurements have revealed no unique shape for K(theta) or theta(h) near saturation, except that there is a rapid increase in K with a small increase in either theta or h. The field-determined theta is lower than the laboratory-determined theta because of trapped air. Extreme variability necessitates numerous measurements, both spatially, temporally, and with depth. The measurements need to be combined with other measurements that cover a broader range of theta or h.

Last Modified: 4/23/2014
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