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Analysis and Testing of a Concentric-Disk Tension Infiltrometer

Dep. of Soil and Crop Sciences, Colorado State Univ., Ft. Collins, CO 80523

R. E. Smith

USDA-ARS, Water Management Research, Ft. Collins, CO

^* Corresponding author (gbutters{at}agsci.colostate.edu).

ABSTRACT

Tension infiltrometers are widely used to measure soil hydraulic properties at soil potentials near saturation in the field. Current methods of hydraulic property analysis using tension infiltrometers rely on secondary measurements of soil water content and/or use of a specific hydraulic conductivity (K(h)) form. In this paper, design criteria and method of application for a concentric disk (CD) infiltrometer are proposed, which can be used to measure saturated and nearly saturated hydraulic conductivities (K(h_o)) and matric flux potential ((h_o)) in the field without secondary measurements or explicit reliance on a specific conductivity form. Using measured pairs of K(h_o) and (h_o), a parameter estimation method is suggested to allow extrapolation of K(h) to lower than measured soil water potentials. The design of the infiltrometer and the accuracy of parameter estimation were tested against results of numerical simulations. We found that an outer-disk radius of 150 mm was sufficient to establish a 50-mm-radius inner-disk of one-dimensional (1-D) infiltration under a wide range of simulated conditions. Identification of input K(h) functions using the suggested method of analysis was excellent, although errors were introduced by nonsteady-state infiltration data. A field test of a CD infiltrometer yielded a promising comparison with K(h) data generated by the instantaneous profile method, an alternative approach but one that is more labor intensive. The advantage of the CD infiltrometer is that, in addition to providing a rapid, direct estimate of K(h_o), the results are not conditional upon an assumed form of K(h) and they may be used to estimate the parameters of a K(h) form of choice.

Received for publication May 9, 1997.