Simple field method for determining unsaturated hydraulic conductivity

A new method is proposed for determining in situ unsaturated hydraulic conductivities from unsaturated infiltration measurements made at several tensions on the same infiltration surface. Wooding's equation for steady-state unconfined infiltration rates is used in calculating hydraulic conductivities. Hydraulic conductivities calculated with the new method are consistent with unit gradient laboratory measurements of saturated and unsaturated hydraulic conductivity. This simple field method is potentially valuable because it is faster than unit gradient laboratory methods, and it is less disruptive of pore continuity than other field infiltration techniques. R OF SOIL WATER INFILTRATION and SUbSUrface water movement are important to researchers developing soil management practices to minimize potential groundwater contamination from land applied chemicals. A simple and rapid field technique of determining field unsaturated hydraulic conductivity would be useful in achieving this objective. Field and laboratory techniques for measurement of unsaturated hydraulic properties of soil were described by Green et al. (1986) and by Klute and Dirksen (1986), respectively. Solution of unsaturated flow problems generally requires experimental determination of the relationship between hydraulic conductivity and water potential or water content. Field methods used to obtain these relationships include the instantaneous profile method, steady-flux methods (with sprinkler irrigation or artificial crusts), sorptivity measurements, and use of tension infiltrometers (Clothier and White, 1981; Ankeny et al., 1988, 1989; Elrick et al., 1988a; White and Perroux, 1987, 1989; Smettem and Clothier, 1989). Because instantaneous profile and steady-flux techniques require laborious installation of tensiometers or neutron probe access tubes, sample numbers and the extensiveness of a site characterization can be limited. Sorptivity is an unsaturated soil parameter sometimes measured in the field (Green et al., 1986). Although sorptivity measurements are fast and simple, these measurements usually require that initial water content be known. White and Perroux (1989) have proposed a laboratory method for calculating unsaturated hydraulic conductivity from sorptivity measurements. Their method, however, requires air drying the sample between measurements at different tensions, which increases experimental time and may cause wetting/drying effects on soil structure. The Guelph infiltrometer (Soilmoisture Equipment Corp., Santa Barbara, CA) compares infiltration rates for difM.D. Ankeny, Daniel B. Stephens & Assoc. , 4415 Hawkins NE, Albuquerque, NM 87109; M. Ahmed, Bangladesh Univ. of Engineering and Technology, Dhaka-1000, Bangladesh; T.C. Kaspar, National Soil Tilth Lab. , Ames, IA 50011; and R. Horton, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Joint contribution from USDA-ARS and Iowa State Univ. Journal Paper no. J-13716 of the Iowa Agric. and Home Economics Exp. Stn. Projects no. 2659 and 2715. Received 6 Nov. 1989. *Corresponding author. Published in Soil Sci. Soc. Am. J. 55:467-470 (1991). ferent radii surface disks and does not require driving a ring. Different soil surface areas, however, are being compared, which may introduce spatial variability associated with the different soil surfaces. A field method to measure in situ hydraulic conductivity at low water tensions is needed for studies of macroporosity and water flow in agricultural soils. The desired criteria for such a method are: 1. Only steady-state infiltration rate measurements are needed. Knowledge of the initial water potential or content should not be required. 2. Soil pore structure should not be disturbed by driving a ring into soil to obtain one-dimensional flow. This way, larger pores are not truncated or collapsed and infiltration through larger pores is less likely to be underestimated. 3. Measurements should be taken on the same soil surface. Measurements taken by using different radii (e.g., Elrick et al. 1988a) are more dependent on the assumption of soil homogeneity. 4. Calculation of hydraulic conductivities should be straightforward. We present a simple scheme for determination of in situ hydraulic conductivity that meets these criteria.