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Soil-Water Properties Computed from Transient Flow Data¹

ABSTRACT

A method is given for calculating water conductivity (K) of unsaturated soil by the use of data obtained during transient changes of moisture content () and suction head (h) along a horizontal soil column. A soil moisture characteristic curve was obtained from the data. Absolute values of the slopes of the latter curve, the differential water capacity (C), pertaining to the experimental values of h were then used to calculate soilwater diffusivity (D) from the equation, D = K/C. The experimental procedure does not involve boundary conditions that, in some procedures, are difficult to evaluate, nor are steady-state and equilibrium-flow conditions required. Data for most soils was obtained in from 8 to 12 days. A program was written in Fortran IV language for use with the IBM 7040 Data Processing System. The solution of the problem was reduced to a routine that saves several hours of hand calculations.

Values of K, C, and D for field cores and laboratory compacted soil columns were measured over the range of 50 h 750 cm of water. Relative error values between measured values of K and those calculated from the equation K = ch^-d were less than 50% in 83% of 288 measurements. Relative error values between measured values of D and those calculated from the equation D = exp (ß) were less than 50% in 74% of the 288 measurements. The constants, c, d, , and ß are empirical for a given soil. Water conductivity values were shown to be independent of hydraulic gradients over a wide range of gradient values.

¹ Contribution from the Dept. of Soils & Plant Nutrition, Univ. of California, Riverside. Presented before Div. S-1, Soil Sci. Soc. Amer., Aug. 23, 1966, at Stillwater, Okla.

² Former Graduate Student and Laboratory Technician and Soil Physicist, Dep. of Soils and Plant Nutrition, Univ. of California, Riverside.

Received for publication June 5, 1967. Accepted for publication July 27, 1967.