Measurement of Hydraulic Properties During Constant Flux Infiltration Field Average

Abstract

Accurate measurement of in situ soil hydraulic properties is important for developing, testing, and applying water and solute transport theory. A method of measuring hydraulic conductivity (K), soil matric pressure head (ψ), and water content (θ) relationships is presented. The procedure uses a series of multipurpose time domain reflectometry (TDR) probes that measure both θ and ψ. The TDR probes are installed vertically and measure the rate of change of local soil water storage (q) along the probe during constant rate water application. The values of q are equal to local soil water flux, and assuming unit gradient, are set equal to K at the steady state θ and ψ measured at long times. The measured values of K, θ, and ψ from different water application rates are combined to obtain average K(θ) and θ(ψ) functions. To test the procedure, multipurpose TDR probes were installed vertically in a sandy soil every 0.15 m in a 7.5‐m‐long transect for a total of 50 probes per depth. This was repeated in parallel transects 0.1 m apart for four depths (0.2 m, 0.4 m, 0.6 m, 0.8 m) and a total of 200 probes. Six different water application rates were applied with subsequent drainage. Average K(θ) and θ(ψ) functions were obtained and used in an analytical solution for constant rate infiltration. Transient water storage changes were accurately predicted for all application rates.