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Estimating Hydraulic Properties of a Fine-textured Soil Using a Disc Infiltrometer

USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012-0010

View larger version (48K): [in a new window]   Fig. 1. Response surface of the objective function [I(t), LAB(h)] in the –Ks parameter plane for cumulative infiltration at -16.0-cm supply pressure in Plot 1 of the no-tillage subsoil. All other parameters were set equivalent to the values obtained for the three-parameter fit. The location of the best fit solution is marked with an "X".

View larger version (35K): [in a new window]   Fig. 2. Response surface of the objective function [I(t), LAB(h)] in the n–Ks parameter plane for cumulative infiltration at -16.0-cm supply pressure in Plot 1 of the no-tillage suboil. All other parameters were set equivalent to the values obtained for the two-parameter fit (Table 2). The location of the best fit solution is marked with an "X".

View larger version (28K): [in a new window]   Fig. 3. Measured cumulative infiltration and water retention data and corresponding two parameter optimized curves for the subsoil no-tillage and native pasture plots (Table 2). Error bars represent 95% confidence limits for the mean water content from soil cores sampled at the 1- to 4- and 6- to 9-cm depth increments in the native pasture plots and 11- to 14- and 15- to 19-cm in the no-tillage plots. The dotted line in no-tillage graphs represents the fitted curves obtained by fitting only the VGM constitutive relationships to the entire infiltration curve at all supply pressure heads (see section, Optimization of K(h) Near Saturation Using Multiple Tension Infiltration Data).

View larger version (29K): [in a new window]   Fig. 4. Measured cumulative infiltraiton and water retention data and corresponding two-parameter optimized curves for the no-tillage surface plots (Table 2). Error bars represent 95% confidence limits for the mean water content from soil cores sampled at the 1- to 4- and 6- to 9-cm depth increments in both plots.

View larger version (16K): [in a new window]   Fig. 5. Measured cumulative infiltration data and corresponding fitted curves for the subsoil no-tillage and native pasture. Inverse fits to cumulative infiltration after the first supply pressure head were obtained using a loglinear piecewise description of the K(h) function (Table 3). Inverse fits for the first supply pressure head are shown in Table 2. Step changes in the supply pressure head are indicated by the symbol in the cumulative infiltration plots. The dotted line in the no-tillage graph represents estimated cumulative infiltration obtained by fitting only the VGM constitutive relationships to the entire infiltration curve at all supply pressure heads.

View larger version (18K): [in a new window]   Fig. 6. Unsaturated hydraulic conductivities at each supply pressure head calculated using Wooding's analysis (symbol) and the corresponding optimized hydraulic conductivity function (line) obtained from four sequential inverse fits (Tables 2 and 3).

View larger version (55K): [in a new window]   Fig. 7. Response surface of the objective function [I(t), SC(z, T)] in the –Ks parameter plane for cumulative infiltraiton at -16-cm supply pressure for the no-tillage surface, Plot 3. All other parameters were set equivalent to the values obtained for the three-parameter fit. At the converged minimum, marked with an "X", error corresponding to the residuals of the infiltration data comprised 75% of the value of [I(t), SC(z, T)].

View larger version (35K): [in a new window]   Fig. 8. Measured water retention data and corresponding optimized retention curves obtained from the inverse fit to infiltration data and final volumetric water contents from extracted soil cores (Table 4). Error bars represent 95% confidence limits for the mean water content from soil cores sampled at the 1- to 4- and 6- to 9-cm depth increments in all plots within a given field.

View larger version (24K): [in a new window]   Fig. 9. Mean measured (symbol) and fitted (line) water contents for TDR probes for surface no-tillage plots. Fitted water contents were obtained by inverse optimization of infiltration data and 45° TDR water contents. Optimized parameters were then used to predict water contents measured by 90° TDR probes. Error bars represent the average 95% confidence interval about the mean water contents measured by TDR.