The variability of the pore‐interaction factor, ρ, for a microscopic model proposed for predicting unsaturated hydraulic conductivity, K(θ), from soil water‐retention, Ψ(θ), data was examined in relation to soil particle‐size data and indices, bulk density, organic C, parameters for the van Genuchten Ψ(θ) function, an index (W) of total energy of drainage and soil‐series, toposequential, and geographic groupings. The ρ exhibited no trend relationship to any of the soil properties tested. However, a change in the distribution of ρ was observed in relation to the geometric‐mean particle diameter (Gd) and other soil textural variables. For Gd < 0.08 mm, variability was much larger than for soils with Gd ≥ 0.08 mm. The latter consisted entirely of soils in the sand and loamy sand textural groups. A similar distributional relationship also occurred for ρ vs. W. Examination of ρ on three sites of the Hecla soil series and on related toposequential soils indicated that classification of ρ on the basis of soil series or of soil‐association groupings is a feasible strategy for ρ parameter estimation on some soils. The exponential factor η for a macroscopic model was also investigated. The η was found to be related to W as an exponential function over the full data range. However, η vs. W was nearly identical to a linear function for W data extending from 0 to 400 cm‐γ (where γ is the density of water). The η was also strongly related to Gd as a power function, and to other textural variables as exponential functions. The increasing slope of the power function for Gd < 0.08 mm indicated a large potential error of prediction for K(θ) on fine‐textured soils.

A physico‐empirical model proposed by L.M. Arya and J.F. Paris in 1981 for predicting the soil moisture characteristic curve using particle size and bulk density data was compared with a commonly used laboratory method using undisturbed core samples and a pressure plate extractor, for precision in estimating in situ water retention measured with paired neutron probe and tensiometer installations. An empirical factor for the Arya and Paris (A&P) model was found to average close to the value (α = 1.38) proposed by its authors. Values for this experiment were different between texture classes. Loam and silt loam values were constant, while values for soils of sand, loamy sand, and sandy loam texture were nonconstant. The empirical factor was calibrated for (i) loam and coarse loam; (ii) silt loam; and (iii) sand, loamy sand, and sandy loam texture groups. The sandy soil values required linear calibration as a function of soil water potential. Results for the calibrated A&P model compared well, overall, with laboratory determinations on soils ranging in texture from sand to loam and silt loam. Laboratory estimates were slightly better for A and B horizon materials, while A&P estimates were better for C horizons. Error for estimation of field volumetric water content using A&P and lab methods was approximately twice the error for estimation of θ using a second field neutron‐tensiometer installation within one meter of the first field apparatus.

Infi 1 tration-rate response 3 Factors affecting infiltration-rate response Comparative index Methods and materials 11 Results and discussion Model development Model 1 (unsaturated flow through an impeding surface layer) 19 Mathematical approach 19 Surface filter-cake layer impedance development Simulation strategy 21 Model-parameter estimation Simulated infiltration-rate response (model 1) Water-table depth Model 2 (ground-water mound intersection of basin floor during early stages of basin operation) Simulated infiltration-rate response (model 2) Effect of basin geometry Effect of initial water-table depth Limiting factors Constant-drawdown approximation Ponded-depth effects on total recharge Conclusions References Glossary of terms ILLUSTRATIONS