468 SSSAJ: Volume 75: Number 2 • March–April 2011 Soil Sci. Soc. Am. J. 75:468–480 Posted online 16 Feb. 2011 doi:10.2136/sssaj2010.0084 Received 22 Feb. 2010. *Corresponding author (nunzio.romano@unina.it). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Using Bimodal Lognormal Functions to Describe Soil Hydraulic Properties Soil Physics C haracterization of soil hydraulic behavior requires knowledge of the com- plex and nonlinear relationships between soil-water suction head, h (i.e., the absolute value of matric pressure head [L]), volumetric water content, θ [L 3 /L 3 ], and hydraulic conductivity, K [L/T]. hese relationships are commonly referred to as the soil hydraulic properties, as deined by the soil water retention function (WRF), θ(h), and the hydraulic conductivity function (HCF), K(h) or K(θ). he soil hydraulic properties are key functions to solving the unsaturated Richards equation that it is usually applied to simulate soil water movement and associated chemical transport in the vadose zone (Hopmans and Schoups, 2005). To facilitate comparisons among diferent soils, the soil hydraulic properties are generally described by analytical relations that have unknown parameters to be determined via experimental and optimization techniques (Kosugi et al., 2002). he van Genuchten θ(h) water retention model, coupled with the semiempirical K(θ) model of Mualem (Mualem, 1976) are the most widely used analytical relations (referred to hereater as the vGM relations). Among the strengths of the vGM relations is that the estimated soil water retention parameters (i.e., the saturated water content, θ s , the residual water content, θ r , and the shape parameters α, n, and m = 1 − 1/n), enables prediction of the unsaturated hydraulic conductivity function from a known hydraulic conductivity matching point at a given saturation or suction, for example, K 0 = K s at h = 0, while ixing the tortuosity parameter, τ, N. Romano* Dep. of Agricultural Engineering and Agronomy Division of Water Resources and Biosystems Engineering Univ. of Napoli Federico II Via Universitá, 100 Naples, Italy P. Nasta Dep. of Land, Air and Water Resources Univ. of California One Shields Avenue Davis, CA 95616-8627 G. Severino Dep. of Agricultural Engineering and Agronomy Division of Water Resources and Biosystems Engineering Univ. of Napoli Federico II Via Universitá, 100 Naples, Italy J. W. Hopmans Dep. of Land, Air and Water Resources Univ. of California One Shields Avenue Davis, CA 95616-8627 Accurate parameterization of the soil hydraulic properties represents a key issue for the modeling of soil water transport processes. he more complex the soil structure, the more crucial this requirement becomes. In dealing with this problem for structured and well-aggregated soils, we have pursued the general objective of developing hydraulic relationships whose parameters characterize the soil’s pore size distributions, thereby providing a physically based framework for the hydraulic relationships of bimodal soils. In our work, we assumed that the soil water retention function is determined by linear superposition of two distinct pore domains, which can be associated with textural and structural retention behaviors, respectively. he composite soil water retention function was described by Kosugi’s lognormal function, with parameters being directly associated with the mean and variance of the soil pore size distribution for each pore domain. he two components of soil water retention were linked by a weighting factor to which a physical meaning can also be given. An important and practical advantage of the proposed bimodal water retention function is that a closed-form analytical expression is obtained for the bimodal hydraulic conductivity function using pore size distribution parameters. his is relevant because we suggest that soil hydraulic properties can be characterized by the soil particle size distribution. Sensitivity analysis and comparisons with experimental data were used to evaluate the proposed bimodal lognormal hydraulic functions and to demonstrate their increased efectiveness in predicting the hydraulic conductivity characteristic of soils. Abbreviations: bHCF, bimodal hydraulic conductivity function; bWRF, bimodal water retention function; HCF, hydraulic conductivity function; PSD, pore size distribution; RMSD, root mean square deviation; vGM, van Genuchten–Mualem; WRF, water retention function.