<p>Hydrological models use soil hydraulic parameters to describe the storage and transmission of water in soils. Hydraulic parameters define the water retention, <em>&#952;(&#968;)</em>, and the hydraulic conductivity, <em>K(&#952;)</em>, functions. These functions are usually obtained by fitting experimental data to the corresponding &#952;(&#968;) and K(&#952;) functions. The drawback of deriving the hydraulic parameters by inverse modelling is that they suffer from equifinality or non-uniqueness, and the optimal hydraulic parameters are non-physical (Pollacco <em>et al.</em>, 2008). To reduce the non-uniqueness, it is necessary to invert the hydraulic parameters simultaneously from observations of both<em> &#952;(&#968;)</em> and <em>K(&#952;</em>), and ensure the measurements cover the full range of <em>&#952;</em> from fully saturated to oven dry, which requires expensive, labour-intensive measurements. &#160;</p><p>We present a novel procedure to derive a unique, physical set of bimodal or dual permeabilityKosugi hydraulic functions,<em> &#952;(&#968;)</em> and <em>K(&#952;)</em>, from inverse modelling. The Kosugi model was chosen given its parameters have direct physical meaning to the soil pore-size distribution. The challenge of using bimodal functions is they require double the number of parameters (Pollacco <em>et al.</em>, 2017), exacerbating the problem of non-uniqueness. To address this shortcoming, we<strong> (1) </strong>derive residual soil water content from the matrix Kosugi standard deviation, <strong>(2) </strong>derive macropore hydraulic parameters from the soil water pressure boundary between macropore and matrix, and <strong>(3)</strong> dynamically constraint the matrix Kosugi hydraulic parameters. We successfully reduce the number of hydraulic parameters to optimize and constrain the hydraulic parameters without compromising the fit of the <em>&#952;(&#968;)</em> and <em>K(&#952;)</em> functions.</p><p>The robustness of the methodology is demonstrated by deriving the hydraulic parameters exclusively from<em> &#952;(&#968;)</em> and <em>K<sub>s</sub></em>data, enabling satisfactory prediction of <em>K(&#952;)</em> without having measured K(&#952;) data. Moreover, having a reduced number of hydraulic parameters that are physical allows an improved characterization of hydraulic properties of soils prone to preferential flow, which is a fundamental issue regarding the understanding of hydrological processes.</p><p>&#160;</p><p><strong>References</strong></p><p>Pollacco, J.A.P., Ugalde, J.M.S., Angulo-Jaramillo, R., Braud, I., Saugier, B., 2008. A linking test to reduce the number of hydraulic parameters necessary to simulate groundwater recharge in unsaturated soils. Adv Water Resour 31, 355&#8211;369. https://doi.org/10.1016/j.advwatres.2007.09.002</p><p>Pollacco, J.A.P., Webb, T., McNeill, S., Hu, W., Carrick, S., Hewitt, A., Lilburne, L., 2017. Saturated hydraulic conductivity model computed from bimodal water retention curves for a range of New Zealand soils. Hydrol. Earth Syst. Sci. 21, 2725&#8211;2737. https://doi.org/10.5194/hess-21-2725-2017</p>
Determination of water retention curves for one residual soil and two distinct non-woven geotextiles
