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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 21, issue 6
Hydrol. Earth Syst. Sci., 21, 2725–2737, 2017
https://doi.org/10.5194/hess-21-2725-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Hydrol. Earth Syst. Sci., 21, 2725–2737, 2017
https://doi.org/10.5194/hess-21-2725-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 09 Jun 2017

Research article | 09 Jun 2017

Saturated hydraulic conductivity model computed from bimodal water retention curves for a range of New Zealand soils

Joseph Alexander Paul Pollacco1, Trevor Webb1, Stephen McNeill1, Wei Hu2, Sam Carrick1, Allan Hewitt1, and Linda Lilburne1 Joseph Alexander Paul Pollacco et al.
  • 1Landcare Research, P.O. Box 69040, Lincoln 7640, New Zealand
  • 2New Zealand Institute for Plant & Food Research Limited, Private Bag 4704, Christchurch 8140, New Zealand

Abstract. Descriptions of soil hydraulic properties, such as the soil moisture retention curve, θ(h), and saturated hydraulic conductivities, Ks, are a prerequisite for hydrological models. Since the measurement of Ks is expensive, it is frequently derived from statistical pedotransfer functions (PTFs). Because it is usually more difficult to describe Ks than θ(h) from pedotransfer functions, Pollacco et al. (2013) developed a physical unimodal model to compute Ks solely from hydraulic parameters derived from the Kosugi θ(h). This unimodal Ks model, which is based on a unimodal Kosugi soil pore-size distribution, was developed by combining the approach of Hagen–Poiseuille with Darcy's law and by introducing three tortuosity parameters. We report here on (1) the suitability of the Pollacco unimodal Ks model to predict Ks for a range of New Zealand soils from the New Zealand soil database (S-map) and (2) further adaptations to this model to adapt it to dual-porosity structured soils by computing the soil water flux through a continuous function of an improved bimodal pore-size distribution. The improved bimodal Ks model was tested with a New Zealand data set derived from historical measurements of Ks and θ(h) for a range of soils derived from sandstone and siltstone. The Ks data were collected using a small core size of 10 cm diameter, causing large uncertainty in replicate measurements. Predictions of Ks were further improved by distinguishing topsoils from subsoil. Nevertheless, as expected, stratifying the data with soil texture only slightly improved the predictions of the physical Ks models because the Ks model is based on pore-size distribution and the calibrated parameters were obtained within the physically feasible range. The improvements made to the unimodal Ks model by using the new bimodal Ks model are modest when compared to the unimodal model, which is explained by the poor accuracy of measured total porosity. Nevertheless, the new bimodal model provides an acceptable fit to the observed data. The study highlights the importance of improving Ks measurements with larger cores.

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Descriptions of soil hydraulic properties, such as soil moisture release curve, θ(h), and saturated hydraulic conductivities, Ks, are a prerequisite for hydrological models. Because it is usually more difficult to describe Ks than θ(h) from pedotransfer functions, we developed a physical unimodal model to compute Ks solely from hydraulic parameters derived from the Kosugi θ(h). We further adaptations to this model to adapt it to dual-porosity structural soils.
Descriptions of soil hydraulic properties, such as soil moisture release curve, θ(h), and...
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