## Improved estimation of
soil water retention characteristics from hydrostatic column experiments

**Andre
Peters* and ****Wolfgang Durner**** **

Institute
of Environmental Sciences, Braunschweig Technical University, Germany

*(*Corresponding
author*)

**Abstract**

The soil water retention
characteristic, q(h), is
required for modeling and predicting water and solute transport in unsaturated
porous media. Commonly, q(h) is determined by relating pressure heads, h, to mean water contents,
q, that are measured in column
experiments under hydrostatic equilibrium conditions and fitting a parametric
retention function to these data pairs. Implicit to this method is the
assumption that the mean water content of the column is equivalent to a point
measurement in the column center. Dependent on the nonlinearity of the vertical
water content distribution, q(z), in the column, this assumption may be invalid and introduces a
systematic error. A sensitivity analysis shows that the magnitude of the error
caused by neglecting the nonlinearity of the water content distribution may
reach several percent if coarse materials with low air entry values and tall
soil columns are investigated. Furthermore, neglecting q(z) yields a smoothed retention characteristic
and thus may lead to wrong conclusions about the most appropriate parametric
model for the water retention characteristic. If the hydraulic conductivity
function K(h) is predicted from such an incorrect retention function it can
differ greatly from the true function. In this paper, we propose to consider
the measured water content of a soil column explicitly as an integral of the
equilibrium water content distribution with depth. We show that this eliminates
systematic parameter estimation errors and leads to improved estimates of the
soil water retention function.

Keywords

soil hydrology; vadose zone; retention function, hydraulic conductivity
function

*Cite as*

Peters, A., and W. Durner (2006): Improved estimation of soil water retention
characteristics from hydrostatic column experiments, Water Resources Research
(in press).

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