Author's personal copy Simplified evaporation method for determining soil hydraulic properties A. Peters * , W. Durner Institute of GeoEcology, Department of Soil Science and Soil Physics, Braunschweig Technical University, Germany Received 26 September 2007; received in revised form 4 March 2008; accepted 3 April 2008 KEYWORDS Hydraulic properties; Evaporation experiment; Soil water flow; Richards equation; Parameter optimization Summary Evaporation experiments are commonly used to derive hydraulic properties of soils. In the simplified evaporation method, as proposed by Schindler [Schindler, U., 1980. Ein Schnellverfahren zur Messung der Wasserleitfa¨higkeit im teilgesa¨ttigten Boden an Ste- chzylinderproben. Arch. Acker- u. Pflanzenbau u. Bodenkd. Berlin 24, 1–7], the weight of a soil sample and pressure heads at two height levels are recorded at consecutive times. The evaluation of these measurements relies on linearization assumptions with respect to time, space and the water content–pressure head relationship. In this article, we inves- tigate the errors that result from the linearization assumptions, and show how systematic and stochastic measurement errors affect the calculation of water retention and hydraulic conductivity data and the resulting fits of soil hydraulic functions. We find that lineariza- tion errors with respect to time are negligible if cubic Hermite splines are used for data interpolation. Linearizations in space lead to minor errors, even in the late stage of evap- oration where strongly non-linear pressure head profiles emerge. A bias in the estimated retention function results from the negligence of a non-linear water content distribution in the sample at the begin of the evaporation process, and affects primarily coarse sands or soils with structured pore systems. This error can be avoided if an integral evaluation of the measurements is used. We introduce an applicable rejection criterion for unreliable hydraulic conductivity data near saturation, based on the error in the hydraulic gradient. Calibration errors of tensiometers lead to biased estimates of hydraulic properties in the wet range, whereas errors in tensiometer installation positions yield biases in the dry range. Random errors in data cause no significant bias, and parametric hydraulic functions can be estimated with small uncertainties, if water retention and conductivity functions are coupled and the underlying model structure is correct. ª 2008 Elsevier B.V. All rights reserved. 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.04.016 * Corresponding author. Tel.: +49 5315930; fax: +49 5315637. E-mail address: a.peters@tu-bs.de (A. Peters). Journal of Hydrology (2008) 356, 147– 162 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol