© 2010 19 th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD. 98 The effect of time-variable soil hydraulic properties in soil water simulations Andreas Schwen A , Gernot Bodner B and Willibald Loiskandl A A Institute of Hydraulics and Rural Water Management, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria, Email andreas.schwen@boku.ac.at B Institute of Agronomy and Plant Breeding, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria. Abstract Modeling soil water dynamics requires an accurate description of soil hydraulic properties, i.e. the retention and hydraulic conductivity functions. Generally, these functions are assumed to be unchanged over time in most simulation studies. However, there is extensive empirical evidence that soil hydraulic properties are subject to temporal changes. In this paper, we implemented temporal changes in the soil hydraulic properties in a Richards’ equation simulation of soil water dynamics. Based on repeated measurement data of the top soil water retention curve, we compared the impact of using constant vs. temporally changing hydraulic functions on water flow simulation for different tillage methods. We observed distinct differences in the soil water content between the simulations for all tillage methods. The results show the remarkable effect of time-variable retention parameters on the soil water dynamics for tilled and non-tilled top soils. Key Words Soil hydraulic properties, temporal variability, soil tillage, pore-size distribution Introduction For many applied questions in the fields of crop production and agronomy, soil water dynamics are of fundamental importance. Modeling can be a valuable tool to optimize its management (Roger-Estrade et al. 2009). However, such soil water modeling requires an accurate description of soil hydraulic properties, i.e. the soil water retention characteristics (WRC) and hydraulic conductivity functions. Generally, these constitutive functions are assumed to be unchanged over time in most simulation studies (Mubarak et al. 2009). However, there is extensive empirical evidence that soil hydraulic properties are subject to temporal changes particularly in the near-saturated range where soil structure essentially influences water flow characteristics (Alletto and Coquet 2009; Daraghmeh et al. 2008; Or et al. 2000). The structure of soil top layers is subject to changes during time, caused by wetting and drying, solution composition, agricultural operations, and biological activity. Soil tillage is used to improve soil structural properties by changing the soil pore-size distribution (PSD). Since these modifications are quite unstable over time, the PSD decreases after tillage (Leij et al. 2002; Or et al. 2000). This effect should be largest for conventional tillage (CT), where the soil is ploughed after harvest every year. Many functions for expressing the WRC have been published (e.g. Brooks and Corey 1964; Van Genuchten 1978). They are compatible with models that describe the relative hydraulic conductivity of soils (e.g. Burdine 1953; Mualem 1976). However, most of these models are empirical curve-fitting equations and do not base on physical fundamentals (Kosugi 1994). In contrast, the soil retention model of Kosugi (1994) bases directly on the lognormal distribution of the soil pore-size distribution (PSD) as described by the Laplace-Young equation (Leij et al. 2002). Recent publications point out that the demand for a new model approach accounts for the temporal variability of the WRC (Alletto and Coquet 2009; Mubarak et al. 2009). In this study, we set up a water flow model that accounts for time variable retention characteristics in the uppermost soil layer. The effect of temporal variability of the WRC on soil water dynamics, as expressed by the volumetric water content, was tested and evaluated for different tillage systems. Governing equations Water flow in unsaturated or partly saturated soils can be described with the Richards’ equation (Richards 1931): h h C K K t z z ∂ ∂ ∂   = -   ∂ ∂ ∂   (1) where h is the soil water pressure head or water potential (dimension L), t the time (T), z the soil depth (L), K is the hydraulic conductivity (L/T) and C is the soil water capacity (/L). C is defined by the slope of the WRC (d/dh), where  is the volumetric water content of the soil (L 3 /L 3 ). In the present study, the WRC in the upper soil is described by Kosugi’s lognormal retention model (Kosugi 1994):