Soil water retention is important for the study of water availability to germinating weed seeds. Six soil water retention models (Campbell, Brooks-Corey, four- and five-parameter van Genuchten, Tani, and Russo) with residual soil water parameter derivations were evaluated to describe water retention for weed seed germination at minimum threshold soil water potential for three hillslope positions. The Campbell, Brooks-Corey, and four-parameter van Genuchten model with modified or estimated forms of the residual parameter had superior but similar data fit. The Campbell model underestimated water retention at a potential less than ？0.5？MPa for the upper hillslope that could result in underestimating seed germination. The Tani and Russo models overestimated water retention at a potential less than ？0.1？MPa for all hillslope positions. Model selection and residual parameter specification are important for weed seed germination by representing water retention at the level of minimum threshold water potential for germination. Weed seed germination models driven by the hydrothermal soil environment rely on the best-fitting soil water retention model to produce dynamic predictions of seed germination. 1. Introduction The soil water retention characteristic (SWRC) is a basic hydrophysical property of the soil that relates the water content of soil water to its energy state [1]. The water content-potential function is fundamental to the characterization of water holding capacity, water retention, and water flow in soil [2, 3]. The SWRC is necessary for modelling fluxes in soil water and is needed for germination studies where soil water is measured on a content basis. The timing of seed germination is a function of soil water potential [4]. As the soil dries, soil water potential is reduced, and it becomes increasingly difficult for seeds to imbibe water. At the minimum threshold (base) water potential, seeds do not imbibe sufficient water to initiate embryo growth and complete the germination process. The minimum threshold water potential at which germination ceases to occur in many agricultural weeds ranges from ？0.1 to ？1.5？MPa [5, 6]. Accurate representation of the SWRC over a wide range of water potential minimum thresholds is required for predictive modelling of seed germination. One of the greatest challenges in characterizing the SWRC for the shallow depth of the seedling recruitment zone (soil layer from which seeds germinate and emerge) across field topography is obtaining the parameters of the soil hydrological property. Determining the SWRC by direct
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