Obtaining soil hydraulic properties for water balance and leaching models from survey data. 1. Water retention

A physico-empirical 2-parameter power law model of the draining water reten tion curve (WRC) based solely on clay content is described and further deve loped using 6 datasets obtained from Australian and New Zealand soils. The slope of the WRC, or pore-size distribution index, is well described by the model but the bubbling pressure, or inflection point is poorly described. Without a good estimation of the bubbling pressure it is not possible to sc ale the physico-empirical model to the WRC. To achieve the scaling, a singl e measured point on the WRC in the unsaturated range is required. The resul ting estimated water contents may be satisfactory for application within br oad-scale leaching risk models and for generalised extrapolation of results from detailed experimental sites but caution is still required for quantit ative applications of nitrate leaching models at a particular site. It is concluded that soil surveys could usefully include a single WRC measu rement in the field at each sampling location to improve their utility for water and chemical transport modelling.