A New Method for Developing Equations Applied to the Water Retention Curve

The soil water retention curve (WRC) represents the relationship between soil suction and water content. Previous equations for representing the WRC do not include a direct relationship between features of the curve and equation fitting parameters. We present a new method for developing equations for representing empirical data that ensures that features of the curve are directly related to fitting parameters in the equation. The method also ensures that the equation has a simple derivative and is readily integrated, an important property if the equation is to be used as part of a numerical model. This method has been applied to the WRC, producing a new equation that is able to accurately represent the entire WRC using a single equation. The direct relationship between the main features of the curve and fitting parameters ensures that the fitting parameters have clear theoretical meaning. Additionally, this direct relationship allows fitting parameters, such as the air-entry value, to be calculated using pedotransfer functions. The equation has been fitted to a wide range of experimental data and has been demonstrated to achieve an excellent fit for sand, silt, and clay soils across the entire suction range from the minimum measured suction value to 10(6) kPa.