Determination of the Tangential Model Soil Water Retention Curves for Various Soil Types.

An analytical soil water retention curve model, referred to as Tangential model, proposed by Kohgo (1995) ensures the continuity of the soil water retention curves (SWRC) slopes. The use of this model requires first selecting three points (E, M and F) along the SWRC, which correspond to the air entry point, the point with the maximum slope of the SWRC, and the point with the largest matric potential below which the change in the degree of saturation is minimal, respectively. Slopes (c_e, c_m and c_f) at those points then need to be determined. This study aims (i) to propose a representative Tangential model curve for a given soil type, and (ii) to develop a pedotransfer function to estimate a Tangential model curve from easily measured soil properties.

SWRCs available from the Unsaturated Soil Hydraulic Properties Database (UNSODA) were used. Tangential model curves were fit to 434 soils (139 Sand, 83 Sandy-loam, 53 Loamy-sand, 41 Sandy-clay-loam, 91 Silt-loam, 27 Silt-clay-loams) using a nonlinear regression method by minimizing the sum of the squared residuals.

For each soil type, the geometric means (GM), the medians (MD), and the arithmetic means (AM) of the coordinates of three points, E, M and F; and the arithmetic means of three slopes, c_e, c_m, and c_f, were obtained to generate representative SWRCs. Mean errors, mean absolute errors, and root mean square errors were then calculated by comparing with observed water retention data.

For most soil types, AM resulted in the largest errors among three, while GM and MD performed similarly. Errors for sandy soils were, in general, larger than those for loamy soils. This study demonstrates that Tangential model curves constructed with mean coordinates represent well the observed curves for most soil types considered in this study.