Soil water retention: I. Introduction of a shape index

Knowledge of soil water retention is fundamental to quantify the flow of water and dissolved substances in the subsurface. Water retention is often quantified with models fitted to observed retention points. Interpretation and conversion of parameters from different models is subjective and prone to error. We examined 461 retention curves from the UNSODA database and 660 from the GRIZZLY database. Parameters of the Brooks-Corey (BC) and van Genuchten (vG) equations were fitted to the retention data. The shape parameters in these functions (lambda, m, and n) are closely correlated to soil texture and may be predicted with so-called pedotransfer functions (PTFs). Among the scale parameters, the saturated water content theta(r) proved to be a robust fitting parameter regardless of parameterization. Reliable optimization of the residual water content Or is more difficult; without any constraint it was negative for 54.4% of the GRIZZLY samples, and its value was strongly correlated to the shape parameters. The BC- and vG-shape parameters are often converted assuming lambda = mn, which is incorrect when X or mn is large (e.g., lambda > 0.8). To facilitate the interpretation, conversion, and optimization of retention parameters, we introduce a water retention shape index P. This index constitutes an integral measure of the slope of the retention curve and characterizes the retention behavior of a particular soil with a single number. A value for the index can be estimated directly from retention data. For the majority of the samples P ranged between 0 and 0.4; rarely did P exceed 3, which is the maximum expected for fractal behavior. The value for P was related to soil texture: fine-textured soils tend to have smaller values than coarse-textured soils. The shape index provides a benchmark for conversion and comparison of parameters.