MECHANISTIC PROCESS-LEVEL CROP SIMULATION MODELS FOR RESEARCH AND ON-FARM DECISION SUPPORT
Title: SIMPLE PARAMETRIC METHODS TO ESTIMATE SOIL WATER RETENTION AND HYDRAULIC CONDUCTIVITY
Submitted to: Development of Pedotransfer Functions in Soil Hydrology
Publication Type: Book / Chapter
Publication Acceptance Date: February 5, 2004
Publication Date: November 25, 2004
Citation: Timlin, D.J., Williams, R.D., Ahuja, L.R., Heathman, G.C. 2004. Simple parametric methods to estimate soil water retention and hydraulic conductivity. In: Development of Pedotransfer Functions in Soil Hydrology. Pachepsky, Y. and Rawls, W., editors. Amsterdam: Elsevier. 30:71-93.
Interpretive Summary: Information on the hydraulic properties of soils is required by a wide range of disciplines, from agricultural sciences to ecology. Information on soil hydraulic properties is especially important for computer models of complex soil water processes and crop growth. These properties include information on how easy it is to remove water from the soil as a function of water content and how rapidly water moves in soil at different water contents. These data are often limited in availability but we show how they can be derived from simpler and more readily available data such as soil texture, organic matter and bulk density. In this study we describe a number of simple methods that have wide applicability and are easily implemented. These approaches require one known water content value, which can be estimated using soil texture and bulk density, available in many databases. The model to generate soil hydraulic properties can be easily incorporated into various soil water or crop growth models. These methods will make it easier for agricultural managers to obtain reasonably accurate estimates of soil hydraulic properties for use in computer simulation models or to evaluate soils for plant productivity or susceptibility to loss of nutrients through leaching.
Although information on the hydraulic properties of soils is required by a wide range of disciplines, from agricultural sciences to ecology, the data are not always widely available. There are extensive data available, however, on organic matter content, bulk density and texture for many soils around the world. Therefore, a great deal of research has been devoted to development of approaches to estimate the soil hydraulic properties either from simpler soil properties, and /or limited data. This paper summarizes several new methods that improve upon the earlier approaches. We describe a similar-media based scaling technique that requires one measured pair of water content/matric potential data and a reference soil moisture release curve. Another method, the Gregson et al. one parameter model (GHM) is based on the log log form of the soil water retention curve, below the air entry value of matric potential, ln(matric potential) = a + b ln (water content), where a and b are the intercept and slope, respectively. A strong linear relationship observed between a and b was expressed as a = p + qb. The parameters p and q fit a wide range of soils so that a single slope and intercept could be used to generate any moisture release curve given one pair of water content-matric potential data. We show that the fit improved if the parameters were grouped by texture class. This relationship was also extended to unsaturated hydraulic conductivity. It is shown that the parameters a and b of the above relationship are related to parameters of the Brooks-Corey equation when data are grouped by texture class. In a large scale watershed study, the use of textural class-name data only to describe soil hydraulic properties provided good simulations of average profile soil water contents when used in the Root Zone Water Quality Model as compared to simulations of water contents using lab measured soil properties.