Soil water retention relationships

Soil water retention (pressure-saturation) relationships are frequently described using the van Gmuchten (VG) equation (2J),... 

It was concluded that T. officinale was positively associated with A. monantha. A milder microclimate within its mat, water retention and nutrient availability in the soil beneath these plants were possible causes for the enhanced performance of the intruder. However, it would be instructive to determine to what extent this relationship is competitive as resources are shared because both species are of the dominant type. Various other Azorella cushion species grow in the periglacial zone along the Andean chain from Venezuela to Argentina (e.g. A. crenata, and A. julianii in the Venezuelan and Colombian paramos), (Vareschi, 1970) and they support other plant species within their round-shaped mats (pers. obsv.). However, no systematic studies of these associations have been conducted yet. 

The effect of all the various phenomena connected with the water circulation between the atmosphere, biosphere and hydrosphere is reflected in general in the water regime of the soil. The water regime includes the water penetration into the soil, its motion and retention in the soil, and its escape from the soil. It is characterized quantitatively by the soil water balance, which may be expressed by the relationship ... 

Many parameters in the modeling of soil physies exhibit nonlinear relationships. One sueh relationship is the water retention rate as a function of matric potential, which defines for water flow a term similar to electric potential for eurrent flow. (See J. Nat. Resour. Life Sci. Educ., Vol. 27, 1998, pp. 13-19 for a deseription of the data set for this example.) Figure 9.45 shows experimental data for soil volume water content vs. matric potential. The data points are approximately equally spaced on a log scale of the independent variable. From the data points plotted on a log scale it seems apparent that the data points should all be weighted equally and this is the default used in all the previous data fittings. This will occur naturally for the data fitting even when the points are non-uniformly spaced along the x-axis. 

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