Several investigations carried out by comparing a great deal of experimental retention data sets highlighted the possibility of describing the drying soil water-retention function reasonably well by employing empirical analytical relations. A closed-form analytical relation can be incorporated much more easily into numerical water flow models than measured values in tabular form. One of the most popular and widely verified nonhys-teretic 6 (h) relations has been proposed by van Genuchten :
where Se = (6 — 6s)/(6s — 6r) is effective saturation; 6s and 6r are saturated and residual water content, respectively; h (m) is the soil water pressure head; and a (1/m), n, and m represent empirical shape parameters. However, attention should be paid to the concept and definition of residual saturation [8, 12]. Usually, 6s and 6r are measured values, whereas the remaining parameter values are computed from measured retention data points by employing nonlinear regression techniques, with the constraints a > 0, n > 1, and 0 < m < 1. A few empirical relations also have been introduced in the literature for analytically describing hysteresis in the water retention function. Basically, it has been proposed that parametric relations practically equal to that of van Genuchten be used, but with different values for the parameters when describing the main wetting or drying curves .
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