A New Model to Analyze Water Retention Characteristics of Forest Soils Based on Soil Pore Radius Distribution

The soil water retention model proposed by Kosugi was modified to have a relatively simple functional form. The resulting water retention model (the lognormal distribution model) contains two parameters which have physical significance on the water retention curve and are related directly to the statistics of the soil pore radius distribution function g(r). The observed retention data sets of 282 undisturbed forest soils taken from Mashimo were analyzed using the lognormal distribution model, and it was shown that the model performs fairly well for every data set. The estimated parameters of the model indicated that the water retention characteristics of the undisturbed forest soils are related to the soil structure more closely than to the soil texture. Many crumb-structure forest soils have greater median of g(r) than the crumb and granular-structure soils. Disturbed loamy soils have smaller median of g(r) than many of the undisturbed forest soils and disturbed sandy soils have smaller g(r) width than the forest soils. The brown forest soils taken from A-horizon generally have the greater median of g(r) than the brown forest soils taken from B-horizon. Some of the brown forest soils have the greater width of g(r) than the black soils. The median of g(r) of brown forest soil taken from A-horizon generally becomes greater as the soil moisture condition becomes wet.