dispersive water model (Seyfreid, 1986). Miscible-displacement

experiments on saturated soil columns and subsequent analysis of

effluent data by the MIM model estimated the mobile-water content at

about 55% of the total soil water. Low Peclet numbers and consequent

high dispersion coefficients indicated a high degree of preferential

water flow that bypassed large portions of the soil water.

 Schulin et al. (1987) used the CD and MIM models to determine

behavior of water in undisturbed columns of soil containing about 55%

by volume gravel. Back-calculation of presented data indicated that

the gravel was not porous and contained no water. The MIM model

calculated the mobile-water content to be about 85% of the total water

present in unsaturated columns maintained at volumetric-water contents

of between 0.135 and 0.175 mL/mL for soils with total porosities

ranging from 0.25 to 0.30 mL/mL. Due to the low immobile-water

fraction, the CD model, which considers all water as mobile, was able

to estimate parameters capable of simulating the experimental BTC

nearly as well as the MIM model.

 Several independently conducted studies have suggested that the

 gravel resulting from mineral dissolution and precipitation in tropical

 stone-line soils is porous (Amouric et al., 1986; Muller and Bocquier,

 1986; Chapter 3.). Although the only apparent study on the

 mobile/immobile-water content of gravelly soil indicates that the

 immobile fraction is relatively small (Schulin et al., 1987), the

 presence of porosity in gravel from tropical stone-line soils would

 suggest that these soils may have a considerable immobile-water

 fraction. The purpose of this study was to use the MIM and CD models