COMPARISON OF FIELD- AND LABORATORY-MEASURED AND PREDICTED HYDRAULIC PROPERTIES OF A SOIL WITH MACROPORES

Abstract

Soil hydraulic properties were determined in situ by the instantaneous profile method on 3 3-m square plots of Dothan soil (plinthic paleudult, fine-loamy siliceous, thermic). Simultaneous measurements of matric tension and moisture content with depth during drainage were made with tensiometers and a field-calibrated neutron probe. Unsaturated conductivity values calculated from the in situ data ranged within an order of magnitude for corresponding horizons in the 3 plots. Variability between plots was greatest at moisture contents near saturation. The large data scatter is attributed to soil heterogeneity and to nonuniform drainage resulting from interaggregate porosity and root channels in the soil profile. Unsaturated conductivities calculated from the instantaneous profile test were compared with values predicted by 3 cut-and-random-rejoin-type pore structure models using laboratory- and field-measured moisture retention data. Laboratory measurements of moisture retention characteristics were made by pressure desorption of 54-mm diameter undisturbed cores. The models of Jackson (1972) and van Genuchten (1980) were both successful in predicting unsaturated conductivities within the scatter of in situ values when field-measured moisture retention data were used and the mean in situ saturated conductivity was employed as a matching point. Corresponding between predicted and in situ conductivities was poorer when using laboratory retention data, reflecting difficulties in obtaining representative undisturbed core samples.