ESTIMATING UNSATURATED HYDRAULIC CONDUCTIVITY OF BRAZILIAN SOILS USING SOIL-WATER RETENTION DATA

Studies to predict the impact of deforestation on climate in Amazonia have stimulated interest in the hydraulic properties of the soils of the region and the surrounding areas. Few directly measured hydraulic conductivity data are available; these were used to derive parameters to allow the unsaturated hydraulic conductivity-water content (K-θ) curve to be obtained from the more easily measured water retention curve. Relative hydraulic conductivity was modeled using the Brooks-Corey formula, whereas the saturated conductivity was calculated as a function of effective porosity from the expression proposed by Ahuja et al. Empirical relationships were fitted to seven different Brazilian soils, from 13 different sites, assembling about 2650 field conductivity values. The data for six of the soil types could be represented by a single set of parameters, with kaolinitic yellow latosolic soils from Amazonia requiring a separate set. Fitted relationships showed very high statistical significance, and the parameters derived were significantly different from those derived by Ahuja et al. for North American soils. To verify the proposed methodology, the parameters were derived using the complete data set (without the kaolinitic yellow latosol), excluding one soil type at a time, and checking the ability of the relationships to predict the conductivity of the soil not included in the fitting. In addition, the parameters obtained from the complete data set predicted hydraulic conductivities from the water release curves of five other Brazilian soils for which measured conductivity values were available.