Unsaturated Soil Hydraulic Conductivity in the Central Amazon: Field Evaluations

Abstract

Determination of hydraulic properties may be required to solve many questions in agriculture and environmental research. Hydraulic properties are spatially and temporally variable, consequently, reliable soil hydraulic characterization is complex and time-consuming. Furthermore, many hydraulic properties are better represented by functions (i.e., unsaturated hydraulic conductivity and soil water retentivity) rather than by the mean values. The variability due to the presence of macropores or soil cracks commonly found in some soil classes in the tropics can enhance the naturally high soil hydraulic variability. A tension infiltrometer is an equipment that can be used to evaluate near-saturated hydraulic conductivity and sorptivity. In structured soil, small changes in soil tension, near the saturation, lead to dramatic changes of the infiltration rates. In this chapter, firstly, a discussion about soil hydraulic parameters and methods more appropriate to evaluate them on tropical soils is provided. Specific sources of errors and procedures to avoid or alleviate them are pointed out. Secondly, unsaturated hydraulic conductivity measurements carried out using a tension infiltrometer are used to compare different land use systems in the Central Amazon. The scaling theory and the statistical techniques of piecewise continuous regression are briefly described and used to analyze the results. Tension infiltrometers show to be relatively cheap, robust and a simple field method to evaluate unsaturated hydraulic conductivity in tropical soils. Since K near saturation is highly variable, the data were better analyzed using the relative hydraulic conductivity and the scaling theory. These small values of the tension in the breakpoint that divide the fluxes dominated by gravity reflect the importance to evaluate unsaturated hydraulic onductivities near saturation.