Comparison of Percolation Test Results and Estimated Hydraulic Conductivities for Mollisols and Alfisols

Percolation tests were performed on an array of biosequences, topo-drainage sequences, and stage of profile development sequences of soils in Iowa and results were compared to estimated subsoil hydraulic conductivities from Soil Interpretation Records. Within a biosequence, Mollisols had higher percolation rates and lower coefficients of variation than Alfisols. The difference in permeability was attributed to higher subsoil clay content, higher subsoil bulk density, more continuous argillans, and stronger subsoil consistence in Alfisols. High coefficients of variation for percolation rates in Alfisols were attributed to large diameter root channels, lateral flow of water on sloping sites, and increased distinctness of subsoil horizon boundaries. Percolation rates of Mollisols commonly exceed Soil Interpretation Records' estimates of hydraulic conductivity by a factor of two or three while percolation rates of Alfisols approximate estimated subsoil hydraulic conductivity. In soils without argillic horizons, percolation test results of topo-drainage sequences are similar and reflect similar morphological characteristics. Soil Interpretation Records assign slower estimated hydraulic conductivity to some topo-drainage sequences as drainage becomes more restrictive. Percolation rates decreased in topo-drainage sequences of soils with argillic horizons as drainage became more restrictive, even though morphological characteristics were quite similar. Differences in percolation rates within topo-drainage sequences may reflect errors inherent in the procedure. Additional permeability data is needed to quantify and more accurately estimate subsoil hydraulic conductivity. Criteria used to establish estimated hydraulic conductivity in Soil Interpretation Records should be applied consistently among biosequences and topo-drainage sequences. Contribution from the Iowa Agric. Exp. Stn.