Abstract
Ranges or “classes” of probable saturated hydraulic conductivity values (Ksat) are listed for all soil series in USDA-NRCS Soil Survey reports. Listed values are not measured, but rather estimated from other soil properties using a pedotransfer function (PTF). To validate the PTF, we compared estimated Ksat classes with measured values in various horizons of nine major soil series of Puerto Rico. For each horizon, a minimum of 9 and usually 16 Ksat measurements were made with Guelph permeameters near locations where soil pedons had been thoroughly described. In most horizons, Ksat was log-normally distributed. The ratios of Ksat values corresponding to one geometric standard deviation above and below the mean were usually less than 10, which is the ratio of upper and lower class boundaries in the Ksat classification system. For most horizons, measured Ksat values were distributed among the rated Ksat class and the next higher class, indicating that the PTF systematically underestimated the Ksat distributions, but by less than an order of magnitude. From the point of view of soil and water management decisions requiring conservative Ksat estimates, the PTF estimates appeared reasonably conservative without deviating from actual values so as to limit the usefulness of the estimates.
Highlights
One of the fundamental physical properties of soil is its saturated hydraulic conductivity (Ksat), defined as the rate of water movement through saturated soil under a unit hydraulic gradient
This has led to estimates of Ksat based on pedotransfer functions (PTFs), which estimate values from correlated soil properties that are readily available in Soil Survey reports, such as texture, bulk density, organic matter, particle size distribution, and structure descriptions
Saturated hydraulic conductivity was measured in situ in various horizons from 9 soil series of Puerto Rico, which had previously been thoroughly characterized by Natural Resources Conservation Service (NRCS) Soil Survey personnel
Summary
One of the fundamental physical properties of soil is its saturated hydraulic conductivity (Ksat), defined as the rate of water movement through saturated soil under a unit hydraulic gradient. The variability is compounded when Ksat measurements are separated by large spatial distances [8,9,10], or taken at different times [11,12] This has led to estimates of Ksat based on pedotransfer functions (PTFs), which estimate values from correlated soil properties that are readily available in Soil Survey reports, such as texture, bulk density, organic matter, particle size distribution, and structure descriptions. Evidence of the usefulness of PTFs for estimating Ksat came from the work of O’Neal [13,14], who developed a set of field clues for estimating Ksat He found good agreement between measured and estimated Ksat values in 68 percent of a total of 271 soil horizons examined
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