SOIL CONSISTENCE AND STRUCTURE AS PREDICTORS OF WATER RETENTION

Authors: Rawls, Walter; Pachepsky, Yakov

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2001
Publication Date: 10/23/2001
Citation: Rawls, W.J., Pachepsky, Y. Soil consistence and structure as predictors of water retention. Soil Science of America Journal. 2002. v. 66. p. 1115-1126.

Interpretive Summary: It is impractical to measure soil water retention for large-scale hydrologic, agronomic, and ecological applications or at the design stages of many projects. Therefore, water retention estimates are often used that take advantage of soil survey results shown in maps. Soil survey profile descriptions routinely include soil structure and soil consistence that currently are not used in water retention estimation because they are give by classes rather than numbers. The objective of this work was to evaluate the potential for structural and consistence properties to serve as predictors of soil hydraulics properties. Total of 2,140 samples from National Soil Characterization Database were used that had soil water retention data, structure characterization with grade, size and shape, consistence characterization with dry and moist consistency, stickiness, and plasticity, and textural class determined in the field and from lab textural analysis. A data exploration technique called regression tree was used to employ structural and consistence parameters represented by classes as predictors. Plasticity class, grade class and dry consistency class were leading predictors of water retention. The accuracy of estimates from structural and consistence parameters was lower than from textural classes. Using soil structural and consistence parameters along with textural classes provided a statistically significant improvement in accuracy of water retention estimates as compared with estimation from texture alone. Soil structural and consistence parameters can serve as predictors of soil water retention because those parameters reflect soil basic properties that affect soil hydraulic properties.

Technical Abstract: It is impractical to measure water retention for large-scale hydrologic, agronomic, and ecological applications or at the design stages of many projects, and water retention estimates are often used. Field soil descriptions routinely include structure and consistence characterization. The objective of this work was to use the NRCS database to evaluate the potential for structural and consistence properties to serve as predictors of soil hydraulics properties. Total of 2,140 samples were found that had (a) values of water contents at -33 kPa and -1500 kPa,(b) structure characterized with grade, size and shape, consistence characterized with dry and moist consistency, stickiness, and plasticity, and (d) textural class determined in the field and from lab textural analysis. Because structural and consistence parameters were represented by categories rather than numbers, regression trees were used for recursive partitioning of the data sets into groups to decrease overall variability measured as the sum of squared errors within groups. Plasticity class, grade class and dry consistency class were leading predictors of water retention at both -33 kPa and -1500 kPa matric potentials. The accuracy of estimates from structural and consistence parameters was lower than from textural classes. Using soil structural and consistence parameters along with textural classes provided a small, although significant improvement in accuracy of water retention estimates as compared with estimation from texture alone. Soil structural and consistence parameters can serve as predictors of soil water retention because those parameters reflect soil basic properties that affect soil hydraulic properties.