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United States Department of Agriculture

Agricultural Research Service

Title: Estimating Water Retention of Sandy Soils Using the Additivity Hypothesis

Authors
item Zeiliguer, A - MOSCOW UNIV, MOSCOW, RUSS
item Rawls, Walter
item Pachepsky, Yakov

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 13, 2000
Publication Date: July 1, 2000

Interpretive Summary: We developed an 'additivity' model assuming that (a) soil water retention is additive and can be approximated by summing up water retention of pore subspaces related to the soil components; (b) the additivity is applicable to gravimetric water contents; (c) water retention related to a textural fraction can be measured on packed samples consisting exclusively of this fraction's particles; (d) water retention of textural fractions contribute in total soil water retention in proportion to the volumes of pore subspaces related to each fraction; and (e) soil structural components; i. e., aggregates or peds do not create their own pore subspaces where capillary or residual water is retained. Water retention of packed cores consisting of soil textural fractions was taken from literature. The resultant model did not have fitting parameters. It was tested with samples of sandy soils that were collected in the UNSODA database from seven states sin the U.S. and seven other countries. In spite of the existence of several sources of errors, the additivity model showed the same or better level of accuracy that regression-based models typically used to estimate soil hydraulic properties. This warrants testing this model with other soil data and improving this model to accommodate various types of soil structure.

Technical Abstract: Soil water retention measurements are relatively time-consuming, and become impractical when hydrologic estimates are needed for large areas. One approach to soil water retention estimation from readily available data is based on the hypothesis that soil water retention is additive and can be approximated by summing up water retention of pore subspaces related to the esoil components. Our objective was to test this hypothesis. In the 'additivity' model used in this work, the main assumptions are: (a) the additivity is applicable to gravimetric water contents, (b) water retention related to a textural fraction can be measured on packed samples consisting exclusively of this fraction's particles, (c) water retention of textural fractions contributes in total soil water retention in proportion to the volumes of pore subspaces related to each fraction. Water retention of packed cores consisting of soil textural fractions was taken from literature. The resultant model did not have fitting parameters. It was tested with samples of sandy soils that were collected in the UNSODA database from seven states in the United States and seven other countries. The median root-mean squared error (RMSE) constituted 0.017 m**3m**-3, 0.023 m**3m**-3, and 0.028 m**3m**-3 in coarse, medium, and fine sands, respectively. These RMSE are at the lower end of the RMSE range for regression-based pedotransfer function found in literature. Slopes and intercepts of the regressions of estimated water contents on measured water did not differ significantly from one and zero at the 0.05 significance level except in one case. The RMSE of the laboratory water retention estimates with additivity model was significantly less than the RMSE of the field water retention estimates from the laboratory water retention data.

Last Modified: 11/27/2014
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