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United States Department of Agriculture

Agricultural Research Service

Title: Determination of Preferential Flow Model Parameters

Author
item Logsdon, Sally

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 10, 2001
Publication Date: July 3, 2002
Citation: Logsdon, S.D. 2002. Determination of preferential flow model parameters. Soil Science Society of America Journal. 66:1095-1103.

Interpretive Summary: Ground water can be contaminated when applied agrochemicals move rapidly through wormholes, cracks, and other large holes in the soil. Computer models are used to predict the rapid movement, but the computer models need a lot of information to run properly. The purpose of this study was to test methods that can be used to obtain the needed information to run the models. We found that useful methods included volume of holes in the soil, how fast water moves through soil, and drainage volume of water from soil. Some of the measurements were not within the constraints of the model indicating that the models need to be revised. The impact of this study is that scientists can use these methods to get the information required by the models. The importance of this work for society is that the models can be used to help us predict when groundwater contamination may occur. Then we can modify our agricultural management practices to reduce contamination.

Technical Abstract: Transient-state preferential flow models are extremely flexible, but require a large number of input parameters. The equations used to describe preferential flow or the macropore domain have not been exhaustively tested with macropore flow and characterization measurements. The objective of this paper was to utilize macropore domain measurements to determine model input parameters. The MACRO model is a two-flow domain model with separate equations for the macropore and micropore domains, but the explicit exchange term between domains assumes an equivalent parallel fracture geometry. The Root Zone Water Quality model (RZWQM) describes macropore flow using Poiseuille's law for cylindrical pores. A few measurements of wet-end hydraulic conductivity (K) - water content - head (h) were available, but a larger data set (55 measurements) of K(h) was utilized. Macropore volume was determined from image analysis of pores and fractures and from desorption. For the MACRO model, the parameters fit the equations best when the cutoff £h£ between macropore and micropore domains was 30 mm. Then the exponent of the K(h) had a mean of 2.1 and a standard deviation (SD) of 1.2, the mean boundary K was 21.1 mm/hr with an SD of 21.7, and the mean saturated K was 158 mm/hr with an SD of 137. The calculated half spacing between fractures ranged from 3 to 100 mm, and the macropore fraction ranged from 0.003 to 0.064 m/m. The macropore fraction calculated from Poiseuille's law only ranged from 0.0008 to 0.0025 m/m. Within constraints these various techniques can be used to determine input parameters in transient-state preferential flow models.

Last Modified: 9/20/2014
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