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Simulation of Soil Water within the Root Zone of a Corn Crop¹

Knowledge of the distribution and movement of soil water in the root zone of a row crop is important in farm and watershed management, and in research on the transport of soil nutrients and pollutants. The objective of this study was to develop a model that would calculate the infiltration, evaporation, transpiration, and deep drainage of soil water in the root zone of a corn (Zea mays L.) crop throughout a growing season, using readily obtainable climatic data as daily inputs. The model differed from existing models in that the effect on the distribution of rainwater due to channeling in the cracks and non-capillary pores of a highly structured soil was considered. This distribution was obtained by adapting equations used to describe analagous distributions in ion exchange chromatography. Two methods of modeling plant uptake of soil water were compared. An empirical approach assumed that the plant took water from the layer in the root zone with the lowest energy level (determined four times daily) and the amount taken was given by a curve that related the ratio of actual and potential transpiration to the water content at that level. A mechanistic approach, similar to that used by Nimah and Hanks in 1973, considered root distribution, hydraulic conductivities, plant resistance, and root-soil contact resistance.

The model was tested using seasonal soil moisture records from 4 years on one soil and a single year on a second soil. Mean squared error of prediction for the seasons ranged from 0.05 to 0.157'. Use of the partial displacement infiltration component improved the accuracy of the results in all but one trial. The two methods of modeling plant uptake of soil water, mechanistic and empirical, differed very little in accuracy.

Key Words: Partial displacement • modeling • Leaf area index

² Graduate student, Dep. of Agron.; professor, Dep. of Math.; graduate student, Dep. of Agron.; assistant professor, School of Biol. Sci.; and professor, Dep. of Agron., Univ. of Kentucky, respectively.

Received for publication December 26, 1978.