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Integrating Chemical Factors with Water and Sediment Transport from a Watershed¹

ABSTRACT

A mathematical model that calculates the movement of a chemical as it is transported through or off of an agricultural watershed is described. Loss of the chemical between storms by degradation or volatilization is described by a first-order rate equation. Simple chromatographic theory is used to describe the chemical distribution in the soil during leaching, assuming a linear adsorption relation and dispersion that is proportional to the square root of the distance moved. The concentration at the surface during the storm is calculated to estimate the amounts lost in runoff water and with interril erosion. It is assumed that rill erosion removes the chemical in proportion to the fraction of the area in rills and to the fraction of the chemical distribution in the soil intercepted by the rills.

Mineralization and uptake are an additional source and sink for nitrate between storms. Lithium bromide movement on a microplot was used to examine some features of the model.

Key Words: mathematical model • pollution

¹ Contribution from the Agricultural Water Quality Management Laboratory, USDA-ARS, Durant, OK 74701.

² Soil Scientist, USDA-ARS, Durant, OK.

Received for publication December 17, 1973.