Technician collects water sample from Walnut Creek watershed in Iowa.  Click the image for additional information about it.

Studying The Differences in Watersheds

A decade-long Agricultural Research Service study of two midwestern watersheds confirms that soil differences affect how water and agricultural chemicals--particularly nitrate fertilizer and atrazine herbicide--move through the soil. Those two chemicals were measured in the study of watersheds in Iowa and Missouri from 1992-2001.

Soil scientists Gene Alberts and Robert Lerch of the ARS Cropping Systems and Water Quality Research Unit in Columbia, Mo.,studied the Goodwater Creek watershed in north-central Missouri. Dan Jaynes, research leader at the ARS National Soil Tilth Laboratory in Ames, Iowa, studied central Iowa's Walnut Creek watershed. Watersheds are geographic areas where the land "sheds" water to a common outlet.

Researchers learned that two watersheds, closely located geographically, can have significantly different water quality issues. Chemical movement depends on their physical properties and how water moves off the land and through the soil. These watersheds have significantly different water movement pathways because their soils vary.

At Walnut Creek, tile drains are needed to grow row crops. The drains intercept rainfall, moving it rapidly into the creek. This results in more rainfall percolating downward through the soil. Because of nitrate's soil mobility, it moves with the percolating water, resulting in high contamination levels of nitrates used as fertilizer.

In Goodwater Creek soils, tile drains do not work well and are not needed for row crop production. Soils within this watershed have a natural clay layer that limits downward percolation of rainfall, resulting in higher levels of surface runoff. Unlike nitrate, atrazine stays near the soil surface where it moves with runoff. This resulted in high atrazine levels in Goodwater Creek, but lower nitrate levels.

Atrazine is a pre-emergence herbicide that is applied to bare soil, which means that it's more susceptible to being washed away without crops to hold it in place.

Crop rotation, cover crops and a nitrogen management plan can be beneficial in central Iowa. In Missouri, surface runoff control practices and a pesticide management plan that includes pesticide incorporation or the use of low-rate pesticides can be of assistance.

ARS is the U.S. Department of Agriculture's chief scientific research agency.