Linking agricultural practices to water quality improvement: The importance of spatial scale in accurately characterizing nonpoint source nutrient loads in Iowa stream systems

Issue

Most research into the effects of nutrient reduction practices on in-stream nutrient loads in Iowa has been performed at either the plot or large watershed scales. However, nutrient exports estimated at these scales can differ significantly from what is actually exported to surface waters from farmlands. This difference is due, in part, to the varied ways water moves over and through the landscape, and to how streams can transform and contribute to nutrient loads. Considering the effects of streams on nutrient loads is important for assessing progress toward meeting statewide nutrient reduction goals. This is especially true for phosphorus, which is strongly influenced by exchange with stream sediments, and a significant amount of which is contributed to by streambed and bank erosion, making it difficult to determine the contribution of this nutrient from agricultural areas when measured on the main channels of larger rivers.

Objective

This project will provide a better understanding of the relative influences that agriculture and natural stream functions have on in-stream nutrient loads in this landscape. Researchers aim to evaluate how nutrient loads and yields change with distance downstream from where agricultural runoff enters local streams, with the goal of identifying where in the stream system the influence of agricultural practices on loads diminishes and where the influence of in-stream processes become more dominant.

Approach

To achieve this objective, researchers will conduct frequent monitoring of in-stream nutrient and suspended sediment loads, with an emphasis on phosphorus, over several years at multiple locations along the main stem of Walnut Creek and its tributaries in Story County. The 5200-hectare Walnut Creek watershed is representative of small to medium-sized catchments within the tile-drained landscape of central and north-central Iowa, and the results from this project are anticipated to be generally applicable to this region.