Integrating Modeling and Management of Agriculturally-impacted Watersheds: Issues of Spatial and Temporal Scale

Project Staff: 

Principal Investigators: Patrick L. Brezonik, Professor, Department of Civil Engineering, K. William Easter, Professor, Department of Applied Economics, Luther Gerlach, Lorin Hatch, David Mulla, W. E. Larson Chair, Department of Soil, Water, and Climate, James Perry, Professor, Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota

Funding: 

U.S. Environmental Protection Agency

Project Duration: 

October 1996 - September 2000

Summary: 

The primary objective of this study was to improve understanding of how biophysical and socioeconomic variables interact in agricultural watersheds of varying scales (sizes) and landscape conditions to affect export of nutrients and to assess their effects on in-stream biological communities. In particular, we focused on evaluating the usefulness of agroecoregions, landscape units roughly comparable in size to major watersheds, in understanding and managing nutrient pollution in large, agricultural drainage basins. A second objective was to assess the role of scientific knowledge about the above issues in decision-making processes affecting local-level land-use management.

The project took a multi-disciplinary approach to study the spatial and temporal scales at which landscape and socioeconomic factors influence water quality degradation in the Minnesota River Basin. We hypothesized that the optimum scale to understand and manage degradation of water quality, aquatic habitats, biotic integrity, and the socioeconomic factors affecting them occurs within agroecoregion boundaries, rather than watershed boundaries. The hypothesis was tested through hierarchical sampling of soil properties, chemical water quality, aquatic habitat, and aquatic biota. In addition, we conducted stakeholder surveys, computer modeling, and statistical evaluations of long-term stream, lake, and ground water quality data.

Agroecoregions are landscape units with relatively uniform crop productivity, climate, geologic parent material, soil drainage, and slope steepness; large watersheds generally are composed of two or more agroecoregions. We found that the variance in soil erosion, stream biotic habitat, stream water quality, lake water quality, and ground water quality was smaller within agroecoregion boundaries than within watershed boundaries. Through linked biophysical and economic modeling, we found that the economic costs of reducing phosphorus loads to streams were lower when best management practices (BMPs) were targeted to specific agroecoregions compared with an untargeted strategy involving entire watersheds. We developed an eight-step framework for restoring and managing watersheds that includes specific steps for defining agroecoregions, prioritizing pollutant loads, and identifying BMPs for each agroecoregion. Finally, we developed a manual describing agroecoregion-specific BMPs to be used in reducing pollutant loads to the Minnesota River Basin. We suggest that watershed management in highly agricultural watersheds will be most effective when watershed boundaries are complemented by agroecoregions to identify and target regions where specific combinations of BMPs for agricultural sediment, nitrogen, and phosphorus abatement are most appropriate.

This project resulted in a wide range of published and presented work, including ten M.S. theses and six Ph.D. dissertations. Collectively, we have presented 23 papers to date and have written 20 technical papers on the findings of the project; abstracts of papers published or presented are found in Chapter 3 of this report. This study was funded by the U.S. Environmental Protection Agency as Project 825290 of the Water and Watersheds Program. Barbara Levinson was the project officer.