Identifying Changes in Land Use and Highly Erodible Soil

As part of the geography team, my task was to map highly erodible soils, and to get an estimate of how much phosphorus went into the waterways due to land use change from 2011 to 2015. This was done using a Geographic Information System (ArcGIS), which is computer software used to map and analyze spatial data. GIS data comes in the form of layers, which contain an attribute table describing the features of the data.

To map highly erodible soils, I first had to download the soils data layer for the state of Wisconsin. This layer uses polygons to visualize the shape and area of the different types of soils. It contained a field in its attribute table with a code for its soil type. These codes vary by county, so in order to find out which ones were highly erodible, I contacted NRCS district conservationists for counties within the Red Cedar watershed and asked them for their list of codes for highly erodible soils. I then selected the different codes from the table using a tool called Select by Attribute, and exported that data into a new layer containing only highly erodible soils. I found there were about 418,000 acres of highly erodible soils in the Red Cedar watershed.

To estimate how much phosphorus was going into the waterways due to changes in land use, I had to find out how much land changed from grassland to cropland. The reason behind this was that no topsoil is lost when there is grass or pasture, but when the land gets farmed and plowed, there is significant loss of topsoil. Plowing leaves the soil exposed to the elements and when it rains, this soil erodes and washed out into the waterways. This soil contains phosphorus, which is what causes the algal blooms. To calculate the change between these two types of land cover, I downloaded satellite imagery from 2011 and 2015 from the USDA. This imagery was classified according to the type of land cover and the different types of crops. I reclassified the two images into two classes, grassland and cropland. I then input these two images in Image Analysis and used the Difference tool, which resulted in a map with three classes, change from grassland to cropland, change from cropland to grassland, and no change. I then calculated the area and found that 8,738 acres changed from cropland to grassland, and 82,047 acres changed from grassland to cropland. Assuming farming at state standards, this means 1,312,752 tons of topsoil were lost from 2011 to 2015. Taking the average soil P test for Dunn and Barron counties of 52ppm, we get 75.25 tons of added phosphorus to waterways. Finally, I reclassified one more time to get only cropland, and overlayed this layer with the highly erodible soils layer and calculated the area. I found that about 85,000 acres, (20%) of highly erodible land are being farmed on.

The maps I produced can help illustrate what happens when there is a big increase in farming. This increase can potentially be explained by rising corn prices. My research found that enrollment in conservation programs such as CREP decreased as prices in corn increased. My map of farmland on highly erodible soil can also help in identifying which areas to target for a bigger push in conservation agriculture.