Cover Crop Mapping Project

Cereal Rye Cover Crop Field – Photo by Jason Johnson

Water quality issues are very important in the State of Iowa. For the last decade, research has been focused on meeting priorities set in the Iowa Nutrient Reduction Strategy. Cover crops have been identified as one potential solution for nutrient management. Cover crops are commonly planted in late summer or early fall around harvest time and terminated in the spring. The goal in planting cover crops is to protect exposed cropland from wind and water erosion during the months when cash crops are not growing.

There is a need by entities in Iowa, including current IowaView partners, to have recent and reliable cover crop data. The goal of this project is to create a repeatable and shareable process for detecting cover crops using publicly available imagery and data products.

NDVI Fall Image Osceola County, Iowa – 11/21/2017
NDVI Early Spring Image Osceola County, Iowa – 4/25/2018
NDVI Late Spring Image Osceola County, Iowa – 5/25/2018

The first year of our research focused on understanding the behavior of cover crops in Iowa, determining a methodology for distinguishing cover crops on the landscape, and developing a model for repetition. The second year goals of the project built upon the first by creating a tool that allows users to input a series of Sentinel 2 images (late fall, early spring, and late spring) and receive an output of potential cover crop locations based on the collective Normalized Difference Vegetation Index (NDVI; this index shows greenness of vegetation on the landscape) value (ranging from -1 to 1, with higher values indicating greater vitality and plant health). The tool was then used to create another deliverable: a statewide map showing potential cover crops.

The tool was developed to operate as three consecutive steps. The first step produced an NDVI image for each of the three Sentinel 2 input images and created a geodatabase for each scene. This geodatabase contains four Sentinel 2 bands, a Normalized Difference Vegetation Index raster for the image, a field boundaries layer that has zonal statistics for each field, and a zonal statistics table.

The toolbox uses the NDVI values to compare pixels in a scene. The project chose NDVI to reveal areas of intentional growth by observing a pattern of greenness from fall through spring. During the months after harvest, it was assumed that fields would be bare in areas not specifically planted to cover crop or covered in permanent vegetation ,such as forest or grass.

The second step examined each pixel of the NDVI images and assigned values indicating low, moderate, and high growth/greenness. Known cover crop planted field data from partner organizations was used to determine the NDVI cutoff values. Then, the values from each of the images were combined to create a composite image with 15 possible values indicating likelihood of cover crop.

The upper image is an example of tool output and the lower image is a generalization of the output data using field boundaries in Osceola County, Iowa. Light blue dots denote fields from a samplecover crop data set. Gray areas indicate No Cover Crop; green areas are Potential Cover Crop and orange areas are Cover Crop.

The final step of the tool reclassified the composite image to three categories based on the sequence of NDVI values: No Cover Crop, Potential Cover Crop, and Cover Crop. These raster values were then summarized by field boundary to give a generalized indication of cover crop potential in the field.

Staff are preparing the toolbox for public release along with guide with documentation. We will be posting these products here as they are completed. Please check back again soon.

A recent poster detailing the project – Spring 2021