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Case Study: Iowa Cities Struggle to Keep Farm Pollution Out of Tap Water
January 11, 2018

Case Study: Iowa Cities Struggle to Keep Farm Pollution Out of Tap Water: Methodology

Methodology

The U.S. Geological Survey national hydrography dataset, the Iowa Department of Natural Resources channelized stream layer, and the Iowa Flood Center top-of-bank data were used to establish a footprint for surface water. Those data were subset to remove all public lands, forest, residential area and any land tracts with less than 10 percent area in cultivated land.

The U.S. Department of Agriculture Farm Service Agency Common Land Units, or CLU, were used to calculate the percent of agricultural area by summarizing pixels of corn and soybeans taken from the USDA’s Cropland Data Layer.

The buffer universe was created by using a 100-foot and 50-foot distance from all agricultural waterways so long as it intersected with CLU classified as agriculture.

For buffer detection, vegetation was detected from the Normalized Difference Vegetative Index, or NDVI, calculated from 5-meter resolution Rapid Eye imagery from Planet. All vegetation was detected between mid-to-late May 2009 and 2010 and mid-to-late May 2015 and 2016. The grass universe was used to calibrate the existence and longevity of buffers within the buffer universe.

Landsat 5 TM and Landsat 8 OLI were used to detect the emergence and vigor of grass using derived NDVI to capture the field level persistence and vigor during the spring and fall. All cover crop acres were subset to areas of corn and soybeans using the USDA’s Cropland Data Layer.