These maps differ from many satellite views. They are generated from a time series, and show, as a first draft, average length of green season from 2014 through 2017 for each 30m Landsat pixel.
Green growing plants reflect a lot of near-infrared radiation, and they absorb red. The ratio, called normalized difference vegetation index (NDVI), is an indicator of the work of photosynthesis. Landsat, which passes over every 16 days, can measure NDVI over each quarter acre pixel when there are no clouds.
The colors on the map represent the number of days in the year that NDVI is observed to be over a threshold, in this case .33 (NDVI is usually between 0 and 1). The brown pixels represent green season shorter than 80. The darker the green, the longer the green season. In general, you need to know the area to interpret this.
How long is the soil foodweb getting fed by carbohydrates from the roots of green and growing plants? When soil life is not being fed, it may starve or go dormant, and the work of growing and maintaining soil structure and aggregation is not being done. This has major implications for water quality, erosion, and soil health.
Length of green season is about soil health principles over time. As you can see from the map, management has major influence.
I will be working hard on developing this as an open and accessible monitoring and feedback tool for land managers, conservation districts and organizations, and watershed groups--for anyone who is interested in soil health and watershed function.
Soil Carbon Coalition is a 501(c)3 nonprofit organization