Central Illinois: Length of green season
Zoom control is at upper left. Adjust transparency with this slider,
to see road map underneath.
Then this slider allows you to see aerial map underneath.
days/yr
NDVI > .4
This map differs from many satellite views. It is generated from a time series, and shows, as a rough first draft, average length of green season from 2014 through 2017.
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 .4 (NDVI is usually between 0 and 1), on that 30-meter square pixel (about the size of a baseball infield, a quarter acre). You can spot a slight difference between the Landsat paths (slightly clockwise from north-south), because the calculation of length of green season depends on timing of observations, and Landsat is in polar orbit with a revisit time of 16 days. So the data is approximate.
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. To interpret this map, local knowledge is important.