Exporting a CSV of Mean EVI Landsat 8 Values using reduceRegion function from Google Earth Engine (GEE)

Question

I am trying to export a CSV file from Google Earth Engine (GEE) that reflects the mean EVI value per census tract. I have script that does this exact thing for NDVI from Landsat 8 but cannot seem to get the code to work for EVI. I understand that in my NDVI script I have specific bands but being unfamiliar with Java I cannot seem to figure out how to edit it for EVI.

Script to Export mean NDVI per Census Tract:

var collection = ee.ImageCollection('LANDSAT/LC08/C01/T1')
    .filterDate('2015-05-01', '2015-09-30')


var count = collection.size();
print('Number of images in collection:', count)

var composite = ee.Algorithms.Landsat.simpleComposite(collection)

var compBands = composite.bandNames()
print('Composite bands:', compBands)

var NDVIcomp = composite.normalizedDifference(['B5', 'B4']).rename('NDVI')

Map.addLayer(NDVIcomp, {min: -1, max: 1, palette: ['blue', 'white', 'green']}, 'NDVI_')
Map.addLayer(UStracts)

var bandNames = NDVIcomp.bandNames()
print('Band names:', bandNames)

//US Census Tracts
var meansCT = NDVIcomp.reduceRegions({
  collection: UStracts,
  reducer: ee.Reducer.mean(),
  scale: 30,
});

Export.table.toDrive({
    collection: meansCT, 
    description: 'US Tracts', 
    folder: 'Aim1', 
    fileNamePrefix: 'UStracts', 
    fileFormat: 'CSV'
})

Answer 1

You should focus your question as much as possible. I this case, I suppose that you actually want to know how to calculate EVI? Also, make sure you have a fully functional script, ideally with a link to the Earth Engine Code Editor script (use the Get Link button). Here, UStracts is undefined, so I cannot execute your script.

Here's one way to calculate EVI:

var composite = ee.Algorithms.Landsat.simpleComposite(collection)
  .select( // Rename used bands
    ['B2', 'B3', 'B4', 'B5'],
    ['blue', 'green', 'red', 'nir']
  )

var EVIcomp = ee.Image().expression(
  'G * ((i.nir - i.red) / (i.nir + C1 * i.red - C2 * i.blue + L))', 
  {
    i: composite.divide(255), // simpleComposite() multiply by 255
    L: 1,
    C1: 6,
    C2: 7.5,
    G: 2.5
  }
).rename('EVI')

https://code.earthengine.google.com/955834ef6f91ae1a3c7977b187d4f950