Google Earth Engine – Why LST Time Series Chart Shows Up as a Line

google-earth-engineland-surface-temperaturetime series

Here is the code I am using to create a time series chart representing the LST over a three month period in 2018. However, the mean LST in the chart appears as a straight line representing one temperature. I don't believe this to be accurate.

 var col = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
.map(maskL8sr)
.filterDate('2018-06-01','2018-08-31')
.filterBounds(geometry);
print(col, 'colection');


//cloud mask
function maskL8sr(col) {
  // Bits 3 and 5 are cloud shadow and cloud, respectively.
  var cloudShadowBitMask = (1 << 3);
  var cloudsBitMask = (1 << 5);
  // Get the pixel QA band.
  var qa = col.select('pixel_qa');
  // Both flags should be set to zero, indicating clear conditions.
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
                 .and(qa.bitwiseAnd(cloudsBitMask).eq(0));
  return col.updateMask(mask);
}

//vis params
var vizParams = {
bands: ['B5', 'B6', 'B4'],
min: 0,
max: 4000,
gamma: [1, 0.9, 1.1]
};

var vizParams2 = {
bands: ['B4', 'B3', 'B2'],
min: 0,
max: 3000,
gamma: 1.4,
};

//image reduction
{
var image = col.median();
print(image, 'true color composite');
Map.addLayer(image, vizParams2);
}

var ndvi = image.normalizedDifference(['B5', 
'B4']).rename('NDVI');
var ndviParams = {min: -1, max: 1, palette: ['blue', 'white', 
'green']};
print(ndvi,'ndvi');
Map.addLayer(ndvi, ndviParams, 'ndvi');


//select thermal band 10(with brightness tempereature), no calculation 
var thermal = image.select('B10').multiply(0.1);
var b10Params = {min: 291.918, max: 302.382, palette: ['blue', 
'white', 'green']};
Map.addLayer(thermal, b10Params, 'thermal');

// find the min and max of NDVI
{
var min = ee.Number(ndvi.reduceRegion({
reducer: ee.Reducer.min(),
geometry: geometry,
scale: 30,
maxPixels: 1e9
}).values().get(0));
print(min, 'min ndvi');
var max = ee.Number(ndvi.reduceRegion({
reducer: ee.Reducer.max(),
geometry: geometry,
scale: 30,
maxPixels: 1e9
}).values().get(0));
print(max, 'max ndvi')
}

//fractional vegetation
{
var fv =(ndvi.subtract(min).divide(max.subtract(min))).pow(ee.Number(2)).rename('FV'); 
print(fv, 'fractional vegetation');
Map.addLayer(fv);
}

//Emissivity

var a= ee.Number(0.004);
var b= ee.Number(0.986);
var EM=fv.multiply(a).add(b).rename('EMM');
var imageVisParam3 = {min: 0.9865619146722164, max:0.989699971371314};
Map.addLayer(EM, imageVisParam3,'EMM');

//LST in Celsius Degree 
function addLST (image) {
  var LST = thermal.expression(
  '(Tb/(1 + (0.00115* (Tb / 1.438))*log(Ep)))-273.15', {
  'Tb': thermal.select('B10'),
  'Ep': EM.select('EMM')
  }).rename('LST');
Map.addLayer(LST, {min: 20.569706944223423, max:29.328077233404645, palette: [
'040274', '040281', '0502a3', '0502b8', '0502ce', '0502e6',
'0602ff', '235cb1', '307ef3', '269db1', '30c8e2', '32d3ef',
'3be285', '3ff38f', '86e26f', '3ae237', 'b5e22e', 'd6e21f',
'fff705', 'ffd611', 'ffb613', 'ff8b13', 'ff6e08', 'ff500d',
'ff0000', 'de0101', 'c21301', 'a71001', '911003'
 ]},'LST');
  return image.addBands(LST);
  
}

var withLST = col.map(addLST);
print('withLST', withLST);


 // Create an image time series chart.
var chart = ui.Chart.image.doySeriesByRegion
({imageCollection: withLST, 
bandName: "LST",
regions: geometry,
//regionReducer: geometry, 
scale: 200, 
yearReducer: ee.Reducer.mean(), 
//seriesProperty, 
startDay: 153, 
endDay: 244});

// Add the chart to the map.
chart.style().set({
  position: 'bottom-right',
  width: '500px',
  height: '300px'
});
Map.add(chart);
Map.centerObject(geometry);

Here is an image of what the time series chart shows up as:
LST Time Series Chart shows single temperature

Best Answer

The problem is to use a reducted collection (ee.Image) instead of a full collection in LST calculation, since thermal comes from image object:

function addLST (image) {
  var LST = thermal.expression(
  '(Tb/(1 + (0.00115* (Tb / 1.438))*log(Ep)))-273.15', {
  'Tb': thermal.select('B10'),
  'Ep': EM.select('EMM')
  }).rename('LST');

Use a function for mapping over the collection without using the reduced collection:

function addLST (image) {
  var LST =  image.expression(
  '(Tb/(1 + (0.00115* (Tb / 1.438))*log(Ep)))-273.15', {
  'Tb': image.select('B10').multiply(0.1),
  'Ep': EM.select('EMM')
  }).rename('LST').select('LST');
  return image.addBands(LST)
  };

Since you already defined image, you can use the following code as well:

function addLST (img) {
  var LST =  img.expression(
  '(Tb/(1 + (0.00115* (Tb / 1.438))*log(Ep)))-273.15', {
  'Tb': img.select('B10').multiply(0.1),
  'Ep': EM.select('EMM')
  }).rename('LST').select('LST');
  return img.addBands(LST)
  };

I discourage the use of image as a product, try to use it only in functions

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