Introduction to NDVI Time-Series

While a single NDVI image provides a snapshot of vegetation health, a **time-series** analysis unlocks the ability to see how forests change over months and years. By plotting the NDVI for a specific location through time, we can track the **phenology**—or seasonal life cycle—of a forest, observing its green-up in the wet season and senescence in the dry season. This is essential for monitoring forest health, detecting the impacts of drought, assessing recovery after disturbances like fires or logging, and identifying subtle, long-term degradation that might not be visible in a single image.

Analysis Workflow

Chart Visualization

The primary output of this analysis is not a map layer, but an interactive chart that appears in the GEE Console. The chart plots NDVI on the y-axis against time on the x-axis, revealing the vegetation's seasonal cycles.

GEE Code Snippet

This script generates a time-series chart of NDVI for a specific point over several years, showing seasonal vegetation cycles.

// NOTE: You must define a 'roi' Point or Polygon before running.
Map.centerObject(roi, 17);
Map.addLayer(roi, {color: 'FF0000'}, 'Region of Interest');

// Function to scale and mask Landsat 8.
function scaleAndMaskL8(image) {
  var qa = image.select('QA_PIXEL');
  var cloudBitMask = 1 << 3;
  var cloudShadowBitMask = 1 << 4;
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0).and(qa.bitwiseAnd(cloudShadowBitMask).eq(0));
  var opticalBands = image.select('SR_B.').multiply(0.0000275).add(-0.2);
  return image.addBands(opticalBands, null, true).updateMask(mask);
}

// Function to calculate NDVI.
function addNDVI(image) {
  var ndvi = image.normalizedDifference(['SR_B5', 'SR_B4']).rename('NDVI');
  return image.addBands(ndvi);
}

// Load and process the image collection.
var l8Collection = ee.ImageCollection('LANDSAT/LC08/C02/T1_L2')
                      .filterDate('2014-01-01', '2025-12-31')
                      .filterBounds(roi)
                      .map(scaleAndMaskL8)
                      .map(addNDVI);

// Create the time-series chart.
var ndviChart = ui.Chart.image.series({
  imageCollection: l8Collection.select('NDVI'),
  region: roi,
  reducer: ee.Reducer.mean(),
  scale: 30,
  xProperty: 'system:time_start'
}).setOptions({
  title: 'Landsat 8 NDVI Over Time',
  vAxis: {title: 'NDVI Value'},
  hAxis: {title: 'Date', format: 'YYYY-MM'},
  interpolateNulls: true,
});

// Print the chart to the console.
print(ndviChart);

Analysis & Results