Sentinel1 water cover_Yearly

// 利用VV VH 推估出水體 正確性需要再研究
// Define ROI for wetland
var coords = [
  [[121.417681921666, 25.1679985333518], [121.42212264459, 25.1638653264896], 
   [121.422161548077, 25.1637361229219], [121.421669436939, 25.163630163993], 
   [121.421440399281, 25.1636005794512], [121.421096642751, 25.1636413205843], 
   [121.420683026734, 25.1637114307947], [121.419374133901, 25.1640261198785], 
   [121.418360829856, 25.1640829598024], [121.417607211616, 25.1639807594822], 
   [121.417466724646, 25.163782852302], [121.417270606106, 25.1635927169186], 
   [121.417148764897, 25.1631368332639], [121.41714080673, 25.1629704654714], 
   [121.417085425471, 25.1629413481011], [121.416902640288, 25.1630097979222], 
   [121.416253909006, 25.1634634219578], [121.41595283846, 25.1643084945578], 
   [121.416177946648, 25.1649199451916], [121.416235434296, 25.165157048303], 
   [121.416238515845, 25.1660648530044], [121.416476181469, 25.1673870830101], 
   [121.417681921666, 25.1679985333518]]
];

var roi = ee.Geometry.MultiPolygon(coords);
Map.addLayer(roi, {color: 'red'}, 'ROI');

// Define the time range
var startDate = '2015-01-01';
var endDate = '2023-12-31';

// Load Sentinel-1 GRD data
var sentinel1 = ee.ImageCollection('COPERNICUS/S1_GRD')
  .filterBounds(roi)
  .filterDate(startDate, endDate)
  .filter(ee.Filter.eq('instrumentMode', 'IW'))
  .filter(ee.Filter.listContains('transmitterReceiverPolarisation', 'VV'))
  .filter(ee.Filter.listContains('transmitterReceiverPolarisation', 'VH'))
  .select(['VV', 'VH']);

// Function to calculate NDWI
var calculateNDWI = function(image) {
  var vv = image.select('VV');
  var vh = image.select('VH');
  var ndwi = vv.subtract(vh).divide(vv.add(vh)).rename('NDWI');
  return image.addBands(ndwi);
};

// Apply the NDWI calculation function to the collection
var ndwiCollection = sentinel1.map(calculateNDWI);

// Calculate yearly mean NDWI
var years = ee.List.sequence(2015, 2023);
var yearlyNDWI = ee.ImageCollection.fromImages(
  years.map(function(year) {
    var start = ee.Date.fromYMD(year, 1, 1);
    var end = start.advance(1, 'year');
    var annualNDWI = ndwiCollection
      .filterDate(start, end)
      .mean()
      .set('year', year);
    return annualNDWI;
  })
);

// Extract NDWI values for charting
var chartData = yearlyNDWI.toList(yearlyNDWI.size()).map(function(image) {
  var ndwiValue = ee.Image(image).reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: roi,
    scale: 10,
    maxPixels: 1e9
  }).get('NDWI');
  return ee.Feature(null, {
    'year': ee.Image(image).get('year'),
    'NDWI': ndwiValue
  });
});

// Create a chart to display NDWI over time
var chart = ui.Chart.feature.byFeature(chartData, 'year', 'NDWI')
  .setChartType('LineChart')
  .setOptions({
    title: 'NDWI Variation from 2015 to 2023',
    vAxis: {title: 'NDWI', minValue: -0.3, maxValue: -0.15}, // Adjust Y axis scale here
    hAxis: {title: 'Year'},
    lineWidth: 2,
    pointSize: 5,
    series: {
      0: {color: 'blue'}
    }
  });

// Display the chart
print(chart);