GEE | 使用植被健康指数(VHI)进行干旱评估并导出逐年VHI指数影像

// 定义感兴趣区var aoi = ee.FeatureCollection("users/geestudy2/yibin");Map.addLayer(aoi)Map.centerObject(aoi, 8)
// 定义分析时间段var startDate = "2010-01-01"; var endDate = "2022-12-31"; var startDate = ee.Date(startDate); var endDate = ee.Date(endDate); 
// 获取MODIS影像集合并选择NDVI波段var filteredNDVI = ee.ImageCollection("MODIS/061/MOD13Q1").select('NDVI');
// ---------------- 计算植被状况指数（VCI） ----------------- //
// 定义计算VCI的函数function calcVCI(image) {  var min = filteredNDVI.reduce(ee.Reducer.min());  var max = filteredNDVI.reduce(ee.Reducer.max());  var vci = image.subtract(min).divide(max.subtract(min)).multiply(100);  return vci.rename('VCI').copyProperties(image, ['system:time_start', 'system:time_end']);}
// 应用函数计算VCIvar VCI = filteredNDVI.filterDate(startDate, endDate).map(calcVCI);
// -------------- 计算温度状况指数（TCI） --------------- //
// 获取MODIS陆地表面温度数据并选择白天温度数据（LST_Day_1km）var LST = ee.ImageCollection("MODIS/061/MOD11A2").select('LST_Day_1km');
// 定义将8天温度数据合并为16天的数据的函数（取平均）function convertLstDatesToModisDates(ndvimg) {  var start = ndvimg.get('system:time_start');  var end = ndvimg.get('system:time_end');
  var composite = LST.filterDate(start, end).mean(); // 使用均值合并影像
  return composite    .set('system:time_start', start)    .set('date', ee.Date(start).format())    .set('system:time_end', end)    .set('empty', composite.bandNames().size().eq(0));}
// 过滤温度数据，匹配NDVI数据的时间段var filteredlst = filteredNDVI.filterDate(startDate, endDate).map(convertLstDatesToModisDates);
// 定义计算TCI的函数function calcTCI(image1) {  var min = LST.reduce(ee.Reducer.min());  var max = LST.reduce(ee.Reducer.max());  var tci = max.subtract(image1).divide(max.subtract(min)).multiply(100);
  return tci.rename('TCI').copyProperties(image1, ['system:time_start', 'system:time_end']);}
// 应用函数计算TCIvar TCI = filteredlst.filterDate(startDate, endDate).map(calcTCI);
// ------------------- 计算植被健康指数（VHI） ---------------------- //
// 定义过滤条件，将VCI和TCI进行配对var filter = ee.Filter.equals({leftField: 'system:time_start', rightField: 'system:time_start'});var join = ee.Join.saveFirst({matchKey: 'match'});
// 将VCI和TCI集合进行配对合并var both = ee.ImageCollection(join.apply(VCI, TCI, filter))  .map(function(img) {    return img.addBands(img.get('match')).set('date', img.date().format('YYYY_MM_dd'));  });
// 计算VHI并将其添加为集合中的一个波段var VHI = both.map(function(img) {  return img.addBands(    img.expression('a1/2 + b1/2', {      "a1": img.select('VCI'),      "b1": img.select('TCI'),    }).rename('VHI')  );});
// 创建每年5月对应的VHI影像列表var imageList = VHI.toList(VHI.size());var my_indices = ee.List([9,32,55,78,101,124,147,170,193,216,239,262,285]); 
// 使用索引选择影像function getImages(list){  var img = ee.Image(imageList.get(list));  return img;} 
// 创建包含所选影像的影像集合var finalCollection = ee.ImageCollection.fromImages(my_indices.map(getImages));
// 将每年的VHI波段提取并裁剪到感兴趣区var VHI_Collection = finalCollection.map(function(image) {  return image.select("VHI").clip(aoi);});
// 导出每年VHI的结果到Google Drivefor (var i = 0; i < my_indices.length().getInfo(); i++) {  var img = ee.Image(VHI_Collection.toList(VHI_Collection.size()).get(i));  var year = 2010 + i;  Export.image.toDrive({    image: img,    description: 'VHI_' + year,    folder: 'VHI_Export',    scale: 500,    region: aoi.geometry(),    crs: 'EPSG:4326',    maxPixels: 1e13  });}
// 可视化VHI的参数var VisParams = {  min: 0,  max: 100,  palette: ['#d73027', '#fdae61', '#ffffbf', '#a6d96a', '#1a9850']};
// 显示2014年的VHI影像var VHI_2020 = VHI_Collection.toList(VHI_Collection.size()).get(10);Map.addLayer(ee.Image(VHI_2020), VisParams, 'VHI 2020');
// ------------------------- 图例 --------------------------//
// 定义图例信息var classInfo = [  {name: '极端干旱', color: '#d73027'},  {name: '严重干旱', color: '#fdae61'},  {name: '中度干旱', color: '#ffffbf'},  {name: '轻度干旱', color: '#a6d96a'},  {name: '无干旱', color: '#1a9850'}];
// 创建图例条目：颜色和标签并排显示function legendEntry(info) {  var color = ui.Panel({style: {    width: '20px',    height: '20px',    backgroundColor: info.color,    margin: '6px 0px 0px 0px'  }});  var label = ui.Label({    value: info.name,  });  return ui.Panel({    widgets: [color, label],    layout: ui.Panel.Layout.flow('horizontal'),    style: {      stretch: 'horizontal',      margin: '-6px 0px 0px 0px'    }});}
// 定义用于保存所有图例条目的面板var legend = ui.Panel({  style: {    position: 'top-left',    padding: '8px 8px 0px 8px'   }});
// 创建图例标题var legendTitle = ui.Label({  value: 'VHI 2020',  style: {    fontWeight: 'bold',    fontSize: '18px',    margin: '0 0 4px 0',    padding: '0'}});
// 将标题添加到面板legend.add(legendTitle);
// 遍历类信息，将每个条目添加到图例面板for (var i = 0; i < classInfo.length; i++) {  legend.add(legendEntry(classInfo[i]));}
// 将图例显示在地图上Map.add(legend);
// ------------------- 绘制每年5月的VHI折线图 ---------------------- //
// 获取每年5月的VHI均值var vhiMeans = finalCollection.map(function(image) {  var mean = image.reduceRegion({    reducer: ee.Reducer.mean(),    geometry: aoi,    scale: 500  }).get('VHI');    return ee.Feature(null, {'VHI': mean});});
// 创建年份列表var years = ee.List.sequence(2010, 2022);
// 将年份和VHI值组合成特征集合var chartData = ee.FeatureCollection(years.zip(vhiMeans.toList(vhiMeans.size())).map(function(val) {  val = ee.List(val);  return ee.Feature(null, {    'year': val.get(0),    'VHI': ee.Feature(val.get(1)).get('VHI')  });}));
// 将特征集合转换为图表var chart = ui.Chart.feature.byFeature(chartData, 'year', ['VHI'])  .setOptions({    title: 'VHI 每年5月平均值',    hAxis: {title: '年份'},    vAxis: {title: 'VHI 值'},    lineWidth: 2,    pointSize: 4,  });
// 显示折线图print(chart);