前言：本文主要介绍Modis下的NDWI数据集的获取。归一化差异水指数 (NDWI) 对植被冠层液态水含量的变化很敏感。它来自近红外波段和第二个红外波段，可用时约为 1.24μm，否则为最接近的可用红外波段。

---

0 数据介绍

NDWI介绍

归一化差异水指数 (NDWI) 对植被冠层液态水含量的变化很敏感。它来自近红外波段和第二个红外波段，可用时约为 1.24μm，否则为最接近的可用红外波段。其值范围为 -1.0 到 1.0。有关详细信息，请参阅 Gao (1996) 。该产品由 MODIS/006/MOD09GA 表面反射率合成图生成。
链接：https://developers.google.com/earth-engine/datasets/catalog/MODIS_MOD09GA_006_NDWI#description

数据集引用

ee.ImageCollection(“MODIS/MOD09GA_006_NDWI”)

MOD09GA介绍

MODIS 表面反射率产品提供了表面光谱反射率的估计值，因为它是在没有大气散射或吸收的情况下在地面测量的。低水平数据针对大气气体和气溶胶进行了校正。MOD09GA 版本 6.1 在正弦投影的每日网格 L2G 产品中提供了 1-7 波段，包括 500m 反射率值和 1km 观测和地理位置统计数据。

数据集引用

ee.ImageCollection(“MODIS/061/MOD09GA”)

1 NDWI天数据下载

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
Map.centerObject(geometry,6);
var dataset = ee.ImageCollection("MODIS/MOD09GA_006_NDWI")
.filter(ee.Filter.date('2020-01-01', '2020-01-03'))
.filterBounds(geometry)
.select('NDWI');
var colorizedVis = {
  min: 0.0,
  max: 1.0,
  palette: ['0000ff', '00ffff', 'ffff00', 'ff0000', 'ffffff'],
};

function exportImageCollection(imgCol) {
  var indexList = imgCol.reduceColumns(ee.Reducer.toList(), ["system:index"]).get("list");
  indexList.evaluate(function(indexs) {
    for (var i=0; i<indexs.length; i++) {
      var image = imgCol.filter(ee.Filter.eq("system:index", indexs[i])).first();
      image = image.clip(geometry);
      Map.addLayer(image, colorizedVis,  indexs[i]);//查看具体的遥感影像
      //tif数据下载
      Export.image.toDrive({
        image: image,
        description: 'modis_ndwi_'+indexs[i],
        fileNamePrefix: 'modis_ndwi_'+indexs[i],
        folder: 'Modis',
        region: geometry,
        scale: 500,
        crs: "EPSG:4326",
        maxPixels: 1e13
      });
    }
  });
}
exportImageCollection(dataset);

2 NDWI月数据下载

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
var dataset = ee.ImageCollection("MODIS/MOD09GA_006_NDWI");
for(var i=2018;i<=2020;i++){
  for(var j=1;j<=12;j++){
    var data_collection = null;
    switch(j){
      case 1:
      case 3:
      case 5:
      case 7:
      case 8:
      case 10:
      case 12:
        data_collection = dataset.filterDate(i+'-'+j+'-01',i+'-'+j+'-31').select('NDWI');
        break;
      case 4:
      case 6:
      case 9:
      case 11:
        data_collection = dataset.filterDate(i+'-'+j+'-01',i+'-'+j+'-30').select('NDWI');
        break;
      case 2:
        data_collection = dataset.filterDate(i+'-'+j+'-01',i+'-'+j+'-28').select('NDWI');
        break;
    }
    var YR_collection = data_collection.mean().clip(geometry);
    Export.image.toDrive({
      image: YR_collection,
      description: i+'-'+j,
      fileNamePrefix: i+'-'+j,
      scale: 500,
      region: geometry,
      crs: "EPSG:4326",//地理投影WGS1984
      maxPixels: 1e13,
      folder: 'Modis'
    })
  }
}

3 NDWI年数据下载

var geometry = ee.FeatureCollection('users/www1573979951/HN_full');
var dataset = ee.ImageCollection("MODIS/MOD09GA_006_NDWI");
for(var i=2000;i<=2000;i++){
  var data_collection = dataset.filterDate(i+'-01-01',i+'-12-31').select('NDWI');
  print(data_collection)
  var YR_collection = data_collection.mean().clip(geometry);
  Export.image.toDrive({
    image: YR_collection,
    description: "HN_NDWI-"+i,
    fileNamePrefix: "HN_NDWI-"+i,
    scale: 500,
    region: geometry,
    maxPixels: 1e13,
    crs: "EPSG:4326",//投影方式
    folder: 'Modis'
  });
}