一、说明
MOD16A2.061是基于Penman-Monteith方程的8天复合蒸散量产品，空间分辨率为500米，时间范围自2001年起至今。基于GEE对ET进行时间变化分析，包括日、月、季节和年，并导出对应的栅格数据。
各位同学可以根据需求更改研究区、时间段和分辨率，当然也可以分析其他产品数据。
二、日变化特征
var geometry = table;Map.centerObject(geometry, 7);var dataset = ee.ImageCollection("MODIS/006/MOD16A2").filterDate('2018-01-01', '2020-12-31').filterBounds(geometry).select('ET');var convertET = function(image) {var img = image.multiply(0.1);return img.set('system:time_start', image.get('system:time_start'));};dataset = dataset.map(convertET);if (dataset.size().getInfo() === 0) {print('Error: No data found for the given time and region');} else {print(ui.Chart.image.series(dataset, geometry, ee.Reducer.mean(), 1000));}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);Export.image.toDrive({image: image,description: 'MODIS_ET_' + indexs[i],fileNamePrefix: 'MODIS_ET_' + indexs[i],folder: 'MODIS_ET_Images',region: geometry,scale: 1000,crs: "EPSG:4326",maxPixels: 1e13});}});}exportImageCollection(dataset);
三、月尺度
var geometry = ee.FeatureCollection('users/yusuhenhao/beijing');Map.centerObject(geometry, 8);var dataset = ee.ImageCollection("MODIS/006/MOD16A2").select('ET');var monthlyET = ee.FeatureCollection([]);for (var i = 2015; i <= 2020; i++) {for (var j = 1; j <= 12; j++) {var startDate = ee.Date.fromYMD(i, j, 1);var endDate = startDate.advance(1, 'month').advance(-1, 'day');var data_collection = dataset.filterDate(startDate, endDate).sum();var monthlySum = data_collection.multiply(0.1).clip(geometry).set({'year': i,'month': j,'system:time_start': startDate.millis()});var meanET = monthlySum.reduceRegion({reducer: ee.Reducer.mean(),geometry: geometry,scale: 1000,maxPixels: 1e13}).get('ET');var feature = ee.Feature(null, {'time': startDate.format('YYYY-MM'),'ET_mean': meanET});monthlyET = monthlyET.merge(ee.FeatureCollection([feature]));Export.image.toDrive({image: monthlySum,description: i + '_' + j + '_ET',fileNamePrefix: i + '_' + j + '_ET',folder: 'MOD17A2H_ET',scale: 1000,region: geometry,crs: "EPSG:4326",maxPixels: 1e13});}}var chart = ui.Chart.feature.byFeature(monthlyET, 'time', 'ET_mean').setChartType('LineChart').setOptions({title: 'Monthly ET (2018-2020)',hAxis: {title: 'Time',slantedText: true,slantedTextAngle: 45},vAxis: {title: 'ET (mm)'},lineWidth: 2,pointSize: 4,colors: ['#1f77b4']});print(chart);
四、季节尺度
春：3-5月，夏：6-8月，秋：9-11月，冬：12-次年2月
var geometry = table;var etCollection = ee.ImageCollection("MODIS/006/MOD16A2").select('ET');var seasons = {'spring': [3, 4, 5],'summer': [6, 7, 8],'autumn': [9, 10, 11],'winter': [12, 1, 2]};var table = ee.List([]);for (var year = 2015; year <= 2020; year++) {for (var season in seasons) {var months = seasons[season];var startYear = year;var endYear = (season === 'winter') ? year + 1 : year;var seasonalCollection = ee.ImageCollection(months.map(function(month) {var startDate = ee.Date.fromYMD(month === 12 ? year : startYear, month, 1);var endDate = startDate.advance(1, 'month');return etCollection.filterDate(startDate, endDate).sum().multiply(0.1); // 单位转换为毫米}));var seasonalET = seasonalCollection.sum().clip(geometry);var meanET = seasonalET.reduceRegion({reducer: ee.Reducer.mean(),geometry: geometry,scale: 500,maxPixels: 1e13}).get('ET');var row = ee.Feature(null, {'Year': year,'Season': season,'Mean_ET': meanET});table = table.add(row);Export.image.toDrive({image: seasonalET,description: year + '_' + season + '_ET',fileNamePrefix: year + '_' + season + '_ET',scale: 500,crs: 'EPSG:4326',region: geometry,maxPixels: 1e13,folder: 'seasonal_ET_data'});}}var featureCollection = ee.FeatureCollection(table);Export.table.toDrive({collection: featureCollection,description: 'Seasonal_ET_Table',fileNamePrefix: 'seasonal_et',fileFormat: 'CSV'});
五、生长季
代码中设置为了4-9月，可以根据研究区的生长季时间修改。
var geometry = table;Map.centerObject(geometry, 6);var dataset = ee.ImageCollection("MODIS/006/MOD16A2").select('ET');var annualET = ee.FeatureCollection([]);for (var i = 2001; i <= 2020; i++) {var startDate = ee.Date.fromYMD(i, 5, 1);var endDate = ee.Date.fromYMD(i, 9, 30);var yearlyData = dataset.filterDate(startDate, endDate).sum();var yearlyET = yearlyData.multiply(0.1).clip(geometry).set({'year': i,'system:time_start': startDate.millis()});var meanET = yearlyET.reduceRegion({reducer: ee.Reducer.mean(),geometry: geometry,scale: 5000,maxPixels: 1e13}).get('ET');var feature = ee.Feature(null, {'year': i,'ET_mean': meanET});annualET = annualET.merge(ee.FeatureCollection([feature]));Export.image.toDrive({image: yearlyET,description: i + "_ET_SZJ",fileNamePrefix: i + "_ET_SZJ",folder: 'Annual_ET',scale: 5000,region: geometry,crs: "EPSG:4326",maxPixels: 1e13});}var chart = ui.Chart.feature.byFeature(annualET, 'year', 'ET_mean').setChartType('LineChart').setOptions({title: 'Annual ET (2001-2005)',hAxis: {title: 'Year'},vAxis: {title: 'Mean ET (mm)'},lineWidth: 2,pointSize: 4,colors: ['#1f77b4']});print(chart);
六、年度变化
var geometry = table;Map.centerObject(geometry, 6);var dataset = ee.ImageCollection("MODIS/006/MOD16A2").select('ET');var annualET = ee.FeatureCollection([]);for (var i = 2001; i <= 2020; i++) {var startDate = ee.Date.fromYMD(i, 1, 1);var endDate = ee.Date.fromYMD(i, 12, 31);var yearlyData = dataset.filterDate(startDate, endDate).sum();var yearlyET = yearlyData.multiply(0.1).clip(geometry).set({'year': i,'system:time_start': startDate.millis()});var meanET = yearlyET.reduceRegion({reducer: ee.Reducer.mean(),geometry: geometry,scale: 5000,maxPixels: 1e13}).get('ET');var feature = ee.Feature(null, {'year': i,'ET_mean': meanET});annualET = annualET.merge(ee.FeatureCollection([feature]));Export.image.toDrive({image: yearlyET,description: i + "_ET",fileNamePrefix: i + "_ET",folder: 'Annual_ET',scale: 5000,region: geometry,crs: "EPSG:4326",maxPixels: 1e13});}var chart = ui.Chart.feature.byFeature(annualET, 'year', 'ET_mean').setChartType('LineChart').setOptions({title: 'Annual ET (2001-2005)',hAxis: {title: 'Year'},vAxis: {title: 'Mean ET (mm)'},lineWidth: 2,pointSize: 4,colors: ['#1f77b4']});print(chart);
七、成为会员
