CWSI（Crop Water Stress Index，作物缺水指数）是基于能量平衡原理计算的，用于衡量植被覆盖区域土壤的干旱程度。CWSI值的计算公式为：CWSI = 1 - ET / PET，其中ET代表实际蒸散量（mm），PET为潜在蒸散量（mm）。CWSI的取值范围在0到1之间，值越接近1表示土壤干旱程度越严重，而值越接近0则表明土壤较为湿润。基于MCD12Q1数据集识别研究区域内的作物区域，采用MOD16A2GF的ET和PET，计算每年夏季的作物CWSI，输出年度变化折线图和栅格数据。![]()
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var geometry = table;
Map.centerObject(geometry, 6);
var time_start = '2001-01-01', time_end = '2024-01-01';var modis = ee.ImageCollection("MODIS/061/MOD16A2GF") .select('ET', 'PET') .filterDate(time_start, time_end);
var lc = ee.ImageCollection("MODIS/061/MCD12Q1") .select('LC_Type1').mode();var crop = lc.eq(12);Map.addLayer(crop.clip(geometry), { palette: ['fbf8f3', 'green'] }, 'Crop Mask', false);
var annualCWSI = ee.FeatureCollection([]);
for (var year = 2001; year <= 2023; year++) { var summer = modis .filter(ee.Filter.calendarRange(6, 8, 'month')) .filter(ee.Filter.calendarRange(year, year, 'year')); var cwsi = summer.map(function(img) { var eq = img.expression('1 - (et / pet)', { 'et': img.select('ET').multiply(0.1), 'pet': img.select('PET').multiply(0.1) }); return eq.rename('CWSI') .copyProperties(img, ['system:time_start']); });
var summerCWSI = cwsi.mean().updateMask(crop); var meanCWSI = summerCWSI.reduceRegion({ reducer: ee.Reducer.mean(), geometry: geometry, scale: 500, maxPixels: 1e13 }).get('CWSI');
var feature = ee.Feature(null, { 'year': year, 'meanCWSI': meanCWSI });
annualCWSI = annualCWSI.merge(ee.FeatureCollection([feature]));
Export.image.toDrive({ image: summerCWSI, description: 'Summer_CWSI_Crop_' + year, region: geometry, scale: 500, crs: 'EPSG:4326', maxPixels: 1e13, folder: 'CWSI_Summer_Crop' });}
var chart = ui.Chart.feature.byFeature(annualCWSI, 'year', 'meanCWSI') .setChartType('LineChart') .setOptions({ title: 'Summer CWSI Mean for Croplands (2001-2023)', hAxis: { title: 'Year' }, vAxis: { title: 'Mean CWSI', minValue: 0.4, maxValue: 0.8 }, lineWidth: 2, pointSize: 3, colors: ['black'] });
print(chart);
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