参考文献：https://doi.org/10.1016/j.scitotenv.2019.135590；我国北方地区植被对水分亏缺的响应特征分析
在大尺度研究中，区域水分亏缺通常被定义为系统或区域的水分供需差额。基于区域水量平衡原理，将降水量（P）与潜在蒸散发量（PET）之差定义为区域水文循环过程中的水分亏缺量（D），计算公式为：D = P - PET。其中，D、P、PET的单位均为毫米（mm）。
采用TerraClimate数据集的降水和潜在蒸散发数据，我们计算了1958-2023年研究区的水分亏缺量，并绘制了降水、蒸散和水分亏缺的年际变化趋势。![]()
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var terraclimate = ee.ImageCollection("IDAHO_EPSCOR/TERRACLIMATE");
var beijing = ee.Geometry.Rectangle([115.42, 39.26, 117.51, 41.05]);
var years = ee.List.sequence(1958, 2023);
var annualData = years.map(function(year) { var startDate = ee.Date.fromYMD(year, 1, 1); var endDate = ee.Date.fromYMD(year, 12, 31); var yearData = terraclimate.filterDate(startDate, endDate).select(['pr', 'pet']); var annualPrecipitation = yearData.select('pr').sum().rename('Precipitation'); var annualPET = yearData.select('pet').sum().multiply(0.1).rename('PET'); var annualDeficit = annualPrecipitation.subtract(annualPET).rename('Deficit'); var annualImage = annualPrecipitation.addBands(annualPET).addBands(annualDeficit) .set('year', year);
return annualImage.clip(beijing);});
var annualCollection = ee.ImageCollection(annualData);
var regionMean = function(image) { var year = image.get('year'); var meanValues = image.reduceRegion({ reducer: ee.Reducer.mean(), geometry: beijing, scale: 4638.3, bestEffort: true });
return ee.Feature(null, meanValues.set('year', year));};
var timeSeries = ee.FeatureCollection(annualCollection.map(regionMean));
var chart = ui.Chart.feature.byFeature(timeSeries, 'year', ['Precipitation', 'PET', 'Deficit']) .setChartType('LineChart') .setOptions({ title: 'Annual Precipitation, PET, and Water Deficit in Beijing (1958-2023)', hAxis: {title: 'Year'}, vAxis: {title: 'Millimeters (mm)'}, lineWidth: 2, pointSize: 4, series: { 0: {color: 'blue', label: 'Annual Precipitation (P)'}, 1: {color: 'red', label: 'Annual PET'}, 2: {color: 'green', label: 'Annual Water Deficit (D)'} } });
print(chart);
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