GEE16: 区域日均降水量计算

1. 区域日均降水量计算

今天分析一个计算区域日均降水量的方法：

数据信息：
  Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) is a 30+ year quasi-global rainfall dataset. CHIRPS incorporates 0.05° resolution satellite imagery with in-situ station data to create gridded rainfall time series for trend analysis and seasonal drought monitoring.

var startDate = ee.Date('2022-01-01');
var endDate = ee.Date('2023-01-01');

// 以重庆为目标
var geometry = ee.FeatureCollection('projects/ee-*****736/assets/Chongqing_Province')
Map.centerObject(geometry,5)
var styling = {color:"red",fillColor:"00000000"}
Map.addLayer(geometry.style(styling),{},"geometry")
var geometry = geometry.geometry()

var dataset = ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY")
  .filterDate(startDate, endDate);

var list_dataset = dataset.toList(dataset.size());
print(list_dataset);

// 计算区域内的日平均值
var getPrecipitation = function(image) {
  var value_precipit = ee.Image(image)
    .reduceRegion(ee.Reducer.mean(), geometry)
    .get('precipitation');
  var precipit_mm = ee.Number(value_precipit); 
  return precipit_mm;
};

// 输出每日平均降水量(mm)
var count = dataset.size();
var precipit_list = dataset.toList(count).map(getPrecipitation);
print("precipitation list", precipit_list);

// 输出所有日期
var allDates = ee.List(dataset.aggregate_array('system:time_start'));
var allDatesSimple = allDates.map(function(date){
  return ee.Date(date).format().slice(0,10);
  });
print('allDates',allDatesSimple);

// 将日期与降水数据组合
var paired = allDatesSimple.zip(precipit_list);
print (paired);

var title = {
  title: 'Daily precipitation',
  hAxis: {title: 'Time'},
  vAxis: {title: 'Precipitation (mm)'},
};

// 构建日平均降水量图表
var chartDaily = ui.Chart.image.seriesByRegion({
  imageCollection: dataset, 
  regions: geometry,
  reducer: ee.Reducer.mean(),
  band: 'precipitation',
  scale: 5566,
  xProperty: 'system:time_start',
  seriesProperty: 'SITE'
}).setOptions(title)
  .setChartType('ColumnChart');
print(chartDaily);

// 将日平均降水量输出CSV文件
var myFeatures = ee.FeatureCollection(paired.map(function(el){
  el = ee.List(el); // cast every element of the list
  var geom = geometry;
  return ee.Feature(null, {
    'date': ee.String(el.get(0)),
    'value':ee.Number(el.get(1))
  });
}));

// Export features, specifying corresponding names.
Export.table.toDrive(myFeatures,
"precipitation", //my task
"GEE_Folder", //my export folder
"daily_precipit",  //file name
"CSV");

结果展示：

2. 降水时间序列

var geometry = ee.FeatureCollection('projects/ee-*****736/assets/Chongqing_Province')
Map.centerObject(geometry ,6)
var CHIRPS= ee.ImageCollection('UCSB-CHG/CHIRPS/PENTAD');


var precip = CHIRPS.filterDate('2022-01-01', '2022-12-31');
var precip1year=CHIRPS.filterDate('2020-01-01', '2022-12-31');

var TS1 = Chart.image.series(precip, geometry, ee.Reducer.mean(),5566, 'system:time_start').setOptions({
          title: 'Precipitation Full Time Series',
          vAxis: {title: 'mm/pentad'},
});
print(TS1);

var TS2 = Chart.image.series(precip1year, geometry,  ee.Reducer.mean(),5566, 'system:time_start').setOptions({
          title: 'Precipitation 1-Year Time Series',
          vAxis: {title: 'mm/pentad'},
});
print(TS2);

结果展示：

3. 降水数据年度时间序列对比分析

// 研究区域边界可视化
var geometry = ee.FeatureCollection('projects/ee-*****736/assets/Chongqing_Province')
Map.centerObject(geometry);
var styling = {color:'red',fillColor:'00000000'}
Map.addLayer(geometry.style(styling),{},'roi')

var dataset = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY')
                  .filter(ee.Filter.date('2017-01-01', '2023-10-01'))
                  .select('precipitation')
                  .filterBounds(geometry)
                  .map(function(image){
                    return image.clip(geometry)
                  });


var Chart1 = ui.Chart.image.doySeriesByYear({
		imageCollection:dataset,
		bandName:'precipitation',
  	region:geometry,
  	regionReducer:ee.Reducer.mean(),
  	scale:5566,
})
print('每年的降水量走势',Chart1)


// 计算每月的平均值
// 自行修改起始年份即可
var years = ee.List.sequence(2017, 2023);
var months = ee.List.sequence(1, 12);
var Monthlymean = ee.ImageCollection(years
  .map(function(y) {
    return months.map(function(m) {
    var perc = dataset
                .filter(ee.Filter.calendarRange(y,y, 'year'))
                .filter(ee.Filter.calendarRange(m, m, 'month'))
                .mean()
                .clip(geometry)
    return perc.set('system:time_start',ee.Date.fromYMD(y,m,1))
                .set('system:index',ee.String(ee.Number(y).int()).cat("_").cat(ee.String(ee.Number(m).int())))
    })
    }).flatten());
print('月平均数据集',Monthlymean)

var Chart2 = ui.Chart.image.doySeriesByYear({
		imageCollection:Monthlymean,
		bandName:'precipitation',
  	region:geometry,
  	regionReducer:ee.Reducer.mean(),
  	scale:5566,
})
print('每年的月度降水量走势',Chart2)

结果展示：
每年的降水量走势：

每年的月度降水量走势：