GEE：绘制累积降雨量折线图、降雨量均值折线图

本文记录了在GEE（GoogleEarthEngine）平台上，使用UCSB-CHG/CHIRPS/DAILY数据集，绘制一个指定点的一段时间内的降水量累计值折线图的代码，也记录了绘制一个区域一段时间内的降水量均值的折线图代码。其中累积降水量使用每日数据累积，降水量均值折线图使用的是每周的降水量数据。即本文记录了将影像集合按天 ‘day’ 合成和按周 ‘week’ 合成的方法。

---

---

一、累积降雨量折线图

操作对象：一个点
研究区：一个点的缓冲区
累计值：一个点一段时期内降水量的累积值

结果展示

代码实现

var chirpsDaily = ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY");

// 定义时间区间
var startDate = ee.Date('2020-06-01')
var endDate = ee.Date('2020-09-30')

var days = endDate.difference(startDate, 'day')
var daysList = ee.List.sequence(1, days)

// Map()在天列表上的函数
var cumulativeImages = daysList.map(function(day) {
	  // 筛选从开始日期到计算当天的集合
	  var begin = startDate
	  var current = startDate.advance(day, 'day')
	  var filtered = chirpsDaily.filter(ee.Filter.date(begin, current))
	  // 使用sum()计算该时期的总降雨量
	  // 确保设置了图像的start_time
	  var cumulativeImage = filtered.reduce(ee.Reducer.sum())
	    .set('system:time_start', current.millis())
	  return cumulativeImage
})

// 把包含累积降雨的图像列表收集起来
var cumulativeCol = ee.ImageCollection.fromImages(cumulativeImages)

// 创建一个上海的点
var point = ee.Geometry.Point(121.1649, 31.0614)

// 累积降雨量图绘制
var chart = ui.Chart.image.series({
	  imageCollection: cumulativeCol, 
	  region: point, 
	  reducer: ee.Reducer.mean(), 
	  scale: 5566,
}).setOptions({
      interpolateNulls: true,
      lineWidth: 1,
      pointSize: 3,
      title: 'Cumulative Monsoon Rainfall at Shanhai (2020)',
      vAxis: {title: 'Cumulative Rainfall (mm)'},
      hAxis: {title: 'Month', format: 'YYYY-MMM'}

});
print(chart);

Map.setCenter(121.1649, 31.0614, 12);
Map.addLayer(point, {}, 'point');

---

二、降水量均值折线图绘制

操作对象：整个影像
研究区：一个矩形区域
均值：整个影像的均值

结果展示

代码实现

var roi = ee.Geometry.Rectangle({
  coords: [[120, 30], [122, 32]],
  geodesic: false
});
Map.addLayer(roi);
Map.centerObject(roi,7)

var yrly_rain = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY') 
                .filter(ee.Filter.calendarRange(2019, 2019, 'year'))
                .sum()
                .clip(roi)
print(yrly_rain)
Map.addLayer(yrly_rain,{},'Yrly Rainfall')

var chirps = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY') 
              // .filter(ee.Filter.calendarRange(2019, 2019, 'year'))

print(chirps.limit(500))

var weeks = ee.List.sequence(1,52)
//print(weeks)

var startD = ee.Date.fromYMD(2019,01,01)

var wkrf = function(Week)
{
	var weekStartD = startD.advance (Week,'week')
	var weekEndD = weekStartD.advance(1, 'week')
	var wk = chirps.filterDate(weekStartD,weekEndD).sum().clip(roi)
	return wk.set({'Week': ee.Number(Week)})
}

var wk_rf = weeks.map(wkrf)
var wk_rfcoll = ee.ImageCollection.fromImages(wk_rf)
print(wk_rfcoll);

var wkrf_series = ui.Chart.image.series({
	imageCollection:wk_rfcoll, 
	region: roi, 
	reducer:ee.Reducer.mean(), 
	scale:5000, 
	xProperty:'Week'
})
print(wkrf_series)
Map.addLayer(wk_rfcoll,{},'wkly Rainfall')