Calculating daily precipitation data from CHIRPS over several years using Google Earth Engine

Question

I am still new to the Google Earth Engine and try to calculate the daily precipitation based on the CHIRPS. I have created a code to calculate monthly precipitation.

Could you advise me on how I change this code to daily precipitation?

var chirps = ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY"),
geometry = /* color: #d63000 */ee.Geometry.MultiPoint();
    
// set start and end year
var startyear = 2018; 
var endyear = 2020; 

// make a date object
var startdate = ee.Date.fromYMD(startyear, 1, 1);
var enddate = ee.Date.fromYMD(endyear + 1, 1, 1);

// make a list with years
var years = ee.List.sequence(startyear, endyear);
// make a list with months
var months = ee.List.sequence(1, 12);

// Specify Country names
var worldcountries = ee.FeatureCollection('FAO/GAUL/2015/level2');

var filterCountry = ee.Filter.eq('ADM2_NAME', 'Kurgantepa district');

var country = worldcountries.filter(filterCountry);

Map.addLayer(country);
Map.centerObject(country, 6);

    
// Get the geometry
var country = country.geometry();


var monthlyPrecip =  ee.ImageCollection.fromImages(
  years.map(function (y) {
    return months.map(function(m) {
      var w = chirps.filter(ee.Filter.calendarRange(y, y, 'year'))
                    .filter(ee.Filter.calendarRange(m, m, 'month'))
                    .sum();
      return w.set('year', y)
              .set('month', m)
              .set('system:time_start', ee.Date.fromYMD(y, m, 1));
                        
    });
  }).flatten()
);


var meanMonthlyP =  ee.ImageCollection.fromImages(
  months.map(function (m) {
    var w = monthlyPrecip.filter(ee.Filter.eq('month', m)).mean();
    return w.set('month', m)
            .set('system:time_start',ee.Date.fromYMD(2018, m, 1)); 
  }).flatten()
);


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


var chartMonthly = ui.Chart.image.seriesByRegion({
  imageCollection: meanMonthlyP, 
  regions: country,
  reducer: ee.Reducer.mean(),
  band: 'precipitation',
  scale: 5000,
  xProperty: 'system:time_start',
  seriesProperty: 'SITE'
}).setOptions(title)
  .setChartType('ColumnChart');

print(chartMonthly);

Answer 1

Following code produces a paired list of dates and daily precipitation values for exporting to Google Drive (for your filterCountry variable) and print a chartDaily for the same region.

var startDate = ee.Date('2018-01-01');
var endDate = ee.Date('2021-01-01');

// Specify Country names
var worldcountries = ee.FeatureCollection('FAO/GAUL/2015/level2');

var filterCountry = ee.Filter.eq('ADM2_NAME', 'Kurgantepa district');

var country = worldcountries.filter(filterCountry);

Map.addLayer(country);
Map.centerObject(country, 10);

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

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

print(list_dataset);

var getPrecipitation = function(image) {

  var value_precipit = ee.Image(image)
    .reduceRegion(ee.Reducer.first(), country)
    .get('precipitation');
  
  var precipit_mm = ee.Number(value_precipit); 

  return precipit_mm;
};

var count = dataset.size();

var precipit_list = dataset.toList(count).map(getPrecipitation);

print("precipitation list", precipit_list);

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(allDatesSimple);

var len = precipit_list.size();

print(len);

var paired = allDatesSimple.zip(precipit_list);

print (paired);

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

var chartDaily = ui.Chart.image.seriesByRegion({
  imageCollection: dataset, 
  regions: country,
  reducer: ee.Reducer.mean(),
  band: 'precipitation',
  scale: 5000,
  xProperty: 'system:time_start',
  seriesProperty: 'SITE'
}).setOptions(title)
  .setChartType('ColumnChart');

print(chartDaily);

var myFeatures = ee.FeatureCollection(paired.map(function(el){
  el = ee.List(el); // cast every element of the list
  var geom = country;
  return ee.Feature(null, {
    'date': ee.String(el.get(0)),
    'value':ee.Number(el.get(1))
  });
}));

//print(myFeatures);

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

After running above code in GEE code editor, result can be observed in following image:

An extract of obtained CSV looks as follows:

system:index,date,value,.geo
0,2018-01-01,0.0,
1,2018-01-02,0.0,
2,2018-01-03,0.0,
3,2018-01-04,1.2671680450439453,
4,2018-01-05,0.0,
5,2018-01-06,0.0,
6,2018-01-07,0.0,
7,2018-01-08,0.0,
8,2018-01-09,0.0,
9,2018-01-10,0.0,
10,2018-01-11,0.0,
11,2018-01-12,0.0,
12,2018-01-13,1.3613150119781494,
13,2018-01-14,0.6806575059890747,
14,2018-01-15,1.7016438245773315,
15,2018-01-16,0.9700509905815125,
16,2018-01-17,0.0,
17,2018-01-18,1.940101981163025,
.
.
.