Iterating over dates to extract precipitation in Google Earth Engine

Question

I'm looking to sum precipitation for a period of 10-days prior to a specified date. This is just an example - in fact, I have around 60 different dates and I don't want to select each the date manually as done below. Is there a way to iterate through the dates?

// CSV file
var coord = ee.FeatureCollection("users/eginamalaj/site_test");

// Select sampling date

var datesampl = '2018-05-06';
var s1 = coord.filter(ee.Filter.eq('date', datesampl));

// Assign time interval
var dates = new Date(datesampl); 
var start = new Date(dates.getTime() - 10*24*60*60*1000); // 10-days before
var end = new Date(dates.getTime() - 1*24*60*60*1000); // exclude sampling day 

// Get image and sum for the time interval
var precImage = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY')
                   .filterBounds(coord)
                   .filterDate(start, end) 
                   .select('precipitation')
                   .sum();
                   
// Extract precipitation for the sampling sites
var precSample = precImage.sampleRegions({
    collection: s1,
    scale: 30,
    geometries: true
});

//////// Repeat above with a different date

var datesampl2 = '2018-09-12'
var s2 = coord.filter(ee.Filter.eq('date', datesampl2));
var dates2 = new Date(datesampl2); 
var start2 = new Date(dates2.getTime() - 10*24*60*60*1000); // 10-days before
var end2 = new Date(dates2.getTime() - 1*24*60*60*1000); // exclude sampling day 
var precImage2 = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY')
                   .filterBounds(coord)
                   .filterDate(start, end) 
                   .select('precipitation')
                   .sum();
var precSample2 = precImage2.sampleRegions({
    collection: s1,
    scale: 30,
    geometries: true
});

                
// Combine them
var combined = precSample.merge(precSample2);
print(combined);

Code and data: https://code.earthengine.google.com/cd0a2c60fb8ddefbb9508abfc1a806bb

Answer 1

You do this by mapping over the feature collection. Unfortunately I think you didn't give access to your script to others, so I can't test it. But this should work:

// CSV file
var coord = ee.FeatureCollection("users/eginamalaj/site_test");

// Select sampling date

var results = coord.map(function(feature){
  var date = ee.Date(feature.get('date'))
  var start = date.advance(-10, 'day')
  var end = date.advance(-1, 'day')
  // Get image and sum for the time interval
  var precImage = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY')
                     .filterBounds(coord)
                     .filterDate(start, end) 
                     .select('precipitation')
                     .sum();
                     
  // Extract precipitation for the sampling sites
  var summed = precImage.reduceRegion({
      reducer: ee.Reducer.sum(),
      geometry: feature.geometry(),
      scale: 30
  });
  return feature.set(summed)
})

print(results)

Most of the time, if you have a workflow that works for a single element out of an ImageCollection or FeatureCollection it is quite straight forward to use the same workflow and automatically map over the entire Collection

Answer 2

I found the problem and fixed it this way

feature = ee.Feature(feature.set('precipitations', max_prcp.get('prcp'),
                                 'sol_rad', mean_sol_radiation.get('srad'),
                                 'temp_max', mean_maxtemp.get('tmax')));  
return feature