Exporting values from Images in ImageCollection to CSV

Question

I am working with rainfall monthly totals for a 30-year period using CHIRPS pentad.

//1.Reference Period
var historical = ee.List.sequence(1991, 2020)
var months = ee.List.sequence(1, 5)
//print(historical)
    
//2.Map over the years and create a monthly totals collection
var monthlyImages = historical.map(function(year){
  return months.map(function(month){
    var filtered = chirps
    .filter(ee.Filter.calendarRange(year, year, 'year'))
    .filter(ee.Filter.calendarRange(month, month, 'month'))
    var monthly = filtered.sum();
    return monthly.set({'month': month, 'year': year})
  })
}).flatten()

print(monthlyImages)

//3. Create an Image Collection for each month of each year
var monthlyCol = ee.ImageCollection.fromImages(monthlyImages)
print(monthlyCol)

//4. Convert to Feature Collection
var montlyRainfall = ee.FeatureCollection(monthlyImages)

// Export.table.toDrive({
//   collection: monthlyRainfall,
//   fileNamePrefix: 'rainfallbymonth',
//   fileFormat: 'CSV'}) 

I have been able to get an Image Collection with 150 images for the months I require. I am looking to export each monthly totals(sum) to a table and obtain results from my geometry.

Transforming my Image Collection to a Feature Collection and exporting to CSV has worked with the correct dates but it just doesn't contain the values from my 'precipitation' band which is the sum that I filtered in the first code of block.

I understand I need to transform my image collection to a feature collection and tell it to assign the dictionary values to my table I just have not figured out how to do it. I am also not sure if I need to Reduce.sum again even though I have filtered my monthly images totals using .sum already.

Answer 1

To transform your image collection to a feature collection your statement:

//4. Convert to Feature Collection
var montlyRainfall = ee.FeatureCollection(monthlyImages)

it doesn't nothing.

You need, for instance, to use 'reduceRegion' method by employing as parameters a reducer, a geometry and a scale; as it can be observed at following lines:

//4. Convert to Feature Collection
var monthlyRainfall = ee.FeatureCollection(monthlyCol.map(function (img){
  
  var value = ee.Image(img).reduceRegion(ee.Reducer.first(),
                                         geometry,
                                         scale)
                            .get('precipitation'); 
  
  var month = ee.Image(img).get("month");
  var year = ee.Image(img).get("year");
  
  var date = ee.String(ee.Number(year).int()).cat('_').cat(ee.Number(month).int());
  
  return ee.Feature(null).set('precipitation', value)
                         .set('date', date);
  
}));

print(monthlyRainfall);

 Export.table.toDrive({
   collection: monthlyRainfall,
   fileNamePrefix: 'rainfallbymonth',
   fileFormat: 'CSV'});

Above function was included in this complete code and, after running it and its corresponding task, you have a CSV file as follows. You can observe the precipitation at corresponding point (chosen arbitrarily in US) by month and year.