Reduce image collection to get annual monthly sum precipitation

Question

I'm using GRIDMET image collection in Google Earth Engine, which has daily climate data from ~1980 to present. I would like to create an image collection of monthly (May-September: each month) sum precipitation for each year (1986-2016). I've been trying to create a function to do so, but can't figure it out.

https://code.earthengine.google.com/ceba35ee52955ec5e174e0293cebb7b8

Answer 1

Here is an example that should get you started:

var modis = ee.ImageCollection('MODIS/MOD13A1');

var months = ee.List.sequence(1, 12);

// Group by month, and then reduce within groups by mean();
// the result is an ImageCollection with one image for each
// month.
var byMonth = ee.ImageCollection.fromImages(
      months.map(function (m) {
        return modis.filter(ee.Filter.calendarRange(m, m, 'month'))
                    .select(1).mean()
                    .set('month', m);
}));
print(byMonth);

Map.addLayer(ee.Image(byMonth.first()));

Obviously you're going to want to replace the mean() with sum(), etc.

Answer 2

Answering your other question about how to get one Image for each month:

// Set years and month
var startYear = 1981;
var endYear = 2019;
var years = ee.List.sequence(startYear, endYear);
var months = ee.List.sequence(1, 12);
// load the image collection
var Daily = ee.ImageCollection('MODIS/MOD13A1').select(1)
  .filterBounds(YourBound);

print(Daily, "Daily prec data from 1981-2019")

// make monthly summed mosaics
// loop over the years and months to get summed monthly images
var byMonth = ee.ImageCollection(ee.FeatureCollection(years.map(function (y) {
  var yearCollection = Daily.filter(ee.Filter.calendarRange(y, y, 'year'));
  var byYear = ee.ImageCollection.fromImages(
    months.map(function (m) {
      var summedImage = yearCollection.filter(ee.Filter.calendarRange(m, m, 'month'))
        .reduce(ee.Reducer.sum());
      var date = ee.Date.fromYMD(y, m, 5).format("MM_dd_YYYY");
      return summedImage.set('system:time_start', ee.Date.fromYMD(y, m, 5))
        .rename(date)
        .reduce(ee.Reducer.mean());
      //.set('month', m).set('year', y); // eventually set year and month 
    }));
  return byYear;
})).flatten());
// filter the empty one out
var outputMonthly = byMonth.filter(ee.Filter.listContains('system:band_names',
    'constant').not())
  .sort('system:time_start').toBands();
print(outputMonthly);

var features = outputMonthly.reduceRegions(YourBound, ee.Reducer.first(), 30);
print(features);

Export.image.toDrive({
  image: features,
  description: "CHIRPS_monthly_mean_1981-2019",
  scale: 3000,
  folder: "CHIRPS_months",
  region: YourBound
});