Google Earth Engine: Multiply an image collection by a different number depending on the month

Question

I am trying to multiply a monthly imageCollection by a different number depending on the month. What I want is to multiply all the pixels of the first 12 months by 0.85, the following 12 months by 0.95, the following 12 by 0.65. All this for a total of 468 months from 1981 to 2019 (the example contains only 2 years, 24 months).

var monthly_sum = ee.List.sequence(0, 2*12).map(function(n) { // .sequence: number of years from starting year to present
  var start = ee.Date('1981-01-01').advance(n, 'month'); // Starting date
  var end = start.advance(1, 'month'); // Step by each iteration

  return ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY")
        .filterDate(start, end)
        .sum()
        .set('system:time_start', start.millis());
});


print(monthly_sum)

var my_list = ee.List([]);

var otherList = ee.List([0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.884748080310752,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95,
0.95
])

Which would be the easiest way to multiply each monthly image by the corresponding multiplicative factor creating a new imageCollection?

Thanks,

Answer 1

You can scale an image collection by a list of scaling factors using the following approach, which constructs a list of (image, scaling factor) pairs, and then multiplies them together.

// Copyright 2020 Google LLC.
// SPDX-License-Identifier: Apache-2.0

var originalCollection = ee.ImageCollection([
  ee.Image(1.0).double(),
  ee.Image(2.0).double(),
  ee.Image(3.0).double(),
  ee.Image(4.0).double(),
]);
var scalingFactors = ee.List([
  1.0,
  10.0,
  100.0,
  1000.0,
]);

// Combine into a list of elements containing an image and a scaling factor.
var MAX_ELEMENTS = 1000;
var zippedList = originalCollection.toList(MAX_ELEMENTS).zip(scalingFactors);

// Map a function over the list to scale each image.
var scaledCollection = ee.ImageCollection.fromImages(
  zippedList.map(function (_list) {
    _list = ee.List(_list);
    var _img = ee.Image(_list.get(0));
    var _scale = _list.getNumber(1);
    var _scaledImage = _img.multiply(_scale);
    return _scaledImage.double();  // Set the data type, so the collection is consistent.
  })
);

// Verify the images are scaled by inspecting the map layer.
Map.addLayer(originalCollection, {}, 'originalCollection');
Map.addLayer(scaledCollection, {}, 'scaledCollection');