Google Earth Engine – MODIS Monthly Evapotranspiration

google-earth-engine

I am trying to get a monthly time series of MODIS evaporation for a point with the script below, however the problem is that it gives me the weekly values for every month.
How can I modify this script to have only the average value of each month?

Script:

var geometry = /* color: #d63000 */ee.Geometry.Point([0, 0]);


var modisPET= ee.ImageCollection("MODIS/006/MOD16A2");
var collection05=ee.ImageCollection(modisPET.filterDate('2006-01-01', '2018-12-31'));
var clipped05=collection05.mean().clip(geometry)
var TS5=Chart.image.seriesByRegion(collection05,geometry,ee.Reducer.mean

(),'ET',2000,'system:time_start').setOptions({title:'PETLong-TermTimeSeries',vAxis:{title:'PET'},});
print(TS5);

Best Answer

Based on this answer: How to create and use a list of unique dates of images in an ImageCollection GEE you can build a list with all unique months and then create a new monthly image collection:

var monthlyMean = uniqueDatesSimple.map(function(date){
  
  var monthStart = ee.Date(date)
  var monthEnd = ee.Date(date).advance(1, 'month').advance(-1,'day')
  
  var subset = collection05.filterDate(monthStart,monthEnd);
  
  
  return ee.Image(subset.mean()).copyProperties(subset.first());
});

https://code.earthengine.google.com/f8d9242c7277939da0f66b4886001313

Related Solutions

[GIS] Finding monthly NDVI anomalies in Google Earth Engine

I think there are two issues here. First and more important one being, you are trying to calculate anomalies for year 1982 to 2017 but the collection you are using GIMMS 3rd generation (NASA/GIMMS/3GV0) has data only upto 2013. So you can not run it till 2017. Another issue is that when you use ee.Filter.calendarRange it requires numbers for start month and end month but since your months variable was updated to contain strings for creating the dictionaries with mean and sds in this line

var months = ee.List(['1','2','3','4','5','6','7','8','9','10','11','12']);

There are now strings not numbers. So when u use months.map the passed elements in variable m are strings. This can be easily fixed by parsing the passed values within the mapping function.

var meanAnomaly = ee.ImageCollection.fromImages(
  years.map(function(y) {
    return months.map(function (m) {
      m = ee.Number.parse(m)
      return NDVI
        .filter(ee.Filter.calendarRange(y, y, 'year'))
        .filter(ee.Filter.calendarRange(m, m, 'month'))
        .mean().subtract(monthmeans.getNumber(ee.String(m)))
        .set('month', m).set('year', y)
        ;
  });
}).flatten());
print("mean anomaly", meanAnomaly)

You can see a working link here. Since your geometry was not available to me, I took the liberty of creating my own.

javascript – Calculating and Plotting Monthly Average NDVI for a Region from MODIS Data Using Google Earth Engine

Looks like you were pretty close, and with the help of Nicholas Clinton's answer here we can arrive at a solution. The main changes I made were:

removing unnecessary variable definitions (first and time0)
adjust how the function mapped to months is defined
added a years variable for the same function mapping technique

All of that said, here's an updated script.

//calculate average monthly NDVI for Ukraine
//get MODIS NDVI
var collection = ee.ImageCollection('MODIS/006/MOD13A1').select('NDVI');

//import country boundaries
var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');

//Subset Ukraine feature from countries.
var ukr = ee.Feature(
  countries
   .filter(ee.Filter.eq('country_na', 'Ukraine'))
    .first()
);
print("ukr",ukr);

//Use a function to iterate through months and calculate average NDVI
var months = ee.List.sequence(1, 12);
print("months",months);
var years = ee.List.sequence(2010, 2020);
print("years",years);

// Map filtering and reducing across year-month combinations and convert to ImageCollection
var yrMo = ee.ImageCollection.fromImages(
  years.map(function (y) {
        return months.map(function (m) {
            return collection
              .filter(ee.Filter.calendarRange(y, y, 'year'))
              .filter(ee.Filter.calendarRange(m, m, 'month'))
              .mean()
              .set('year',y)
              .set('month',m);
        });
    }).flatten());
print("yrMo",yrMo);

//Define chart parameters
var chartParam = {
 title: 'Monthly average NDVI',
  hAxis: {title: 'Time'},
  vAxis: {title: 'Average NDVI'},
};

//Plot the chart
var chart = ui.Chart.image.seriesByRegion({
  imageCollection: yrMo,
  regions: ukr,
  reducer: ee.Reducer.mean(),
  scale: 500,
  xProperty: 'system:index',
  seriesProperty: 'PROJECT'
});
//Print chart to console
print(chart.setOptions(chartParam));

Best Answer

Related Solutions

[GIS] Finding monthly NDVI anomalies in Google Earth Engine

javascript – Calculating and Plotting Monthly Average NDVI for a Region from MODIS Data Using Google Earth Engine

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