Exporting daily climatology data from NEX-GDDP product on Google Earth Engine

Question

I am using NEX-GDDP for obtaining daily climatology (Precipitation, Temp min and Temp max) data of the 21 GCM models in the period 2018-01-01 to 2099-12-31, for certain points. I made this script, for one model in one scenario

//Dataset
var dataset = ee.ImageCollection('NASA/NEX-GDDP')
              .filter(ee.Filter.date('2018-01-01', '2099-12-31'))
              .filterMetadata('scenario','equals','rcp45')
              .filterMetadata('model','equals','MPI-ESM-LR')

//Points of interest 
var Acomayo = ee.Geometry.Point([-71.689166667, -13.921388889]),
var Machupicchu = ee.Geometry.Point([-72.545555556, -13.166666667]),
var Urubamba = ee.Geometry.Point([-72.129116546, -13.323123791]),
var Pisac = ee.Geometry.Point([-71.849444444, -13.415833333]),
var Ccatcca = ee.Geometry.Point([-71.56, -13.609722222]),
var GranjaKcayra = ee.Geometry.Point([-71.875, -13.556666667]),
var Pomacanchi = ee.Geometry.Point([-71.5357971, -14.027777778]),
var Sicuani = ee.Geometry.Point([-71.236944444, -14.253333333]);

var pts = ee.FeatureCollection(ee.List([ee.Feature(Acomayo),ee.Feature(Machupicchu),ee.Feature(Urubamba),ee.Feature(Pisac)
                                       ,ee.Feature(Ccatcca),ee.Feature(GranjaKcayra),ee.Feature(Pomacanchi),ee.Feature(Sicuani)]));

//Export to table .CSV
// Empty Collection to fill
var ft = ee.FeatureCollection(ee.List([]));

//Without removal of null values ----------------------------------
//Function to extract values from image collection based on point file and export as a table 
var fill = function(img, ini) {
// type cast
var inift = ee.FeatureCollection(ini);

// gets the values for the points in the current img
var ft2 = img.reduceRegions(pts, ee.Reducer.first(),30);

// gets the date of the img
var date = img.date().format("yyyy/MM/dd");
var scenario = img.get("scenario");
var model = img.get("model");


// writes the date in each feature
var ft3 = ft2.map(function(f){return f.set("date", date, "scenario", scenario, "model", model)});
// merges the FeatureCollections
return inift.merge(ft3);
};

// Iterates over the ImageCollection
var newft = ee.FeatureCollection(dataset.iterate(fill, ft));
//print(newft);

// Export

Export.table.toDrive({
  collection: newft,
  description: 'GCM_diario',
  folder: 'Downscalling_Diario',
  fileFormat: 'csv'
});

The scripts work fine for two days and two points, but for the current points and period of time I need, it still working after 5 hrs. To reduce the amount of data I think these ideas:

Average the daily data of the 21 GCMs models in the product, and make it one ImgaeCollection, so there is only a need to separate by scenario.
Export the ImageCollection of every variable (Pp, Tmin, Tmax) to a NetCDF only the area that contains points (don't know if it is possible).

geometry = ee.Geometry.Polygon(
        [[[-72.77555636882136, -12.867571480133547],
          [-72.77555636882136, -14.670820732958893],
          [-70.69914035319636, -14.670820732958893],
          [-70.69914035319636, -12.867571480133547]]], null, false);

If there is another way to download this data I open to do it.

score 2 · Accepted Answer · answered Nov 20 '19 at 16:17

Using .iterate() can be very memory intensive and is prone to memory errors. A more straight forward approach to this would be to select a specific point you want to focus on, loop over all of the days of interest, and use .reduceRegion() to get the information desired. You can then export the time series as a CSV and convert it to whichever format you want.

Here is an example that gets all variables for all models and scenarios:

// specify start and end date
// Change as needed
var startDate = ee.Date('2018-01-01');
var endDate = ee.Date('2019-01-01');

// get the dataset between date range and extract band on interest
var dataset = ee.ImageCollection('NASA/NEX-GDDP')
                  .filter(ee.Filter.date(startDate,endDate));

// get projection and band information
var firstImage = dataset.first();
var bandNames = firstImage.bandNames();
var proj = firstImage.projection();

var point = ee.Geometry.Point([-71.689166667, -13.921388889])

// calculate number of days to map and extract data for
var n = endDate.difference(startDate,'day').subtract(1);

// map over each date and extract all climate model values
var timeseries = ee.FeatureCollection(
  ee.List.sequence(0,n).map(function(i){
    var t1 = startDate.advance(i,'day');
    var t2 = t1.advance(1,'day');
    var dailyColl = dataset.filterDate(t1, t2);
    var dailyImg = dailyColl.toBands();
    // rename bands to handle different names by date
    var bands = dailyImg.bandNames();
    var renamed = bands.map(function(b){
      var split = ee.String(b).split('_');
      return split.slice(0,2).cat(split.slice(-1)).join('_');
    });
    // extract the data for the day and add time information
    var dict = dailyImg.rename(renamed).reduceRegion({
      reducer: ee.Reducer.mean(),
      geometry: point,
      scale: proj.nominalScale()
    }).combine(
      ee.Dictionary({'system:time_start':t1.millis(),'isodate':t1.format('YYYY-MM-dd')})
    );
    return ee.Feature(point,dict);
  })
);
print(timeseries);

// get properties to chart (all climate models)
var props = timeseries.first().propertyNames().removeAll(['system:time_start','system:index','isodate']);

// Make a chart of the results.
var chart = ui.Chart.feature.byFeature(timeseries, 'system:time_start', props.getInfo());
print(chart);

Map.addLayer(point);
Map.centerObject(point,6);

// export feature collection to CSV
Export.table.toDrive({
  collection: timeseries,
  description: 'NEX-GDDP-timeseries',
  fileFormat: 'CSV',
});

You may run into memory errors in the code editor when working with long date ranges (2018-2099) but the export should work. Also, keep in mind that the Earth Engine exports are a little simplistic so exporting point by point would be the best approach, 1) to avoid memory errors and 2) keep the resulting CSV to one point. You can merge all the points together and export the time series per point in one file but that may be difficult to work with once exported...

Here is a working link: https://code.earthengine.google.com/139432f76ae3f6a81b1459762325ef7f

Exporting daily climatology data from NEX-GDDP product on Google Earth Engine

1 Answers1