Annual evapotranspiration

Use mod16 to investigate yearly variability in ET

Step 1: import the mod16 image collection

Step 2: Set the date variables

// set start and end year
var startyear = 2000;
var endyear = 2014;

// make a date object
var startdate = ee.Date.fromYMD(startyear, 1, 1);
var enddate = ee.Date.fromYMD(endyear + 1, 1, 1);

// make a list with years
var years = ee.List.sequence(startyear, endyear);

Step 3: Set the geographic location

// Specify Country names
var country_names = ['Indonesia'];

// Get the country boundaries
var countries = ee.FeatureCollection('ft:1tdSwUL7MVpOauSgRzqVTOwdfy17KDbw-1d9omPw');

// Filter for country
var Indonesia = countries.filter(ee.Filter.inList('Country', country_names));

// Get the geometry
var Indonesia = Indonesia.geometry();

// Get ET 
var ET = mod16.select("ET")

Step 4: Calculate the yearly ET

var annualET = ee.ImageCollection.fromImages(
 years.map(function (year) {
 var annual = ET
 .filter(ee.Filter.calendarRange(year, year, 'year'))
 .sum()
 .multiply(0.1);
 return annual
 .set('year', year)
 .set('system:time_start', ee.Date.fromYMD(year, 1, 1));
}));

Step 5: Make a chart of the data

var title = {
 title: 'Annual ET',
 hAxis: {title: 'Time'},
 vAxis: {title: 'ET (mm)'},
};

var chart = ui.Chart.image.seriesByRegion({
 imageCollection: annualET, 
 regions: Indonesia,
 reducer: ee.Reducer.mean(),
 band: 'ET',
 scale: 2500,
 xProperty: 'system:time_start',
 seriesProperty: 'SITE'
}).setOptions(title)
 .setChartType('ColumnChart');

 print(chart);

Step 6: Calculate the mean of the period

var annualMean = annualET.mean().clip(Indonesia);

 

Step 7: show the map

var ETViz = {
min: 1200,
max: 1800,
palette: '000000, 0000FF, FDFF92, FF2700, FF00E7'
};


Map.centerObject(Indonesia, 5);
Map.addLayer(annualMean,ETViz, 'mean yearly ET');

See an example here.

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