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For a paper I am writing I need to know the extend of all drylands on earth. Instead of getting third party data I decided to calculate this myself so I keep in control of all steps of the process.

Online the definition of drylands according to Koppen can be easily found:

Multiply the average annual temperature in Celsius by 20, then add

(a) 280 if 70% or more of the total precipitation is in the spring and summer months (April–September in the Northern Hemisphere, or October–March in the Southern), or

(b) 140 if 30%–70% of the total precipitation is received during the spring and summer, or

(c) 0 if less than 30% of the total precipitation is received during the spring and summer.

If the annual precipitation is less than 50% of this threshold, the classification is BW (arid: desert climate); if it is in the range of 50%–100% of the threshold, the classification is BS (semi-arid: steppe climate)

I tried implementing these factors and calculating the arid and semi-arid areas but I am pretty sure that France, the Amazon rain forest, Hungary and even Thailand are no drylands

I have also taken the median of the extend of the TRMM dataset (2000 - 2017) to eliminate inter annual variability but this did not seem to make any difference.

The code I am using is:

function ClassifyYear(precipitationdata, temperaturedata, year){

  var data = precipitationdata.filterDate(year + "-01-01", (year+1)+"-01-01");
  var Temp = temperaturedata.filterDate(year + "-01-01", (year+1)+"-01-01");
  var AvTemp = Temp.mean().subtract(273.15); //Average Annual temperature

  var PPTns = data.filterDate(year+"-03-20", year+"-09-22").sum(); //Precipitation in the summer of the Northern Hemisphere
  var TotPPT = data.sum();
  var PPTss = TotPPT.subtract(PPTns); //Precipitation in the summer of the Southern Hemisphere

  var mask30 = PPTns.lte(TotPPT.multiply(0.3)); //Masking pixels where summer PPT is less then 30% of the total PPT
  var mask70 = PPTns.lte(TotPPT.multiply(0.7)).subtract(mask30); //Masking pixels where summer PPT is less then 70% of the total PPT
  var mask100 = mask70.remap([0,1], [1,0]); //Masking pixels where summer PPT is more then 70% of total PPT

  var Threshold = AvTemp.multiply(20).add(mask70.multiply(140)).add(mask100.multiply(280)); //Calculating Threshold PPT
  var KoppenDry = TotPPT.lte(Threshold); //If PPT is less then Threshold -> Dryland
  var KoppenArid = TotPPT.lte(Threshold.multiply(0.5)).rename("arid"); //if PPT less then 0.5*Threshold -> Arid
  var KoppenSemi = KoppenDry.subtract(KoppenArid).rename("semi-arid"); //Dryland - Arid = Semi-Arid
  var Koppen = KoppenArid.addBands(KoppenSemi);

  return Koppen;
}

var list = [];
for (var year = 2000; year <= 2017;year++){ //Taking the median climate classification

  var result = ClassifyYear(TRMM.select("precipitation"),GLDAS.select("Tair_f_inst"), year);
  list.push(result);
}
print(list);

Map.addLayer(ee.ImageCollection(list).median());

Does anyone see a mistake that I am missing or knows more about the Koppen classification and finds what wrong with the calculation? The results I am getting:

  • Are you sure you aren't interpreting the colours backwards. The green corresponds pretty well to places where it is known to rain a lot, France, Indonesia, the Amazon included. – John Powell Apr 5 '18 at 9:41
  • When I use the inspector the check the values for the individual pixels, red stands for arid lands and the light blue for semi-arid. Also in Indonesia and the Philippines, where it is known to rain a lot, no values were given. So I assume the problem is rather in the calculation than in the misinterpretation of the colors :) – Sam Apr 5 '18 at 10:02
  • Can you include the link with the regular SE Image-Share option (Imgur)? This way you ensure it to be available in the future for people finding this question. – Kersten Apr 7 '18 at 20:21
1

The problem was that the TRMM dataset is an indicator for the precipitation intensity every 3 hours and not the amount of precipitation. an rain intensity of 10mm/h would have been read as 10mm of rain in 3 hours but this is false and makes the amount of precipitation approximately 3x smaller, resulting in a lot of land that is (semi)arid. The precipitation has to be corrected to represent the amount of precipitation in 3 hours.

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