Conditional statements with formulas in Google Earth Engine

Question

I would like to apply an expression to an image based on conditions. The part of my code that I need to do this in is where i need to calculate runoff Q2 based on values of AMC and S:

var runoff_func = function(image) {

  var AMC = image.select('precipitation_1')
  var ppt = image.select('precipitation')
  var AMCreplaced = S_image2_slope_corrected.where(AMC.lte(13), S_image1);
  var AMCreplaced2 = AMCreplaced.where(AMC.gt(28), S_image3)
  var s_value = AMCreplaced2.select('S_image2_slope_corrected')

  // adjusted Q based on Verma et al., 2018

  var Q2 = image.expression(
    '((ppt-0.12*S_image + 0.19*(sqrt(ppt*S_image))*(ppt-0.12*S_image +0.38*S_image))/(ppt+0.88*S_image)+(0.38*S_image))', {
      'ppt': image.select('precipitation'),
      'S_image': AMCreplaced2.select('S_image2_slope_corrected')
    });

  var Q3 = Q2.where(ppt.lt(s_value.multiply(0.2)), zeroImage)
  return Q3.clip(Drakensberg).rename('runoff')
    .copyProperties(image, ["system:time_start"]);
};
var runoff = MergedRain_AMC.map(runoff_func)

print(runoff)

I would like to calculate Q2 based on the following conditions:

If ppt <= (0.12*S_image2 - 0.19*S_image2) then Q = 0
If (0.12*S_image2 -P)<= 0.19 *S_image2 <0.12*S_image2_slope_corrected  then Q = ((ppt-0.12*S_image + 0.19*(sqrt(ppt*S_image))*(ppt-0.12*S_image +0.38*S_image))/(ppt+0.88*S_image)+(0.38*S_image))
If 0.12*S_image2 <= 0.19*S_image2<= 1.12*S_image2 then

Q = ppt (1- (1.12*S_image2 - 0.19*S_image2)**2/ppt*(1.12*S_image2 - 0.19*S_image2)+S_image2*(S_image2+0.19*S_image2))

Here is the link to my code:

https://code.earthengine.google.com/?scriptPath=users%2Fmorriskolawole%2Ftest%3ASCS_CN_Verma%20method_cathment

Answer 1

You can use ee.Image.where() for conditionals. Note that if more than one condition match, the last will get applied. Using ee.Image.expression() makes the conditions a bit more readable when they're complex. To make it more concise, I created a function to evaluate your specific variables.

var Q = ee.Image(-1) // Value to use when none of the conditions match
  .where(
    evaluate('ppt < (0.12*S_image2 - 0.19*S_image2)'), 
    0
  )
  .where(
    evaluate('(0.12*S_image2 -P)<= 0.19 *S_image2 <0.12*S_image2_slope_corrected'), 
    evaluate('((ppt-0.12*S_image + 0.19*(sqrt(ppt*S_image))*(ppt-0.12*S_image +0.38*S_image))/(ppt+0.88*S_image)+(0.38*S_image))')
  )
  .where(
    evaluate('0.12*S_image2 <= 0.19*S_image2<= 1.12*S_image2'), 
    evaluate('ppt*(1- (1.12*S_image2 - 0.19*S_image2)**2/ppt*(1.12*S_image2 - 0.19*S_image2)+S_image2*(S_image2+0.19*S_image2))')
  )

function evaluate(expression) {
  return ee.Image().expression(expression, {
    ppt: ppt,
    S_image: S_image,
    S_image2: S_image2,
    S_image2_slope_corrected: S_image2_slope_corrected,
    P: P
  })
}

https://code.earthengine.google.com/d2579a461fad8ca25c8bef674aee7524