I want to apply an algorithm to calculate the clorophyll-a concentration of a reservoir using sentinel 2.To do this i need to create a mask using sentinel's 2 band 11(infrared) to separate the water pixels from pixels with larger reflectance values than the water.This is the algorithm: (If B11 <=0.05 then 109.17*((1/B4-1/B5)*B6+ 19.03 else 0).But i can't use if and else in qgis.