The Crop Stress Index is the ratio of actual evapotranspiration (AET) to potential evapotranspiration (PET) express as:
CSI = 1-(AET/PET)
AET and PET are calculated within the Versatile Soil Moisture Budget (VSMB) model using temperature and precipitation data and a crop-specific biometeorological time scale model to estimate growth stage (Robertson, 1968), with crop specific phenological and crop water extraction coefficients taken from Chipanshi et al 2013. The WDI ranges between 0 and 1, with a value closer to 1 indicating higher stress Crop Stress Index is modelled for each climate station using measured precipitation and temperature