where \(\text{ETa}_{\text{ij}}\) is the actual evapotranspiration value at position i at time j, \({\overset{\overline{}}{\text{ETa}}}_{j}\) the average actual evapotranspiration of all positions in space at time j,\(\sigma\) the standard deviation and np the number of pixels. Low TSI values indicate zones with high temporal stability of ETa, while high TSI values indicate zones of low stability over time.