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.