where \(n_{\text{irr}}\) is the index of the soil layer corresponding to\(z_{\text{irrig}}\), \({z}_{i}\) (mm) is the depth of the soil layer\(i\) and \(\theta_{target,i}\) is the target volumetric SM value in a
given soil layer. Similarly, \(w_{\text{wilt}}\) is calculated as the
sum of SM at wilting point of each soil layer: