A term based on the computation of an annual moisture budget by C. W. Thornthwaite (1955), and calculated from the aridity and humidity indices, as Im = 100 × (S − D)/PE, where Im is the moisture index, S is the water surplus in months when precipitation exceeds evapotranspiration, D is the water deficit in months when evapotranspiration exceeds precipitation, and PE is the potential evaporation.