The index which refers to moisture deficit or surplus in an area is called:

Solution (By Examveda Team)

The term used instead of 'moisture budget', e.g. by C. W. Thornthwaite (1955), and calculated from the aridity and humidity indices, as I m = 100 × (S − D)/PE, where I m is the moisture index, S is the water surplus, D is the water deficit, and PE is the potential evapotranspiration.