This study investigates the impacts of climate seasonality, i.e., the seasonal cycle of precipitation (P) relative to that of potential evaporation (PET), on surface water supply and the long-term water partitioning and proposes an augmented aridity index considering climate seasonality in addition to climatic mean. Evaporation tends to be favored over streamflow at long-term timescales when both cycles occur in tandem (in-phase seasonality), while the opposite occurs (less evaporation, more streamflow) when the two cycles are out-of-phase. This study proposes a straightforward approach to incorporating the seasonality effects on the mean annual water balance into the Budyko framework, by revising the water availability (A) in the formulation of the aridity index (Φ). We hypothesize the Budyko curve represents catchments with uniform monthly values of P, leading to a mathematical formulation of A that better represents the coupled, land-atmosphere nature of the water availability. Our results also provide a simple mathematical framework for incorporating the seasonality into the aridity index, thus reducing the dimensionality of the long-term water balance problem through an aridity-seasonality index (Φ’). The formulation used here was able to improve the explanatory power of the Budyko framework for 328 catchments within the continental US, being proved as a useful strategy for the incorporating climate variations into its formulation in addition to climatic mean.