Most previous studies and applications in deriving or applying FFC are underpinned by the stationarity assumption. To examine the theoretical robustness of this basic assumption, we analyzed the observed FFCs at hundreds of catchments in the contiguous United States along the gradients of climate conditions and human influences. The shape of FFCs is described using three similarity indices: mean annual floods (MAF), coefficient of variance (CV), and a seasonality index defined using circular statistics. The characteristics of catchments are quantified with a small number of dimensionless indices, including particularly: 1) the climatic aridity index, AI, which is a measure of the competition between energy and water availability; 2) reservoir impact index, defined as the total upstream reservoir storage capacity normalized by the annual streamflow volume. The linkages between these two sets of indices are then explored based on a combination of mathematical derivations of the Budyko formula, simple but physically based reservoir operation models, and other auxiliary data. It is found that the shape of FFCs shifts from arid to humid climate, and from periods with weak human influences to periods with strong influences. The seasonality of floods is found to be largely controlled by the synchronization between the seasonal cycles of precipitation and solar radiation in pristine catchments, but also by the reservoir regulation capacity in managed catchments. Our findings may help improve flood-risk assessment and mitigation in both natural and regulated river systems across various climate gradients.

https://doi.org/10.1002/2013WR014374