• step change;
  • flood frequency curve;
  • flood estimation;
  • storage threshold

[1] Previous work has shown that non linear catchment response related to a storage threshold may translate into a step change in the flood frequency curve. The aim of this paper is to understand the controls of this step change for catchments where runoff is generated by the saturation excess mechanism and a clear separation between a permanently saturated region and a variably saturated region with spatially uniform storage deficits exists. The magnitude of the step change is quantified by the maximum of the second derivative (curvature) of the flood peaks with respect to their return period. Sensitivity analyses with a stochastic rainfall model and a simple rainfall runoff model show that the magnitude of the step change decreases with increasing temporal variability of antecedent soil storage, and increases with increasing area of the variably saturated region. The return period where the step change occurs is very similar to the return period of the rainfall volume that is needed to exceed the storage threshold. Diagrams are presented that show the joint effects of spatial and temporal storage variability on the magnitude and return period of the step change. The diagrams are useful for assessing whether step changes in the flood frequency curve are likely to occur in catchments where the runoff generation characteristics are as examined here and the flood records are too short to indicate a step change.