Consider the flood series plotted below. If this flood had occurred so
that the peaks were in different water years, only one could be used. So
the water year should be changed in this case.
Flood events are not always well modeled as stationary
For example, dam construction and changes in landuse and
land management can change the probability of floods.
Climate change due to increasing greenhouse gases also
changes in flood probabilities.
And even when climate and landuse/land management are
apparently stable, flood probabilities can change in time.
For example, the El Nino-Southern Oscillation (ENSO)
affects flood probabilities in some locations.
Most routine analyses of flood probabilities assume stationarity. But
in specialized analyses it is possible to account for the presence of
However, given climate change, many experts recommend only using
the last 30 years of flood data.