Normal Versus Extreme Hydrologic Events
The immense variability observed in precipitation and streamflow leads one to wonder what constitutes an ‘extreme’ event. For example, most rivers tend to flood (i.e., water completely fills the channel and spills out onto adjacent floodplain) every one to five years. River discharge during such events is often on the order of 10 times the mean annual flow and often 100 to 1000 times greater than the lowest flows. In that context, perhaps they are extreme. However, considering them within the context of all the floods that occur over a century, we refer to floods that occur every one to five years as ‘common floods’ (e.g., all the events below ~ 25,000 cfs for the Lehigh River in Figure 4 shown earlier). So, labeling an event as ‘extreme’ requires some timescale context. Similarly, what we consider ‘extreme’ varies from place to place. For example, a rainfall event that delivers 5 cm of precipitation is quite rare in Utah but is nearly a daily occurrence in parts of Hawaii.
While there is no formal, universal definition for what hydrologists consider to be ‘extreme’ events, there are numerous ways we can assess precipitation and streamflow events within the appropriate context (timescale and location) to determine how they compare with ‘normal’ conditions.
Notice that the distribution of flood events in Figure 4 (on a previous page) has a strong right (also called positive) skew, meaning a long tail to the right of the graph. This positive skew is common in flood frequency data. It is tempting to label the two events that exceed 90,000 cfs as extreme events, but for many rivers there is no clear cut-off. Instead, hydrologists commonly determine the rarity of an event by calculating the frequency with which the event has occurred in the past. They use that frequency as an estimate of the probability that it will occur in the future, as discussed in the example of the Lehigh River. This is useful way to make predictions, but note that climate change prevents us from using the past to predict the future. If the entire event distribution shifts due to climate change, the event probabilities also change. We will address this issue towards the end of the module.
In any case, terms like ‘extreme’ may be useful for news headlines and catchy titles for scientific presentations, but nature doesn’t easily fit into boxes like ‘extreme’ and ‘normal’. Instead, hydrologists tend to use more well-defined terminology to characterize hydrologic events according to their frequency, duration, and magnitude as well as the spatial extent. Events that occur infrequently (i.e., events of low probability) are the ones to watch out for!