Part 4 – Now the Cravats

Ten or more years of data are needed to calculate a frequency analysis determining recurrence intervals. The more historical data the better; a hydrologist will have more faith in an analysis of a river with 30 years of record than one based on 10 years of record.

Also, recurrence intervals at a given location can change, particularly if there are significant upstream operational changes to an impoundment or flow diversion (if one exists) or surrounding land development (rural to urban land uses) resulting in increased paved or impervious surfaces where water runs off and becomes streamflow rather than being absorbed into the soil.

It is also possible that a precipitation event and streamflow event may not correlate. This can be due to the extent of a rainfall event in a watershed. Rainfall may be measured at one point within a watershed and because rainfall is intensely localized and not uniform throughout a basin, the anticipated streamflow may not occur. Similarly, rainfall amounts throughout a basin can differ greatly from the rainfall amount measured at one or two locations because they simply did not receive the same amount of precipitation.


Several factors can independently influence the cause-and-effect relation between rainfall and streamflow.

Finally, another factor affecting the cause-and-effect relationship between rainfall and streamflow is soil saturation before the storm event (antecedent precipitation). Existing conditions prior to a storm event can influence the amount of stormwater runoff into a stream. Dry soils allow greater infiltration of rainfall and reduce the amount of runoff entering the stream. Conversely, soil that is already wet from previous rains has a lower capacity for infiltration, allowing more runoff to enter the stream.

So, in case you thought hydrologists have it all down tightly, many factors need to be carefully weighed when it comes to predicting a flood event. The issue is not whether a flood will occur; that appears certain. The issue for us is “when” will the next flood occur?

You shall remember that you were a slave in Egypt; and you shall be careful to observe these statutes. - Deuteronomy 16.12

“Rain falls on the just and the unjust.” Matthew 5.45