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Droughts are complex phenomena, which can be defined according to intensity and duration. The relationship between intensity and duration defines many different types of events, all of which may be considered droughts. It is difficult to compare a growing season with almost no rain to a multi-year period with below average precipitation. Even if these two types of events coincide, their recurrence intervals may be very different.
One way to indicate the relative severity of a hydrologic event is by calculating its frequency or recurrence interval. The recurrence interval is the average length of time between 2 events of a given magnitude and duration. It tells how likely an event is, but says nothing about when it will come. Recurrence intervals can be calculated for selected time scales and durations. The recurrence interval for the annual 7-day low flow may vary significantly from the 90-day low flow or from the recurrence interval of a multi-year surface-water drought event.
The 7 (90)-day lowflow is the annual minimum 7 (90)-day consecutive average discharge that is equaled or exceeded on average every x number of years (where x is the recurrence interval) (Maidment, 1993). Each year from April 1-March 31 is considered independently while calculating 7-day or 90-day lows. A 7-day low flow with a large recurrence interval can indicate low ground water levels. A 90-day low flow can indicate a lack of precipitation during a summer growing season.
The recurrence intervals for the 6 lowest years on record were calculated at selected stations for both the 7-day low-flows and the 90-day low-flows. The 6 lowest years may or may not coincide between stations or between 7 and 90 day lows. It is important to note that there may be multi-year periods, which have below average precipitation or surface-water, but they do not show up in the tables because no single year within that time period at that station was extremely low. 2002 low-flow statistics included the 7-day low, but not the 90-day low because data were not yet available.
A map of approximate regional recurrence intervals for 7-day surface-water low flows shown below is for 2001. Click here for regional recurrence intervals for historic droughts
| Station # | Station Name | Period Of Record |
|---|---|---|
| 01014000 | St John River at Fort Kent | 1927-2002 |
| 01015800 | Aroostook River near Masardis | 1958-2002 |
| 01030500 | Mattawamkeag River near Mattawamkeag | 1935-2002 |
| 01031500 | Piscataquis River at Dover Foxcroft | 1903-2002 |
| 01022500 | Narraguagus River at Cherryfield | 1949-2002 |
| 01047000 | Carrabassett River nr North Anson | 1926-2002 |
| 01052500 | Diamond River near Wentworth, NH | 1942-2002 |
| 01057000 | Little Androscoggin River near South Paris | 1914-2002 |
| 01060000 | Royal River at Yarmouth | 1950-2002 |
| 01064500 | Saco River near Conway, NH | 1904-2002 |
| 01038000 | Sheepscot River at North Whitefield, ME | 1939-2002 |
| 01010000 | St. John River at Ninemile Bridge, ME | 1951-2002 |
| 01054200 | Wild River at Gilead | 1965-2002 |
| 01055000 | Swift River near Roxbury, ME | 1930-2002 |