Low flow is the “flow of water in a stream during prolonged dry weather,” according to the World Meteorological Organization. Many states use design flow statistics such as the 7Q10 (the lowest 7-day average flow that occurs on average once every 10 years) to define low flow for the purpose of setting permit discharge limits.
What factors affect variations in flow?
- Rainfall and snowmelt
- Land use/land cover (e.g., the permeability or imperviousness of surrounding land)
- Water control structures (e.g., reservoirs and dams)
- Water intakes (e.g., for drinking water and industrial cooling)
- Water discharges (e.g., from industries, utilities, and wastewater treatment plants)
- Geological characteristics (e.g., groundwater flow and stream slope)
Do low flows occur at the same time each year?
Most streams will illustrate annual variation that can be explained by seasonal changes in snowmelt, rainfall, and other factors. For many areas in the country, the lowest flows often occur near the end of the summer or beginning of fall. However, each stream is different and any particular year can be an anomaly in terms of if and when low flows occur. The magnitude and duration of low flows can vary significantly from year to year.
Why does a low flow year begin in April while a water year begins in October?
The U.S. Geological Survey defines a “water year” as the 12-month period from October 1 through September 30 of the following year. This definition is appropriate for dealing with water supply and high flow statistics since typically the lowest flows of the year occur in the fall months. However, for low flow calculations, April 1 through March 31 is typically used.