cfs and acre-feet

cfs: a rate of flow

A cubic foot per second (cfs) is a cube of water one foot on each side — about 7.48 gallons — passing a point every second. Stream gauges report flow in cfs, and Colorado water rights for ditches are decreed in cfs: a 10 cfs right lets the ditch divert up to 10 cubic feet each second while the right is in priority.

One cfs equals about 448.8 gallons per minute. A small irrigation ditch might carry a few cfs; the Colorado River near the Utah line can carry thousands in spring runoff.

Acre-feet: a volume

An acre-foot (AF) is the amount of water that would cover one acre to a depth of one foot: 43,560 cubic feet, or about 325,851 gallons. Reservoir storage, annual diversions, compact deliveries and municipal supplies are counted in acre-feet. Large figures use thousands of acre-feet (KAF) or millions (MAF).

Converting between them

The working rule: multiply cfs by about 2 to get acre-feet per day (1.9835 exactly). The reverse: one acre-foot per day is about half a cfs (0.504 cfs).

The live box above applies this to today's flow at a Division 5 gauge, so you can see what the number means in volume.

Worked examples

• A ditch with a 2 cfs right that is in priority for 100 days can divert about 2 × 1.98 × 100 ≈ 397 acre-feet over the season.
• A reservoir release of 50 cfs for a week moves about 50 × 1.98 × 7 ≈ 694 acre-feet.
• A town that needs 1,000 acre-feet a year uses, on average, about 1.4 cfs (1,000 ÷ 724).

Why both are used

A river is water in motion, so you measure how fast it passes: cfs. A reservoir is water at rest, so you measure how much is there: acre-feet. Water rights reflect the same split. A direct-flow right is decreed as a rate (cfs) that can be diverted while in priority. A storage right is decreed as a volume (acre-feet) that can be stored in a reservoir.

When people compare the two — “the ditch took 1,200 acre-feet this season” — they have added up daily flows over time.

How gauges measure cfs

A gauge records the water level (stage) every few minutes. Hydrographers periodically measure the actual flow by wading or boating across the channel and measuring depth and velocity at many points. Those measurements build a rating curve that turns stage into discharge. Rating curves shift when the channel changes, so recent readings are provisional and can be revised; ice in winter can make readings unreliable. USGS and DWR both operate gauges in Colorado, and their real-time values carry that provisional label.

Percent of median

A flow number alone does not say whether the river is high or low for the season. BasinMind compares each USGS gauge's current flow with the USGS median daily flow for the same calendar day over the gauge's period of record. 100% means typical for the date; 50% means half the usual flow. DWR telemetry stations do not come with that median, so for them BasinMind shows the flow without a percentage.

Other units you will meet

• Miner's inch — an old unit still found in some historic decrees; its value has varied by state and era, so check the decree.
• Snow water equivalent (inches) — the depth of water in the snowpack; see Snowpack and SWE.
• Gallons per minute — common for wells and pumps.