Lesson 4 of 9 · about 17 minutes
How Irrigation Works: Canals, Acre-Feet and Efficiency
You'll be able to
- Tell a canal company from an irrigation district and explain shares and assessments
- Convert cfs to gallons per minute and acre-feet per day, and acre-feet to gallons
- Compare application efficiency for flood, furrow, sprinkler and drip systems
- Explain why higher efficiency can reduce aquifer recharge
Southern Idaho farms on water that fell as snow in Wyoming and on the high country months earlier. Reservoirs store it, rivers carry it, canals spread it, and farmers apply it by the inch. The ARS research lab at Kimberly puts it bluntly: few crops are grown in southern Idaho without irrigation. Its summary, which draws its acreage from the 1997 Census of Agriculture, counts about 3,188,000 irrigated acres in the state, and notes that the irrigated land produces almost 80 percent of the state's crop value.
Who moves the water
| Type | What it is | Who decides | How it pays |
|---|---|---|---|
| Mutual canal company | A private corporation. Shares of stock carry the right to a portion of the company's water. | Shareholders elect a board | Assessments per share |
| Irrigation district | A public entity formed under state law, with defined boundaries | Voters in the district elect a board; board meetings are public | Assessments levied on land in the district |
| Groundwater district | A public entity of well owners, often formed to carry out a mitigation plan | Elected board | Assessments on members |
| Individual right holder | A farm with its own decreed or licensed right, its own pump or ditch | The owner | Pumping power bills, ditch maintenance |
Public irrigation districts produce budgets, minutes and assessment rolls you can request. Mutual canal companies are private, so their records are harder to get, though the water rights they hold are public at IDWR.
The units, worked out
Two units do almost all the work. An acre-foot is a volume. Cubic feet per second is a rate. USGS defines an acre-foot as 325,851 gallons and one cfs as 7.48 gallons per second. Everything else follows.
Worked example: the conversions that come up constantly
cfs to gallons per minute. 7.48 gallons per second times 60 seconds = 448.8 gallons per minute. Call it 449.
cfs to acre-feet per day. 7.48 times 86,400 seconds in a day = 646,272 gallons. Divide by 325,851 = 1.983 acre-feet. So 1 cfs for one day is just under 2 acre-feet. Field people round it to 2.
A canal's season. An invented canal diverts 400 cfs for 180 days. 400 times 1.983 times 180 = 142,776 acre-feet.
Acre-feet to inches on a field. 1 acre-foot on 1 acre is 12 inches deep. So 300 acre-feet spread over 125 acres is 300 divided by 125 = 2.4 feet, or 28.8 inches.
Acre-feet to gallons. 1,000 acre-feet times 325,851 = 325,851,000 gallons, about 326 million.
| From | To | Multiply by |
|---|---|---|
| cfs | gallons per minute | 448.8 |
| cfs for 1 day | acre-feet | 1.983 |
| acre-feet | gallons | 325,851 |
| acre-feet | cubic meters | 1,233 |
| million gallons per day | acre-feet per day | 3.069 |
How much water a crop uses
A crop's water use is mostly evapotranspiration: water evaporating from soil and leaves. It depends on the crop, the weather and the length of the season. Alfalfa cut three or four times in a long hot season uses far more than spring grain harvested in July. IDWR and the University of Idaho publish crop water use estimates by weather station. Use those for local numbers rather than any single statewide figure. In the examples below, crop needs are invented for the math.
Efficiency: flood, furrow, sprinkler, drip
Application efficiency is the share of applied water that ends up stored in the root zone where the crop can use it. The rest runs off the end of the field or sinks below the roots.
| System | Application efficiency | Source |
|---|---|---|
| Wild flooding | 15 to 35 percent | University of Idaho CIS 1055 |
| Furrow | 35 to 60 percent | CIS 1055 |
| Surface irrigation in general | 30 to 50 percent | USDA-ARS Kimberly |
| Wheel line or hand line | 60 to 75 percent | CIS 1055 |
| Center pivot | 70 to 85 percent (CIS 1055); 75 to 85 percent (ARS) | Both |
| Linear move | 80 to 87 percent | CIS 1055 |
| Drip | 90 to 95 percent; over 95 percent for buried drip | CIS 1055; ARS |
The ARS summary counts about 2,187,000 sprinkler acres in Idaho (67 percent), about 992,000 surface-irrigated acres (31 percent), and only about 1,080 acres of drip. Those are 1997 Census of Agriculture figures, so they are a historical snapshot. Sprinkler and drip acreage has grown since; for current numbers, use the latest USDA NASS Census of Agriculture and its irrigation tables. The two sources use slightly different ranges, which is normal; efficiency depends on soil, slope, pressure and management.
Worked example: same field, two systems
An invented 125-acre field on the Halvern place needs 24 inches of water stored in the root zone over the season (a made-up figure for the math).
Furrow at 45 percent. Applied = 24 divided by 0.45 = 53.3 inches. Over 125 acres: 53.3 divided by 12 = 4.44 feet, times 125 = 555 acre-feet.
Center pivot at 80 percent. Applied = 24 divided by 0.80 = 30 inches. 30 divided by 12 = 2.5 feet, times 125 = 312.5 acre-feet.
Difference. 555 minus 312.5 = about 243 acre-feet less diverted or pumped.
Now the part most discussions miss. The crop used the same 24 inches either way: 2 feet times 125 acres = 250 acre-feet consumed. Under furrow, the other 305 acre-feet ran off or soaked down. Some returned to a drain or stream for a downstream user. Some recharged the aquifer. Under the pivot, only about 62.5 acre-feet went past the roots. So the switch cut diversions by about 243 acre-feet while consumption stayed at 250, and recharge fell.
This is why the ARS Kimberly summary says much of southern Idaho's groundwater came from past inefficient irrigation, and that without that recharge water tables would fall and spring flows would drop. Sprinklers let farmers grow more with less diversion and less labor, and they reduce erosion and fertilizer loss. They also changed the water balance in ways the delivery call is still sorting out.
Common mix-ups
- Claim: Switching to sprinklers frees up water for new users. What the evidence shows: It reduces diversions, but the water saved was often someone else's return flow or recharge. Consumptive use stays about the same, or rises if the farmer now waters more acres.
- Claim: Flood irrigation wastes water. What the evidence shows: It applies more per inch stored, but the excess largely stays in the basin. "Waste" depends on whether anyone downstream or underground was counting on it.
Reporting angle
- Ask an irrigation district for its diversion records at the headgate and its deliveries at turnouts. The gap is canal loss, and canal loss is recharge.
- When an efficiency program claims "water saved," ask: saved from diversion or from consumption? Where did that water go before?
- Request district budgets and assessment rates per acre over five years. Rising assessments tell a story about aging canals.
- Convert every number to one unit before you compare. Show the math in the story or a sidebar.
Research angle
- University of Idaho Extension publication CIS 1055: extension guidance on irrigation.
- USDA ARS irrigation overview: a basic government explainer.
- USGS Water Science glossary: definitions for cfs, acre-feet and more.
- IDWR Surface Water Coalition delivery call: how delivery shortfalls are documented.
Check yourself
1. One cfs running for 24 hours delivers about how many acre-feet?
7.48 gallons per second times 86,400 seconds is 646,272 gallons. Divide by 325,851 gallons per acre-foot: about 1.98 acre-feet.
2. What range of application efficiency does USDA-ARS give for center pivot sprinklers?
The ARS Kimberly summary lists 75 to 85 percent for center pivots, 30 to 50 percent for surface irrigation and over 95 percent for buried drip.
3. A field's crop needs 24 inches of water stored in the root zone. With a furrow system at 45 percent efficiency, about how much must be applied?
Applied = needed divided by efficiency: 24 divided by 0.45 is about 53.3 inches.
4. Where does the extra water applied by a low-efficiency system mostly go?
Water not stored in the root zone runs off or sinks below it. ARS notes much of southern Idaho's groundwater came from past inefficient irrigation.
5. What is the key structural difference between a typical irrigation district and a mutual canal company?
Irrigation districts are local government units organized under state law with assessment powers. Mutual canal companies are private corporations whose shares carry water.
6. About what share of Idaho's irrigated acres were sprinkler-irrigated in the 1997 Census of Agriculture figures the ARS summary uses?
ARS lists about 2,187,000 sprinkler acres of 3,188,000 irrigated acres, about 67 percent, with about 31 percent surface irrigated.
Terms in this lesson
- Acre-foot
- The water needed to cover one acre one foot deep: 325,851 gallons, or 1,233 cubic meters.
- Cubic feet per second (cfs)
- A flow rate. One cfs is 7.48 gallons passing a point each second.
- Application efficiency
- The share of applied irrigation water that ends up stored in the crop root zone.
- Consumptive use
- Water withdrawn that is evaporated, transpired by plants or built into crops, and so removed from the local water supply.
- Irrigation district
- A public entity formed under state law to deliver irrigation water, governed by an elected board, with power to assess land.
- Return flow
- Irrigation water that runs off or seeps back to a stream or aquifer and can be used again downstream.
Sources
- USGS Water Science School glossary checked 2026-10
- USDA-ARS Kimberly, Irrigation in southern Idaho (Bjorneberg) checked 2026-10
- University of Idaho Extension CIS 1055, Irrigation Systems for Idaho Agriculture (Neibling) checked 2026-10
- IDWR, Surface Water Coalition Delivery Call overview checked 2026-10