89
Cows one acre-foot covers at 10 gallons a day
01
The Unit
An acre-foot is one acre of water, one foot deep: 325,851 gallons. An acre is 43,560 square feet, a little smaller than a football field with its end zones (57,600 square feet). New Mexico counts reservoirs, canals, and water rights in acre-feet, so Lake Level Watch does too.
Conversions used on this page
1 acre-foot = 325,851 gallons = 12 acre-inches. 1 acre-inch = 27,154 gallons. A flow of 1 cubic foot per second for one day moves about 1.98 acre-feet.
One acre-foot is about
| Use | What one acre-foot covers | Section |
|---|---|---|
| Pecans | 0.2 to 0.3 acre for a year, or 340 to 440 lb of nuts | 2 |
| Alfalfa | 0.25 acre for a year, or about 1.75 tons of hay | 2 |
| Cotton | 0.4 acre for a year, or 435 to 560 lb of lint | 2 |
| Cattle | 45 to 89 cows drinking for a year | 3 |
| People on a Sierra County water system | 4 to 11 people for a year | 4 |
| Motel | About 12 rooms for a year | 5 |
| RV park with hookups | About 9 sites for a year | 5 |
| Restaurant | 18 to 30 seats for a year | 5 |
| Data center (upper-end reports) | About 1 day of a medium-sized one; about 1.6 hours of a hyperscale one | 5 |
| Lake surface | One acre of open water loses about 5 acre-feet a year to evaporation | 6 |
Each row is worked out in its own section, with the source and the range.
02
The Farm — What a Crop Needs
A crop's water need is its consumptive use: the water the plants take up and release over the season. Consumptive use is need, not delivery. How much a farm actually receives from the canal and its wells is a separate question, listed at the end of this section as in development.
In a 2001 NMSU survey of the Elephant Butte Irrigation District, pecans, alfalfa, and cotton were planted on almost 80 percent of irrigated acres. Green chile and onions are also grown in the valley.
| Crop | Needs per acre (acre-feet) | Yield per acre | One acre-foot grows | Acres one acre-foot covers | Status |
|---|---|---|---|---|---|
| Pecans | 3.3 – 5.0 | 1,450 – 2,000 lb | 340 – 440 lb | 0.2 – 0.3 | Published |
| Alfalfa | about 4.0 | about 7 tons of hay | about 1.75 tons | 0.25 | Published |
| Cotton | about 2.5 | 1,090 – 1,400 lb of lint | 435 – 560 lb | 0.4 | Published |
| Green chile | about 3.1 | about 33,000 lb | about 10,500 lb | 0.3 | Estimate |
| Onions | about 2.1 | about 40,000 lb (800 sacks of 50 lb) | about 19,000 lb | 0.5 | Estimate |
Why some rows are ranges and two are estimates
Pecans. NMSU's benchmark is about 5.0 acre-feet per acre for mature trees. The Office of the State Engineer's 2020 report cites 3.3 to 4.3 acre-feet for the growing season. Both are published, so the table shows the full range. The yield range runs from the county's ten-year average (about 1,450 lb) to the benchmark (2,000 lb). Cotton. NMSU's report lists lint yield as 1,089 lb in one place and 1,402 lb in another, so the table shows both. Chile and onions. These rows are worked out from NMSU research: the chile figure from a drip-irrigation yield function (1977–81 trials), and the onion figure from the state's average yield of about 800 sacks per acre. They are marked as estimates until a published per-acre figure replaces them.
What the farmer gets
In developmentThe next layer sets these needs beside what the Elephant Butte Irrigation District delivers each season: orders, allotments, and the groundwater farms pump to make up the difference.
03
The Ranch — Cattle Drink Little
The Office of the State Engineer counts 10 gallons per head per day for beef cattle: 9 for drinking and 1 for other uses. At that rate a cow uses 3,650 gallons a year, which is 0.011 acre-feet. One acre-foot covers 89 cows for a year. A lactating cow in 70-degree weather drinks closer to 20 gallons a day (NMSU and National Research Council figures), and at that rate one acre-foot covers 45.
45
Cows one acre-foot covers at 20 gallons a day
76 – 107
Acre-feet of rain falling on the range one cow needs in a year
The forage comes from rain, not from the river or a well. NMSU's carrying capacity for the southwest ranching area is 5 to 7 animal units per section, year-round. A section is 640 acres, so each cow needs 91 to 128 acres. At 10 inches of rain a year, a round figure for the region, that land receives 76 to 107 acre-feet. That is 6,800 to 9,500 times what the cow drinks.
Where Sierra County's livestock water came from, 2020
| Source | Acre-feet | Share |
|---|---|---|
| Groundwater | 339 | 88% |
| Surface water | 44 | 12% |
| Total livestock | 384 | 100% |
Most ranch water in Sierra County is pumped from the ground. It does not come from the river. The Office of the State Engineer's total covers cattle, horses, sheep, hogs, and poultry.
04
Homes — People on a Water System
The Office of the State Engineer reports each water system's withdrawals and the people it serves. Dividing one by the other gives gallons per person per day. These rates are total system withdrawals divided by residents served.
| System, 2020 | People served | Gallons per person per day | Acre-feet a year | People one acre-foot serves |
|---|---|---|---|---|
| Truth or Consequences | 6,521 | 205 | 1,498 | 4.4 |
| Elephant Butte Water System | 1,431 | 105 | 169 | 8.5 |
| City of Elephant Butte | 759 | 99 | 84 | 9.0 |
| Garfield MDWCA | 979 | 94 | 103 | 9.5 |
| Self-supplied domestic (wells) | 1,392 | 80 | 125 | 11.2 |
Self-supplied domestic uses the state's default of 80 gallons per person per day. All of these systems draw groundwater.
Across Sierra County, public water systems withdrew 1,893 acre-feet in 2020 for the 10,184 people they served, an average of 166 gallons per person per day. The county's 2020 census population was 11,576.
Domestic wells
A single-household domestic well may divert up to 1.0 acre-foot a year (19.27.5.9 NMAC). The State Engineer can raise that to 3.0 acre-feet where combined domestic wells will not impair existing water rights, and new wells in a domestic well management area are generally capped at 0.25 acre-foot.
05
Businesses — From a Motel to a Data Center
Everyday local businesses
Septic-system rules assume a daily sewage volume for each kind of business, called a design flow. Design flow is a close stand-in for the water a business uses. The rates below come from Minnesota's rule and Douglas County, Colorado's table, not New Mexico's, so treat them as approximate. They leave out employees.
| Business | Design flow | What one acre-foot covers | A typical size |
|---|---|---|---|
| Motel | 75 gal per room per day | about 12 rooms | 50 rooms, full: 4.2 acre-feet a year |
| RV park with hookups | 100 gal per site per day | about 9 sites | 100 sites, full: 11.2 acre-feet a year |
| Restaurant | 30 – 50 gal per seat per day | 18 to 30 seats | 60 seats: 2.0 – 3.4 acre-feet a year |
Motel: Douglas County, 75 gal per room; Minnesota, 38 gal per guest. RV park: both sources, 100 gal per site with sewer hookup. Restaurant: Minnesota, 30 gal per seat (open 16 hours or less); Douglas County, 50 gal per seat (one or two meals).
A large water user, for contrast
Data center water use depends mostly on the cooling method. Evaporative cooling is the most common and uses the most water. Liquid cooling, immersion cooling, and closed-loop systems use much less. The figures below are upper-end reports, not a measure of any particular facility.
| Data center size | Reported use | Acre-feet a year | One acre-foot equals |
|---|---|---|---|
| Medium-sized | up to about 110 million gal a year (about 301,000 a day) | about 338 | about 1.1 days of use |
| Large (hyperscale) | up to 5 million gal a day (1.8 billion a year) | about 5,600 | about 1.6 hours of use |
Against Sierra County's 2020 totals, a medium-sized data center at the upper-end figure would equal 18 percent of all public water supply and 32 percent of all self-supplied commercial use. A hyperscale one would equal about 3 times the public supply, 5 times the commercial use, and 28 percent of all irrigation.
06
The Lake — What Evaporates
Water leaves a reservoir through its surface. A Bureau of Reclamation study estimated gross evaporation at Elephant Butte of 57.6 to 62.1 inches a year (1994–96), which is 4.8 to 5.2 acre-feet for every acre of open water. That is the same range as an acre of mature pecans (3.3 to 5.0 acre-feet).
79,669
Acre-feet net evaporation, Elephant Butte, 2020
29,666
Acre-feet net evaporation, Caballo, 2020
4.6×
Sierra County's reservoir evaporation compared with every other use combined, 2020
Net evaporation is gross evaporation minus the rain that fell on the lake. Together the two reservoirs lost 109,335 acre-feet in 2020. Every other use in Sierra County added up to 23,707 acre-feet. Elephant Butte's evaporation alone was 42 times the county's public water supply.
How evaporation is measured
- Pan on the shore. This is Reclamation's official basis at Elephant Butte: a Class A evaporation pan on land, multiplied by 0.7. Studies rate pans among the least accurate methods for reservoirs.
- Models. Weather and lake models calculate evaporation from wind, humidity, and temperature.
- Towers and satellites. An eddy covariance tower measures one spot on the lake. NMSU researchers have paired it with remote sensing to cover the whole surface.
- Water budget. Inflow, minus outflow, minus the change in storage, leaves evaporation. Errors in every other term collect in the answer.
The methods differ. For 1994–96, two of them gave totals 17,600 to 22,000 acre-feet apart. That is about the size of all irrigation in Sierra County in 2020 (20,235 acre-feet).
What can be recovered
- Surface films. Reports cite 10 to 25 percent reductions in general. Field tests on larger reservoirs ranged from about 9 to 63 percent, and the highest came from a 10-hour test under ideal conditions. Wind pushes the film to the shore and it breaks down in days, so it has to be reapplied. A Texas lake test measured about 15 percent, with a statistical range of 0 to 26 percent.
- Covers. Floating covers reduce evaporation on the area they cover. Cost scales with the acres covered, and a reservoir the size of Elephant Butte has a very large surface.
- Where water is stored. Evaporation follows surface area, not volume. Underground storage loses almost nothing to evaporation. Arizona's water bank had recharged over 4.46 million acre-feet through 2020. Albuquerque dedicated the state's first permitted aquifer storage and recovery project at Bear Canyon in 2014.
07
Sierra County — 2020 Compared With 1995
The Office of the State Engineer publishes water use for every county. These are Sierra County's withdrawals in acre-feet.
| Category | 2020 | 1995 | Change |
|---|---|---|---|
| Public water supply | 1,893 | 2,467 | down 23% |
| Self-supplied domestic | 125 | 119 | about the same |
| Irrigated agriculture | 20,235 | 20,566 | about the same |
| Livestock | 384 | 560 | down 31% |
| Commercial (self-supplied) | 1,056 | 546 | nearly doubled |
| Mining | 14 | 18 | about the same |
| Industrial | 0 | 25 | none reported in 2020 |
| Reservoir evaporation (net) | 109,335 | — | — |
- Irrigation is 85 percent of the county's withdrawals other than evaporation, and 55 percent of that irrigation water was pumped from the ground. About 4,687 acres were irrigated.
- Truth or Consequences' golf course runs on treated effluent, which the report does not count separately.
- 2020 precipitation was well below average, so crop water needs were generally higher than in a typical year.
- Power generation also reported 0 acre-feet in 2020.
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