One acre-foot of water is the amount of water required to cover 1 acre of land to a depth of 1 foot. This measurement is commonly used to describe volumes of water, especially related to resources like reservoirs, aqueders, irrigation, and runoff. Understanding what an acre-foot represents helps conceptualize the immense quantities involved with water resources.
Quick Overview
Here are some key facts about 1 acre-foot of water:
- Equals 326,000 gallons
- Equals 43,560 cubic feet
- Enough to meet the needs of a typical family for 1 year
- Enough to irrigate about 1 acre of farmland
- A football field 1 acre in size covered by 1 foot of water
Visualizing an Acre-Foot of Water
Since an acre-foot is an abstract concept, it helps to visualize what this volume looks like:
- If spread over an acre of land, it would be a layer 1 foot deep.
- Stacking 326,000 gallons would create a cube of water about 89 feet on each side.
- Pooling 43,560 cubic feet together would be a cube about 66 feet per side.
- Filling up a football field 1 acre in size to a depth of 1 foot requires 1 acre-foot of water.
Acre-Foot Equivalents
An acre-foot is equivalent to:
- 325,851 gallons
- 43,560 cubic feet
- 1,233 cubic meters
- About 326,000,000 pounds (163,000 tons) of water
The acre-foot is commonly used by irrigation districts, water utilities, government agencies, and other entities that manage water as a resource. By using a standard volume measurement, water supplies and usage can be quantified and managed.
Residential Water Use
In residential terms, about 1 acre-foot of water can supply a typical suburban family for 1 year. This includes both indoor household uses as well as outdoor irrigation. The exact amount depends on usage habits, such as the number of residents, appliances, amount of landscaping, and climate.
On average, a single individual uses about 1,800 gallons per month. So a family of 4 would use about 7,200 gallons per month or 86,400 gallons per year. This is about 0.27 acre-feet per year.
Outdoor Water Usage
Outdoor watering for lawns, gardens and landscaping often accounts for 50% or more of a household’s usage. In hot, dry climates this can be much higher.
Typical residential lawns require about 18,000 gallons per 1,000 square feet over a year. So a 5,000 sq ft suburban lawn would need 90,000 gallons. Add in extra for a garden and other landscaping and a household could easily use 150,000+ gallons just for irrigation in a dry climate.
Agricultural Water Use
For agricultural irrigation, 1 acre-foot is approximately the amount of water needed to irrigate 1 acre of farmland. However, this can vary significantly depending on the crop, soil, climate, and irrigation method. Some examples:
- Corn: 1 – 2 acre-feet per acre
- Wheat: 2 – 3 acre-feet per acre
- Alfalfa: 4 – 5 acre-feet per acre
- Vegetables: 2 – 6 acre-feet per acre
- Orchards: 3 – 4 acre-feet per acre
- Vineyards: 1 – 3 acre-feet per acre
In areas where rainfall is insufficient to grow crops, irrigation must supplement precipitation. Modern agriculture often relies on withdrawing groundwater or diverting surface water to provide enough irrigation throughout the growing season.
Water Requirements Vary by Region
The amount of water needed depends heavily on climate factors. Cool, wet climates may only need irrigation to supplement rainfall now and then. Hot, dry regions may require extensive irrigation continuously throughout the growing season.
For example, parts of the upper Midwest may only need 5-10 inches of irrigation water applied over an entire season. Meanwhile, arid states like Arizona or California may need 40 inches or more applied via irrigation during a single growing season.
| State | Typical Inches of Irrigation Water Needed Per Acre |
|---|---|
| North Dakota | 5 |
| Iowa | 10 |
| Kansas | 15 |
| Texas | 20 |
| California | 40 |
One inch applied over an entire acre equals 27,152 gallons of water. So you can see how crops in drier climates like California require significantly more irrigation water to account for less rainfall.
Water Usage by State
The USGS tracks water usage for each state. The major categories are:
- Thermoelectric: power plant cooling
- Irrigation: agricultural irrigation
- Public supply: residential, commercial, industrial uses
- Aquaculture: raising aquatic animals like fish for food
- Mining: extracting minerals and fossil fuels
- Livestock: watering animals
The top 5 water using states in 2015 were:
| State | Total Water Use (million acre-feet) |
|---|---|
| California | 41.7 |
| Texas | 17.6 |
| Idaho | 16.8 |
| Florida | 12.7 |
| Illinois | 12.1 |
The mix of water usage varies significantly by state based on climate, population, agriculture, economy, and geography. For example:
- California uses 9.6 million acre-feet per year for irrigation, accounting for over 20% of national irrigation water use.
- Texas uses about 5 million acre-feet per year for irrigation, mainly for cotton, corn, sorghum, and wheat.
- Idaho uses 15.3 million acre-feet for irrigation, the highest total nationally, supporting extensive food production.
- Florida uses about 40% of its water for irrigation, supplemented by high rainfall.
- Illinois uses about 6.3 million acre-feet for thermoelectric power plant cooling.
State-by-State Water Usage
| State | Total Water Usage (million acre-feet) | Top Category | Percentage of Total |
|---|---|---|---|
| California | 41.7 | Irrigation | 23% |
| Texas | 17.6 | Irrigation | 29% |
| Idaho | 16.8 | Irrigation | 91% |
| Florida | 12.7 | Public Supply | 40% |
| Illinois | 12.1 | Thermoelectric | 48% |
Reservoir Storage Capacity
Large reservoirs are measured in acre-feet to quantify their storage capacity. Some examples:
| Reservoir | Full Storage Capacity |
|---|---|
| Lake Mead | 26 million acre-feet |
| Lake Powell | 24 million acre-feet |
| Lake Sakakawea | 23.8 million acre-feet |
| Lake Oahe | 23.6 million acre-feet |
However, reservoirs are rarely at full capacity. Changing weather patterns, droughts, water withdrawals, and evaporation can significantly reduce storage compared to the official full capacity.
Lake Mead Storage
For example, Lake Mead is currently at 28% of its full capacity as of 2019. Here are the current storage numbers for Lake Mead:
- Full capacity: 26.1 million acre-feet
- Current storage: 7.5 million acre-feet
- Percent full: 28%
This illustrates how actual water availability can differ greatly from the full reservoir capacity. Planning and managing water resources uses both current and maximum storage numbers.
Groundwater Aquifers
The amount of water stored in groundwater aquifers is also often measured in acre-feet. Some major aquifers in the United States include:
| Aquifer | Estimated Storage (million acre-feet) |
|---|---|
| Ogallala | 3,000 |
| High Plains | 400 |
| Mississippi Embayment | 1,000 |
| California Central Valley | 850 |
This groundwater storage is extremely valuable for agriculture, municipalities, and residential use. However, unsustainable use can deplete aquifers faster than they recharge.
Declining Water Tables
Many aquifers around the world are declining due to excessive pumping for irrigation. As groundwater levels fall, wells dry up and pumping costs increase. Examples include:
- The Ogallala Aquifer has fallen over 100 feet in some regions.
- Aquifers underlying Beijing, China have fallen nearly 200 feet.
- The California Central Valley aquifers have declined dangerously in some areas.
Careful monitoring and management is crucial to balance aquifer withdrawals with recharge rates to ensure long-term availability.
Flow Rates of Rivers
The flow rate of rivers and streams is also measured in acre-feet and reflects how much water passes by a location over time.
Discharge is the volume of water flowing past a point per unit time. It is usually stated as cubic feet per second (cfs). This discharge can be converted to acre-feet per day or year.
For example, the average discharge of the Colorado River near the Arizona border is around 18,000 cfs. Over a year, this is:
- 18,000 cubic feet per second
- x 86,400 seconds per day
- = 1,555,200,000 cubic feet per day
- About 12.7 million acre-feet per year
Seasonal flows vary throughout the year for most rivers. Peak discharge usually occurs in spring with snowmelt. Rivers in arid climates can vary dramatically between flood and drought conditions.
Average Annual Streamflow of Major U.S. Rivers
| River | Annual Discharge (million acre-feet) |
|---|---|
| Mississippi | 436 |
| Columbia | 198 |
| Colorado | 14 |
| Delaware | 6 |
As you can see, the mighty Mississippi outpaces all other U.S. rivers by a large margin, discharging over 400 million acre-feet annually past New Orleans.
Water Usage Regulations
With increasing demands being placed on limited freshwater supplies, usage regulations are becoming more commonplace. Understanding standard measurements like the acre-foot helps officials develop appropriate water regulations.
Policies that limit water use may be implemented through:
- Permitting – Drilling wells or diverting water requires government approval.
- Limits – Caps placed on total water allotments for a property.
- Pricing – Higher water rates discourage wasteful usage.
- Restrictions – Bans or limits on specific types of usage like lawn watering.
Urban water suppliers that fail to promote efficient water use may be subject to fines or intervention by state officials. Homeowners and businesses must comply with any local water regulations or face penalties.
Example Water Regulations
- California: Monthly water budgets limit outdoor irrigation. Fines for exceeding allocations.
- Arizona: Must reduce per capita water use by set amounts each 5 years.
- Nevada: Limits on golf course grass acreage. Required use of recycled water if available.
- Texas: Rainwater harvesting and graywater recycling systems are encouraged.
- Massachusetts: Bans excessive outdoor watering, irrigation systems required to have sensors.
Water regulators walk a fine line between preserving supplies and causing undue harm to residents, businesses, agriculture, and ecosystems.
Conclusion
An acre-foot is a unit of volume commonly used to measure large amounts of water usage, resources, and flows. While it represents a huge quantity of water, the acre-foot scale helps put perspective on massive water systems like reservoirs, aquifers, and rivers.
When officials allocate water rights, plan reservoirs, or regulate usage, they utilize acre-feet to develop equitable and sustainable policies. For individuals, understanding how much water you use or conserve makes more sense when converted to acre-feet.
While we may take it for granted, fresh clean water is a precious resource that must be used wisely. The acre-foot lends context to this finite supply and our costly demand that continues rising. Together, through informed decisions and efficient practices, we can work to sustain water resources at the community level.