Water Rights Calculator
Calculate acre-feet allocation, required diversion rate, and duty of water for irrigation water rights.
About this calculator
Western US water rights are measured in two different units that this calculator ties together: a diversion rate (how fast you can pull water, in cubic feet per second or gallons per minute) and a volume allocation (how much total water you're entitled to use over a season, in acre-feet). The calculator starts from your duty of water — the acre-feet allowed per acre per season under your right or your crop's typical need — and multiplies it by irrigated acreage to get the net volume actually required at the field. Because water is lost in unlined ditches and canals before it ever reaches the crop, the calculator then grosses that figure up using a conveyance-loss percentage; a system with 20% conveyance loss needs 25% more water diverted at the source than what arrives at the field.
Dividing gross acre-feet by the length of the irrigation season converts the volume into the continuous diversion rate needed, using the fixed conversion that 1 CFS flowing for one day delivers 1.9835 acre-feet. If you enter your actual decreed water right in CFS, the calculator flips the logic around: it computes how many acres your right can actually serve and what percentage of your calculated need it covers, which is the real question most water-rights holders are trying to answer. Keep in mind this is a planning tool, not a legal determination — actual prior-appropriation rights, seasonal curtailment, and district-specific delivery rules can all further limit what water is actually available regardless of what's decreed on paper.
Inputs
Results
Gross need (acre-ft)
150
Required diversion (CFS)
0.5
How to Use This Calculator
- Enter irrigated acres and your water right's duty of water in acre-feet per acre.
- Set irrigation season length in days and conveyance loss percentage for your delivery system.
- Enter water cost per acre-foot from your district or ditch company.
- Optionally enter your decreed diversion rate in CFS to check adequacy.
- Review Gross need (acre-ft), Required diversion (CFS/GPM), and Annual water cost to verify your water right covers your needs.
How the result changes with Irrigated acres
| Irrigated acres | Gross need (acre-ft) | Required diversion (CFS) |
|---|---|---|
| 20 | 75 | 0.25 |
| 30 | 112.5 | 0.38 |
| 60 | 225 | 0.76 |
| 100 | 375 | 1.26 |
What each input means
- Irrigated acres
- Total irrigated acreage under the water right.
- Duty of water (AF/acre)
- Acre-feet per acre per season. Varies by crop and region: 2-4 typical.
- Irrigation season (days)
- Length of the irrigation season in days.
- Conveyance loss (%)
- Water lost in canals/ditches before reaching field. Lined=5-10%, unlined=20-40%.
- Water right (CFS)
- Your decreed diversion rate in cubic feet per second. Leave 0 to calculate requirement.
- Water cost ($/AF)
- Cost per acre-foot from your water district or ditch company.
What each result means
- Net need (acre-ft)
- Net water needed at the field based on duty of water.
- Gross need (acre-ft)
- Total diversion needed including conveyance losses.
- Required diversion (CFS)
- Continuous diversion rate needed over the irrigation season.
- Required diversion (GPM)
- Diversion rate in gallons per minute.
- Serviceable acres
- Acres your water right can serve (based on entered or calculated CFS).
- Right adequacy (%)
- Percentage of water need covered by the water right.
- Gross volume (gallons)
- Total seasonal volume in gallons.
- Annual water cost ($)
- Estimated annual cost based on gross allocation.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersIrrigated acres = 40, Duty of water (AF/acre) = 3, Irrigation season (days) = 150, Conveyance loss (%) = 20 = 6 input(s) provided
- Calculate Gross needGross need = netAllocationAF * conveyanceMultiplier150 = 150
- Calculate Required diversionRequired diversion = grossAllocationAF / (irrigationSeasonDays * 1.9835)0.5 = 0.5
- Calculate Net needNet need = irrigatedAcres * dutyOfWater120 = 120
- Calculate Required diversionRequired diversion = requiredCFS * 448.83226 = 226
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What's the difference between a diversion rate and an acre-foot allocation?
Diversion rate (CFS or GPM) is how fast water can be pulled from the source at any given moment, while an acre-foot allocation is the total volume you're entitled to use over the whole irrigation season. This calculator connects the two by spreading your seasonal acre-foot need evenly across the season length in days, using the fixed conversion that 1 CFS running for a full day delivers 1.9835 acre-feet.
Why does conveyance loss increase the required diversion rather than the amount used at the field?
Conveyance loss represents water that disappears in unlined ditches and canals before it ever reaches the crop, so it has to be added at the source, not the field. The calculator applies a conveyance multiplier of 1 ÷ (1 − loss%) to the net field requirement — at 20% loss that multiplier is 1.25, meaning 25% more water must be diverted upstream than what actually arrives at the field.
How does the calculator decide if my decreed water right is adequate?
If you enter your actual CFS water right, the calculator converts it to an equivalent seasonal acre-foot volume (CFS × season days × 1.9835) and compares that against your gross calculated need to produce the 'right adequacy' percentage. It also divides your right's seasonal volume by duty of water and the conveyance multiplier to report how many acres it can actually serve, which is often the more useful number for planning.
What is 'duty of water' and where should that number come from?
Duty of water is the acre-feet of water allowed (or needed) per irrigated acre per season, and it's the key multiplier this calculator applies against your acreage to get net demand. It varies by crop, climate, and soil, and for a legal water right it's often set by your decree or district rather than derived from crop water use — check your water right documentation or local extension office for the figure that applies to your situation.
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