How long does flood irrigation take to apply a set depth?
Flood irrigation, also called surface irrigation, moves water across the soil by gravity. In a basin the water ponds inside dikes on level ground. In a border it flows down a gently sloped strip between low ridges. Either way, the water delivered has to equal the depth you want times the area, so the run time follows from how fast water arrives.
Depth is the number that matters agronomically, because it tells you how far the water can wet the root zone. Volume is what your pump or ditch actually delivers. The table converts between the two, and the calculator does the same arithmetic for your numbers.
| Quantity | Equals |
|---|---|
| 1 acre-inch (1 in over 1 acre) | 27,154 gal |
| 1 acre-foot | 325,851 gal (12 acre-inches) |
| 1 cfs | 448.83 gpm |
| 1 cfs for 1 hour | about 0.992 acre-inch |
| 1 hectare-millimeter (1 mm over 1 ha) | 10 m³ (10,000 L) |
| 1 L/s | 3.6 m³/h |
For pure volume conversions, see the acre-inches calculator and the acre-feet calculator. Both sit in the agriculture calculators group.
Check delivery rules first. Irrigation districts and water rights often set delivery schedules, allotments or flow limits. Where they apply, they take priority over a calculated set time.
How does this flood irrigation calculator work?
The calculator uses a volume balance: the water delivered (flow × time) equals the water needed (area × depth). It assumes the whole delivered volume counts as applied.
US units
Water needed
Gallons = Area (sq ft) × Depth (in) × 144 ÷ 231
One inch over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches. Area entered in acres is multiplied by 43,560.
Time to apply a depth
Minutes = Gallons ÷ Flow (gpm)
Depth from a set time
Depth (in) = Flow (gpm) × Minutes ÷ (Area (sq ft) × 144 ÷ 231)
Flow entered in cfs is multiplied by 448.8312 to get gpm.
Metric units
Water needed
Liters = Area (m²) × Depth (mm)
One millimeter over one square meter is one liter. Area entered in hectares is multiplied by 10,000.
Time and depth
Minutes = Liters ÷ Flow (L/s) ÷ 60
Depth (mm) = Flow (L/s) × 60 × Minutes ÷ Area (m²)
Flow entered in m³/h is multiplied by 1,000 ÷ 3,600 to get L/s.
Depths and rates show two decimals in US units and one in metric. Volumes are exact conversions, so they are not rounded to "about" figures. Times are rounded for reading. Your own inputs are shown back exactly as typed. Switching units or a flow or time unit converts your entries so the physical answer stays the same, apart from display rounding.
When this calculator isn't the right tool
- Furrow irrigation, where water is split among furrows. See the furrow irrigation calculator.
- Sprinkler and pivot systems, which apply water at a set rate per hour. See the center pivot calculator.
- Choosing how much depth to apply, which depends on soil and crop. See the irrigation application depth calculator.
- Modeling advance, cutoff and uniformity, which needs soil intake data and field surveys that a simple volume balance can't capture.
Flood irrigation calculation examples
Time to apply a depth (US)
A 1-acre basin gets 4 inches of water from a pump delivering 905 gpm.
- Area: 1 ac × 43,560 = 43,560 sq ft.
- Water needed: 43,560 × 4 × 144 ÷ 231 = 108,617.1, shown as 108,617 gallons (4.00 acre-inches).
- Time: 108,617.1 ÷ 905 = 120.02 minutes, shown as about 2 hr.
- Check: the inflow is 905 ÷ 448.8312 = 2.016 cfs, and 4 in over 2.00 hr is 2.00 in/hr.
Depth from a set time (metric)
A 0.6 ha border receives 60 L/s for 3 hours.
- Area: 0.6 × 10,000 = 6,000 m².
- Time: 3 hr × 60 = 180 minutes.
- Water delivered: 60 × 60 × 180 = 648,000 liters (648 m³).
- Depth: 648,000 ÷ 6,000 = 108.0 mm.
The same 108 mm is about 4.25 inches, so the two examples show similar applications in different units.
How do I use the result in the field?
Measure the inputs
Area: measure the irrigated basin or border between dikes, not the whole field. Inflow: use a meter, a bucket test for small flows, or the rate your supplier states. Small errors in flow become equal errors in time, so it is worth measuring. Depth: take it from the soil water the crop has used and how deep the roots go. The management allowed depletion calculator and the available water capacity calculator help set it, and the crop water requirement calculator shows how much the crop uses.
Apply it
Treat the calculated time as a starting point. Note when the water reaches the far end of the border or fills the basin, and how much runs off. If water reaches the end early or ponds there, shorten the set or cut off the flow sooner. If the far end stays dry, check leveling and infiltration. The soil infiltration rate calculator and the soil infiltration rate chart explain how intake varies by soil.
Common mistakes
- Mixing up gpm and cfs. One cfs is about 449 gpm.
- Entering gross field acres when only one basin or border is irrigated at a time.
- Entering the full supply flow when it is split among several basins at once.
- Treating the delivered depth as the depth stored in the root zone.
To see what the water costs, use the irrigation water cost calculator. To check whether your pump can supply the flow, use the irrigation well capacity calculator.
What changes flood irrigation results?
| Factor | Effect | What to adjust |
|---|---|---|
| Soil intake rate | Fast-absorbing soil takes water in before it reaches the end. Slow soil sends more water to the tail end. | Watch advance and adjust the set time or flow. |
| Slope and leveling | Uneven ground pools water in low spots and leaves high spots dry. | Correct grade where you can. Expect uneven depth until then. |
| Field length | Longer runs take longer to cover, so the upper end soaks longer than the lower end. | Consider a higher flow or shorter runs. |
| Inflow accuracy | Time changes in direct proportion to the flow you enter. | Measure flow at the field, not at the pump. |
| Runoff and deep percolation | Water leaving the field or passing below the roots isn't stored. | Apply the result, observe losses, then adjust. |
| Root zone and soil storage | They limit how much depth the soil can hold. | Set the target depth from soil and crop, not from convenience. |
If you want to put a number on losses, the irrigation efficiency calculator is the next step.
Frequently asked questions
How long does it take to flood irrigate an acre?
It depends on your inflow and the depth you want to apply. One acre-inch is 27,154 gallons, so divide the gallons needed by your flow in gallons per minute. For example, 4 inches over 1 acre at 905 gpm takes about 2 hours, before any losses.
Try the GPM per acre calculator to see how supply compares with area.
How do I convert cfs to acre-inches per hour?
One cubic foot per second delivers about 0.99 acre-inch per hour, so the two are often treated as roughly equal. The exact figure is 448.83 gpm × 60 minutes ÷ 27,154 gallons per acre-inch, or about 0.992. Use the exact factor when timing matters over long sets.
Does this calculator include runoff and deep percolation losses?
No. It counts all the water you deliver as applied. Real basins and borders lose some water to runoff at the tail end and to deep percolation below the roots, and uneven infiltration along the field changes how much the soil actually stores. Treat the result as the delivered depth and adjust from your own field observations or an efficiency estimate.
How do I measure my inflow?
Use a flow meter on the pump or turnout if you have one. For small flows, a bucket test works: time how long it takes to fill a container of known volume, then divide volume by time. For ditch or canal deliveries, use the measured flow or the rate stated by your water supplier.
The bucket test calculator does the division for you.
Can I use this calculator for furrow irrigation?
Only as a rough volume check. The same balance of flow × time = area × depth holds, but furrows are irrigated a few at a time, with flow split among them and infiltration that depends on furrow spacing and shape. Use the furrow irrigation calculator for that.
See the furrow irrigation calculator.
How long should I leave the water on?
This calculator gives the time for the delivered volume to equal your target depth over the whole area. In practice the water may reach the far end before that time, and the flow may be cut off earlier, depending on soil, slope and field length. Watch how far the water advances during your first irrigations and adjust the set time from what you see.
The irrigation set time calculator covers set times more generally.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Volume, flow and unit conversions checked in both directions, US to metric and back.
- Both worked examples calculated by hand and matched to the calculator's output.
- Formulas follow standard volume-balance and continuity hydraulics.
Sources
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 623, Irrigation, Chapter 4: Surface Irrigation. [VERIFY URL]
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 652, Irrigation Guide. [VERIFY URL]
- Food and Agriculture Organization of the United Nations. Irrigation Water Management: Irrigation Methods (Training Manual No. 5). [VERIFY URL]
- Walker, W. R., and G. V. Skogerboe. Surface Irrigation: Theory and Practice. Prentice-Hall, 1987.