How much water does a furrow irrigation set apply?
In furrow irrigation, water runs down the length of an open channel between crop rows and soaks into the soil as it advances. The amount applied is measured as a gross depth: the total volume delivered to a set, spread evenly over the area those furrows cover, expressed in inches or millimeters just like rainfall. "Gross" matters here because this average counts every gallon that entered the furrow, including water that ran off the far end or moved past the root zone; it isn't the same as the net depth the crop actually stored.
University of Nebraska-Lincoln (UNL) Extension frames it as a management check: once you know your stream size per furrow and how long you ran it, you can work out whether that combination applied a reasonable amount of water. As a rough guide from the same source, gross applications above about 1.5 to 2 inches on sandy, fast-draining soils, or above about 2.5 to 3 inches on medium- to fine-textured soils, generally mean more water went on than a fully depleted root zone can hold.
| Soil texture | Typical ceiling | Why |
|---|---|---|
| Sandy, fast intake | about 1.5–2 in (38–50 mm) | Holds less water per foot of root zone; excess drains below the roots quickly. |
| Medium to fine, slower intake | about 2.5–3 in (64–76 mm) | Holds more water per foot, but slow intake means long sets can still overshoot. |
These figures are a general planning check, not a verdict this calculator applies to your result. Whether a given depth is appropriate for your field also depends on your crop's root depth and how much of that root zone was already dry, which is a question for the crop water requirement calculator rather than this one.
How this furrow irrigation calculator works
The calculator turns the water flowing into one furrow over your set time into a volume, then spreads that volume over the strip of ground one furrow covers to get a depth. It repeats that per-furrow depth across however many sets it takes to reach every furrow in the field.
1. Gross depth applied per set
US units
Gross depth (in) = 1,155 × Inflow per furrow (gpm) × Set time (hr) ÷ [Furrow length (ft) × Furrow spacing (in)]
From University of Nebraska-Lincoln Extension G1338, Managing Furrow Irrigation Systems. The constant folds together the volume-to-depth conversion for gallons, feet and inches, so it only holds when stream size is gpm, time is hours, length is feet and spacing is inches.
Metric units
Gross depth (mm) = 360,000 × Inflow per furrow (L/s) × Set time (hr) ÷ [Furrow length (m) × Furrow spacing (cm)]
The metric constant comes from the same continuity relationship (volume = flow × time; depth = volume ÷ area), converted for liters, seconds, meters and centimeters. It gives the same physical answer as the US formula for equivalent inputs.
2. Sets needed to cover the field
Number of sets
Sets = ROUND UP( Total furrows ÷ Furrows irrigated per set )
If your equipment can run more furrows at once than the field has, the whole field waters in a single set.
3. Total time and flow per set
Total time and flow
Total time to finish the field = Sets × Set time
Flow needed per set = Inflow per furrow × Furrows running at once
Rounding policy
Gross depth is shown to 2 decimal places in US units (inches) and 1 decimal place in metric (millimeters), matching the precision of the inputs the formula uses. Sets are whole numbers, always rounded up, since a partial set still needs its own pass. Flow needed per set is shown to 1 decimal place in gpm or 2 decimal places in L/s.
When this calculator isn't the right tool
It estimates gross depth and set count for open, graded furrows. Use a different tool for:
- Basin or border (flood) irrigation, which floods a wide strip or diked check rather than individual furrows. See the flood irrigation calculator.
- Center pivot or other sprinkler-based field irrigation. See the center pivot calculator.
- Working backward from a target depth to a set time instead of forward from a set time to a depth. See the irrigation set time calculator.
- Checking whether your well or pump can supply the flow a set needs. See the irrigation well capacity calculator.
- Advance time, cutoff ratio or intake-rate-based design, none of which this calculator models; see the practical guidance and factors sections below for the concepts, and consult an irrigation designer for field-specific design work.
Furrow irrigation calculation example
A field has furrows 1,320 feet long, spaced 30 inches apart, with 100 furrows total. The irrigator opens 100 gates at once, each running 7.5 gpm, for a 12-hour set.
- Gross depth: 1,155 × 7.5 gpm × 12 hr ÷ (1,320 ft × 30 in) = 103,950 ÷ 39,600 ≈ 2.63 inches.
- Sets needed: 100 furrows ÷ 100 furrows per set = 1 set.
- Total time: 1 set × 12 hours = 12 hours.
- Flow needed per set: 7.5 gpm × 100 furrows = 750 gpm.
The same calculation in metric
A field has furrows 400 m long, spaced 75 cm apart, with 80 furrows total. The irrigator runs 20 furrows at once, each at 0.5 L/s, for a 10-hour set.
- Gross depth: 360,000 × 0.5 L/s × 10 hr ÷ (400 m × 75 cm) = 1,800,000 ÷ 30,000 = 60.0 mm.
- Sets needed: 80 furrows ÷ 20 furrows per set = 4 sets.
- Total time: 4 sets × 10 hours = 40 hours.
- Flow needed per set: 0.5 L/s × 20 furrows = 10 L/s.
Both examples match what the calculator above returns for the same inputs.
Measuring your inputs and applying the result
Measuring furrow length and spacing
Furrow length is simply the run from the inlet, where water enters, to the tail end of the row; pace it off or use a known field dimension. Furrow spacing is the center-to-center distance between watered furrows, which usually matches your row spacing unless you irrigate every other furrow, in which case enter double the row spacing, since each running furrow is then wetting twice the strip of ground.
Measuring inflow per furrow
If you know your total pump or head-ditch flow and the number of furrows currently running, divide the total by the count to get an average gpm or L/s per furrow. That average is only accurate if the delivery system has minimal leaks and every furrow gets roughly the same opening size. For a direct reading on one furrow, the bucket test calculator turns a fill-time measurement straight into a flow rate.
Applying the result
Once you have the gross depth and set count, plan sets back to back so each group of furrows gets the same stream size and set time; the total time to finish the field is simply sets multiplied by set time. Compare the flow needed per set against your pump, well or head-ditch capacity before you commit to a furrows-per-set number, since that comparison decides whether the plan is achievable on the day. The irrigation well capacity calculator and the gpm per acre calculator both help with that check.
Common mistakes
- Assuming every furrow gets the same stream size. Uneven gate or siphon-tube openings mean the average inflow you calculated doesn't apply evenly across the set; check individual furrows, not just the total.
- Ignoring the every-other-furrow doubling. Forgetting to double the spacing entry when you skip furrows overstates the area watered and understates the true gross depth.
- Running a stream size too large for the furrow. UNL Extension notes most furrows can't carry much more than about 50 gpm without water overtopping the banks; if your calculated flow per furrow is near or above that, split it across more, smaller streams instead.
- Treating gross depth as the whole picture. A correct average depth can still be poorly distributed if water takes too long to reach the far end of the furrow; see the factors below for how run length and set time interact with that.
Factors that change the result
The inputs above give an average depth and a set count, but several field conditions decide whether that average is actually a good plan.
| Factor | Effect | What to adjust |
|---|---|---|
| Soil intake rate | Faster-draining soils absorb more water near the inlet before the stream reaches the far end. | Shorten furrow length or set time on sandy, fast-intake soils. |
| Furrow length | Longer runs take longer for water to reach the end, so the inlet end tends to get more water than the tail end. | UNL Extension guidance generally keeps runs under about 600 ft on sandy soils, up to about 1,300 ft on medium soils and up to about 2,600 ft on low-intake soils. |
| Furrow slope | Steeper slopes move water faster but raise the risk of erosion and reduce the non-erosive stream size. | Keep stream size below the non-erosive maximum for your slope; reduce inflow on steeper ground. |
| Stream size (inflow per furrow) | Larger streams advance faster and water more evenly, but too large a stream overtops the furrow. | Keep well under the roughly 50 gpm (about 3.15 L/s) furrows can typically carry (UNL Extension). |
| Furrows per set | Running more furrows at once spreads a fixed water supply thinner across each one. | Balance available total flow against how many furrows you want running simultaneously. |
| Set time and cutoff ratio | Longer sets apply more depth but also lose more water to deep percolation near the inlet once the front has already reached the end. | Extension guidance ties set time to a cutoff ratio (advance time ÷ set time) that varies by soil texture and whether runoff is reused. |
| Every-other-furrow irrigation | Wets a wider strip per running furrow, effectively doubling the spacing. | Double the furrow spacing entry if you're only running every other furrow. |
Advance time and cutoff ratio decide how evenly a correct gross depth is actually distributed along the furrow; this calculator doesn't model them. The soil infiltration rate chart is a starting point for estimating your soil's intake rate, and the soil water balance calculator can help track moisture between irrigations once you know roughly how much each set applies.
Frequently asked questions
What is gross application depth in furrow irrigation?
Gross application depth is the average depth of water a set puts over the field area it covers, found by dividing the volume delivered by the area of the furrows that received it. It counts every gallon or liter that entered the furrows, whether the crop's root zone actually stored it or it ran off or moved below the roots. It is not the same as the net depth the crop actually uses.
How do I find my furrow's inflow rate (stream size)?
If you know your pump or head-ditch flow and how many furrows are running, divide the total flow by the number of furrows to get gallons per minute, or liters per second, per furrow. If you'd rather measure a single furrow directly, time how long it takes to fill a bucket of known volume and use that flow rate instead.
How many sets do I need to irrigate a field?
Divide the total number of furrows in the field by the number of furrows you can run at once, and round up. For example, 100 furrows run 20 at a time takes 5 sets. Each set uses the same stream size and set time as the others unless you change your inflow or timing partway through.
How long should a furrow irrigation set run?
Long enough for water to advance to the end of the furrow and for the target depth to infiltrate, but not so long that excess water is lost to deep percolation near the inlet. Extension guidance ties this to a cutoff ratio: the time water takes to reach the end of the furrow divided by the total set time, with target ratios that vary by soil texture and whether runoff is reused.
What's the maximum stream size a furrow can carry?
University of Nebraska-Lincoln Extension notes that most furrows cannot carry much more than about 50 gallons per minute without water overtopping the furrow banks, and the safe limit drops further on steeper slopes. Very small stream sizes cause their own problems, since they can take too long to reach the end of the furrow and apply water unevenly.
How long should furrows be?
It depends mainly on soil intake rate. University of Nebraska-Lincoln Extension guidance generally keeps runs under about 600 feet on sandy, fast-draining soils, up to about 1,300 feet on medium-textured soils, and as long as 2,600 feet on soils with a low intake rate. Longer runs on soils that absorb water quickly tend to apply far more water at the inlet end than the far end.
Does this calculator account for furrow intake rate or advance time?
No. It calculates the average, or gross, depth applied across the whole furrow from your inflow and set time, but it does not model how fast water advances down the furrow or how evenly it infiltrates along its length. Uneven advance means the inlet end usually receives more water than the far end, so the actual depth varies along the row even when the gross average is correct.
Can I use this for basin or border (flood) irrigation instead of furrows?
Not directly. Basin and border irrigation flood a wide strip or diked area rather than individual furrows, so the area per set is worked out differently. Use the flood irrigation calculator for those layouts instead.
See the flood irrigation calculator.
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
- The gross depth formula and both worked examples were verified by hand against UNL Extension's own published example.
- The metric formula was cross-checked against the US formula using equivalent inputs to confirm both give the same physical depth.
- The 50 gpm stream-size caution and furrow length guidance are quoted from the source cited below, not estimated.
Sources
- Yonts, C.D., Eisenhauer, D.E., Varner, D.L. University of Nebraska-Lincoln Extension. Managing Furrow Irrigation Systems (G1338).
- ATTRA / National Center for Appropriate Technology. Measuring and Conserving Irrigation Water.
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 623, Chapter 4, "Surface Irrigation." [VERIFY URL]