Furrow Cutoff & Stop at the Right Point
Waters evenly
Cut off furrow inflow too late and water spills off the tail; too early and the tail end stays dry. Enter your run to get when to cut off (the cutoff ratio), the cutoff time, application efficiency and the tail-water vs deep-percolation loss split — cut off near 80% of length on loam for about 85% efficiency.
Enter your furrow run
Next: cut off inflow when the wetting front reaches about 79.7% of the furrow (≈ 143 m, around 21.1 min after turn-in). Recession then finishes the run, giving roughly 84.6% application efficiency with only 5.3% tail water.
Advance model: x = p·√Q·t^r (Walker / Kostiakov-Lewis form), required intake from Z = k·τ^a (NRCS NEH-623 Ch.4–5). Efficiency from the advance ratio vs the ~0.4 design optimum. Planning-grade — confirm with a field advance test (time the front to ¼, ½, ¾, full).
Furrow cutoff — key facts
- Typical cutoff ratio
- ≈ 60–90% of length (≈ 80% loam)
- Advance model
- x = p·√Q·t^r (Walker form)
- Intake model
- Z = k·τ^a (Kostiakov)
- Design advance ratio
- ≈ 0.3–0.5 (≈ 0.4 optimum)
- Best application efficiency
- ≈ 70–85%
- Late cutoff
- tail-water spill
- Slow advance
- tail end stays dry
- Privacy
- Runs in your browser; nothing uploaded
Why the cutoff point makes or breaks furrow efficiency
In furrow irrigation the head end starts soaking the moment water is turned in, but the tail end gets nothing until the advancing front reaches it. If you keep the inflow running until the front is well past the tail, the head over-soaks and water spills off the end as tail water. If you cut off too soon, the front never reaches the tail and the end of the field stays dry. The cutoff ratio — how far the front has gone when you stop the inflow — is the single lever that balances these two losses.
This tool models the advance with the volume-balance power law x = p·√Q·t^r (the Walker / Kostiakov-Lewis form) and the intake with the Kostiakov equation Z = k·τ^a, both fitted by soil intake family (USDA-NRCS NEH Part 623 Ch.4–5). It returns the recommended cutoff ratio and time, the advance ratio, the application efficiency and distribution uniformity, and the split of losses between tail water and deep percolation. Use it to set a starting cutoff and to test how length, stream size and target depth move efficiency before you go to the field.
Soil intake family & advance behaviour
| Soil intake class | Basic intake (mm/h) | Kostiakov k, a | Behaviour |
|---|---|---|---|
| Sand (very high intake) | 50 | 12, 0.62 | Water soaks fast; front crawls; cut off late or the tail goes dry. |
| Sandy loam (high intake) | 30 | 9.5, 0.58 | Fast intake; needs a brisk advance to reach the tail before cutoff. |
| Loam (medium intake) | 18 | 7, 0.55 | Balanced — the classic furrow case; cut off near 80% of length. |
| Clay loam (low intake) | 10 | 5, 0.52 | Slow intake; front races; cut off later but watch tail water. |
| Clay (very low intake) | 5 | 3.6, 0.5 | Very slow intake; front races to the end; biggest tail-water risk. |
Representative intake-family values from USDA-NRCS NEH Part 623 Ch.4 (Intake families) & Ch.5 (Surface Irrigation), and Walker & Skogerboe (1987). Used relatively to drive the advance ratio and cutoff recommendation; confirm with a field advance test.
Stop at the right point
Get the cutoff ratio and time instead of guessing when to shut off.
Cut tail-water waste
See how a late cutoff spills water off the field tail.
Reach the tail end
Avoid the dry tail a sluggish advance leaves behind.
Tune the stream
Test inflow and length to land the advance ratio near 0.4.
Frequently Asked Questions
When should I cut off furrow inflow?+
Cut off when the advancing wetting front reaches a fraction of the furrow length — the cutoff ratio — typically about 60–90% of the way down, near 80% on loam. Cutting off there lets recession finish the run while stored water keeps infiltrating, so the tail end still wets without the head over-soaking. This tool computes the exact cutoff ratio, distance and time for your soil, length, inflow and target depth.
What is the cutoff ratio in furrow irrigation?+
The cutoff ratio is the fraction of the furrow length the front has covered when you stop the inflow. A ratio of 0.8 means cut off when water has advanced 80% down the furrow. On fast-advancing heavy soils you cut off later (higher ratio) so the tail gets water; on slow-advancing sandy soils a brisk stream and earlier cutoff avoid over-soaking the head.
What is the advance ratio and why does it matter?+
The advance ratio R is the advance time (front to the end) divided by the required intake opportunity time to refill the target depth. R near 0.3–0.5 is the design sweet spot — the front reaches the tail well before the head finishes soaking, giving even watering. A high R (slow front) starves the tail end; a very low R (racing front) risks tail-water spill if you cut off late.
What application efficiency can furrow irrigation reach?+
Well-managed graded furrow irrigation reaches about 70–85% application efficiency; this tool caps the best case at 85%. Poorly timed runs — cutting off too late (tail water) or with a sluggish advance (under-irrigated tail) — drop well below that. The efficiency here is driven by how close your advance ratio sits to the ~0.4 optimum and how the cutoff is set.
How is the cutoff time calculated?+
First the advance time is found from the volume-balance power law x = p·√Q·t^r (the Walker / Kostiakov-Lewis form), fitted by soil intake family. The cutoff time is then the time for the front to reach the recommended cutoff distance (cutoff ratio × length). For a 180 m loam furrow at 1 L/s refilling 75 mm, the front reaches the ~80% cutoff in roughly 21 minutes.
What is the required intake opportunity time?+
It is how long a point in the furrow must have water over it to infiltrate the target depth, from the Kostiakov intake equation Z = k·τ^a inverted for τ. For loam (k=7, a=0.55) refilling 75 mm, that is about 75 minutes. The whole design balances giving the tail end enough opportunity time against not over-soaking the head.
Why does heavy soil need a later cutoff than sandy soil?+
Clay and clay-loam infiltrate slowly, so the front races down the furrow over near-sealed ground (low advance ratio) — you cut off later (a higher ratio) to give the slow-infiltrating tail enough opportunity time, while watching for tail-water spill. Sandy soils infiltrate fast, so the front crawls; you need a brisk stream and may cut off earlier to avoid drowning the head.
How do I split losses between tail water and deep percolation?+
The inefficiency (1 − application efficiency) divides into tail water (runoff off the end) and deep percolation (water past the root zone, mostly at the over-soaked head). Cutting off late and using a big stream skews the loss to tail water; cutting off early on a slow soil skews it to deep percolation at the head. The tool reports both shares so you can target the bigger one.
Does a bigger stream improve furrow efficiency?+
A larger inflow speeds the advance so the front reaches the tail sooner (lower advance ratio), which improves uniformity — up to a point. Too big a stream advances so fast that a late cutoff spills heavy tail water, and very large streams can erode the furrow. The tool lets you test inflow against length to find the stream that puts the advance ratio near the 0.4 optimum.
Should I use cutback or surge with the cutoff?+
Yes — after the cutoff, reducing inflow (cutback) or switching the surge valve to its soak cycle holds water in the furrow to finish infiltrating without spilling out the tail. Cutting off completely is the simplest option; cutback and surge recover even more efficiency, especially where the advance ratio is low and tail water is the dominant loss.
Is 80% distribution uniformity good for furrow irrigation?+
For surface irrigation, a distribution uniformity around 70–85% is good. Surge and a well-set cutoff commonly lift furrow uniformity into that band from the 50–70% typical of poorly managed continuous runs. This tool estimates a uniformity proxy from your advance ratio so you can compare set-ups before going to the field.
Does this replace a field advance test?+
No — it is a planning-grade tool to set a starting cutoff ratio and compare lengths, streams and target depths. Always confirm with a field advance test: time the front to the quarter, half, three-quarter and full points, then tune the cutoff, because real intake varies with tillage, residue, soil moisture and season.