Tail-Water Pit & Capture the Runoff, Reuse It
Recovers runoff
Surface irrigation sheds 20–45% of the water off the low end of the field as tail water. Enter your set and this tool sizes the recovery pit, the pump-back and the water and money you reuse each season by catching that runoff and returning it to the field head.
Enter your irrigation set
Next: excavate a pit of about 634 m³ (20.6 m × 20.6 m × 1.5 m, with freeboard), install a pump-back of at least 40.3 L/s, and route the tail ditch into it. You will re-use about 6528 m³/season worth 326 — line the pit if seepage is high.
Method: runoff = applied × tail-water fraction (NRCS NEH-623 / CPS-447); pit live storage = runoff × capture factor, footprint = built volume ÷ dig depth, pump-back = live storage ÷ pump-back hours. Planning-grade — confirm runoff with a field tail-water measurement and check seepage/permitting.
Tail-water recovery — key facts
- Runoff (graded furrow)
- ≈ 20–45% of applied (≈ 30% typical)
- Runoff formula
- applied depth × area × tail-water fraction
- Pit live storage
- ≈ runoff of the largest set
- Pit footprint
- built volume ÷ dig depth
- Pump-back
- live storage ÷ pump-back hours
- Reuse efficiency
- ≈ 85% (15% pit / re-lift loss)
- Best payoff
- graded furrow & border on heavy soil
- Privacy
- Runs in your browser; nothing uploaded
Why a recovery pit turns runoff back into a crop
In surface irrigation — graded furrow, border or flood — water has to keep running so the stream reaches the far end of the field. By the time the tail end is wet, the low end is overflowing, and a large share of the water you pumped simply runs off the bottom of the field. On graded furrow systems that tail water is often 20–45% of everything you applied: water you paid to pump, leaving the farm and carrying your sediment and nutrients with it.
A tail-water recovery pit catches that runoff in a basin at the field's low corner, and a pump-back line lifts it back to the field head to be applied again. This tool sizes the whole structure: it computes the runoff per set from the tail-water fraction (USDA-NRCS NEH Part 623 and Conservation Practice 447), sizes the pit to one set's peak runoff with freeboard, derives the footprint at your dig depth, and sizes the pump-back to empty the pit between sets. It then reports the water and money you reuse per season and whether recovery is worth building for your method and soil.
Tail-water (runoff) fraction by surface method
| Surface method | Typical runoff | Range | Note |
|---|---|---|---|
| Graded furrow (on slope) | 30% | 20–45% | Highest tail water — water runs off the low end; prime case for recovery. |
| Graded border / border-strip | 22% | 15–35% | Sheet flow off the lower end; moderate-to-high runoff. |
| Wild / uncontrolled flood | 35% | 25–50% | Poorly controlled — large, uneven tail water. |
| Level / near-level furrow | 10% | 5–18% | Low runoff — most water infiltrates in place. |
| Level basin (diked) | 3% | 0–8% | Near-zero runoff — basin is closed; recovery rarely needed. |
Soil intake multiplier on runoff
| Soil intake class | Runoff multiplier | Effect |
|---|---|---|
| Sandy / high intake | 0.7 × | Absorbs more — less runoff |
| Loam / medium intake | 1 × | Reference soil |
| Clay loam / low intake | 1.2 × | Sheds more — more runoff |
| Clay / very low intake | 1.35 × | Sheds more — more runoff |
Planning-grade values consistent with USDA-NRCS NEH Part 623 (Surface Irrigation) Ch.4–5 and NRCS Conservation Practice Standard 447 (Irrigation System, Tailwater Recovery). Confirm runoff with a field tail-water measurement before final design.
Right-size the pit
Get the pit volume and dimensions from one set's peak runoff.
Match the pump-back
See the L/s needed to empty the pit between irrigation sets.
Value the water saved
Translate recovered runoff into m³ and money per season.
Know when to skip it
Level basins shed little — fix runoff at source instead.
Frequently Asked Questions
How big a tail-water recovery pit do I need?+
Size the pit to the runoff of your largest irrigation set, not the whole season. The tool computes runoff = applied water × the tail-water fraction (≈30% on graded furrow), takes about 55% of that as the peak live storage one set produces, then adds 20% freeboard. For 4 ha at 80 mm on graded furrow loam that is roughly 960 m³ of runoff per set and a built pit near 630 m³ — about a 20 m × 20 m × 1.5 m basin.
What fraction of irrigation water runs off as tail water?+
On graded (sloping) furrow systems, 20–45% of the applied water commonly runs off the low end as tail water, with a typical value near 30%. Graded borders lose about 15–35%, wild flood up to 50%, while level furrow (5–18%) and diked level basins (near 0%) lose very little. Tighter clay soils shed more than sandy soils because water infiltrates more slowly.
How much water can tail-water recovery save me?+
Recovery re-uses the runoff minus pit and re-lift losses — this tool applies an 85% reuse efficiency. Capturing a 30% runoff fraction and reusing 85% of it returns about 25% of all the water you apply. Over a season of 8 irrigations on 4 ha that is roughly 6,500 m³ of reusable water, which at typical pumping costs pays back the pit and pump within a few seasons.
What pump-back capacity do I need?+
Size the pump to empty the pit's live storage within the time between sets. The tool divides the live storage by your pump-back hours and adds 10% overhead: a 528 m³ live volume emptied over 4 hours needs about 145 m³/h, or roughly 40 L/s. A smaller pump just means a larger pit (more storage) — there is a trade-off between pit size and pump size.
Should I line the recovery pit?+
Line the pit if your soil is sandy or seepage is high — an unlined pit on permeable ground can lose much of the captured water before you pump it back, defeating the purpose. On clay or clay-loam soils an unlined pit usually holds water well enough. The tool's reuse efficiency (≈85%) assumes a reasonably tight or lined pit; raise the loss allowance for leaky ground.
Is tail-water recovery worth it on a level basin?+
Usually no. Level (diked) basins are closed and lose almost no tail water (near 0%), so there is little runoff to recover — the tool rates these "low payoff." Recovery shines on graded furrow and graded border systems where 20–45% of the water runs off. If your runoff fraction is below about 10%, fix runoff at the source (leveling, shorter runs, surge or cutback) before building a pit.
How is the pit footprint calculated from the volume?+
Footprint area = built pit volume ÷ dig depth. A 630 m³ built pit dug 1.5 m deep needs about 420 m² of surface, which is roughly a 20.5 m × 20.5 m square (or any equivalent rectangle). Digging deeper shrinks the footprint but raises the pump-back lift; most farm pits run 1.5–2.5 m deep.
What is the difference between a recovery pit and a recovery pond?+
They are the same idea at different scales — a small in-field sump or pit captures the runoff from one field, while a larger recovery pond may serve several fields and store water across multiple sets. NRCS Conservation Practice 447 covers both as "tailwater recovery." This tool sizes the per-field pit to one set's runoff; for a multi-field pond, sum the contributing sets.
Does tail-water recovery improve irrigation efficiency?+
Yes — re-using runoff raises whole-farm application efficiency because water that would have left the field is applied again instead of pumped fresh. Recovering 25% of applied water effectively cuts your net diversion by a similar amount. It does not fix uneven application within the field; pair it with leveling, surge or cutback to reduce the runoff in the first place.
How quickly does a tail-water recovery pit pay back?+
Payback depends on water cost and runoff volume. With 6,500 m³ reused per season worth, say, $0.05/m³, that is about $325/year of water value; a modest pit-plus-pump installation often returns its cost within 2–5 seasons, faster where water or pumping energy is expensive. Enter your water value to see the annual recovered value for your field.
Can I use captured tail water on any crop?+
Generally yes for the same field, but tail water can carry sediment, nutrients and any applied chemicals, so it is best returned to the field it came from or to a similar crop. Let sediment settle in the pit, and avoid applying recovered water that may carry residual herbicides to a sensitive following crop. Recovery also keeps those nutrients and sediment on your farm rather than in downstream water.
Does this replace a field tail-water measurement?+
No — it is a planning-grade design tool. The runoff fractions come from NRCS surface-irrigation references and your method and soil, but real runoff varies with stream size, set time, slope and tillage. Confirm your actual tail-water volume with a flume or timed measurement at the field outlet before final pit and pump sizing.