Center Pivot & Stop the Outer Span Running Off
Checks depth per pass
A center pivot applies the same depth everywhere, but the outer span delivers it fastest — enter your pivot length, flow, depth and soil texture to see the instantaneous rate at the end tower and whether it exceeds soil intake and runs off.
Enter your pivot & soil
Next: cut the gross depth per pass (run faster, more frequent passes), fit a low-pressure spray package with a wider wetted band, or add residue/basin tillage to lift intake. Aim to bring the outer rate of 50 mm/hr at or below Loam's 13 mm/hr.
Outer rate i(R) = depth · 2πR / (T · wetted-width); allowable = soil basic intake × slope factor. A pivot applies the same depth everywhere but the outer span delivers it far faster.
Center pivot application rate — key facts
- Depth per pass
- 1000 · Q · T ÷ (π · R²)
- Time per revolution
- depth · π · R² ÷ (1000 · Q)
- Outer rate i(R)
- depth · 2πR ÷ (T · wetted width)
- System capacity
- 1000 · Q · 24 ÷ (π · R²)
- Runoff starts when
- rate > soil basic intake
- Loam intake
- ≈ 13 mm/hr
- Sand intake
- ≈ 50 mm/hr
- Clay intake
- ≈ 2.5 mm/hr
- Slope cuts intake
- 0.4–0.8× on slopes
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Soil basic intake rate by texture
Runoff under a pivot is a race between how fast water lands (the application rate) and how fast the soil takes it in (the basic intake rate). These NRCS-family intake rates set the bar your outer-span rate must stay under. On slopes, multiply by the slope factor below.
| Soil texture | Basic intake (mm/hr) | Available water (mm/m) | Pivot note |
|---|---|---|---|
| Sand | 50 | 70 | Tolerates high rates |
| Loamy sand | 38 | 90 | Tolerates high rates |
| Sandy loam | 25 | 120 | Tolerates high rates |
| Fine sandy loam | 18 | 140 | Watch the outer span |
| Loam | 13 | 170 | Watch the outer span |
| Silt loam | 10 | 190 | Watch the outer span |
| Silty clay loam | 6 | 180 | Runoff-prone — slow the pivot |
| Clay loam | 5 | 175 | Runoff-prone — slow the pivot |
| Silty clay | 3 | 160 | Runoff-prone — slow the pivot |
| Clay | 2.5 | 150 | Runoff-prone — slow the pivot |
| Field slope | Intake factor | Effect |
|---|---|---|
| 0–3% (flat) | 1.0× | Full intake |
| 3–6% (gentle) | 0.8× | Allowable rate cut to 80% |
| 6–10% (moderate) | 0.6× | Allowable rate cut to 60% |
| >10% (steep) | 0.4× | Allowable rate cut to 40% |
Sources: USDA-NRCS National Engineering Handbook Part 623 intake families; Kansas State (MF-2853) and University of Nebraska (EC732) center-pivot design references.
Why the edge of the field floods first
A center pivot is a single rotating pipe. The depth it lays is uniform across the radius, but the geometry isn't: a point near the end-tower travels a far longer circle each revolution than a point near the pivot point, so the wetted band sweeps past it much faster. That depth is therefore delivered in a shorter window at the edge, giving a much higher instantaneous application rate in millimetres per hour. On most soils the outer span applies water two-to-five times faster than the inner field.
Runoff is simply that outer rate beating the soil's ability to soak water in. Once the application rate climbs above the basic intake rate, water can't infiltrate fast enough — it ponds, then runs downslope, carrying soil and fertiliser with it and leaving the very edge it floods under-irrigated. This tool computes the depth per pass, the hours per revolution, the system capacity, and the outer-span rate, then compares that rate to your soil's slope-adjusted intake so you know whether to slow the pass, widen the spray pattern, or improve intake before the next irrigation.
How to use it
- 1Enter the pivot length (radius), the system flow in m³/h, and the target gross depth per pass.
- 2Enter the sprinkler package's wetted-band width — wider bands spread the depth over more time and cut the peak rate.
- 3Choose your soil texture and the field slope to set the allowable intake rate.
- 4Read the outer-span instantaneous rate, the depth per pass, and the hours per revolution.
- 5If the outer rate exceeds soil intake, slow the pass for a smaller depth, widen the band, or improve intake — then re-check.
Frequently Asked Questions
How do I calculate a center pivot's application rate?+
Per-pass gross depth comes from the flow and the time over the wetted circle: depth (mm) = 1000 × flow (m³/h) × time (h) ÷ area (m²), where area = π × pivot length². To deliver a target depth, the time per revolution is depth × area ÷ (1000 × flow). The instantaneous rate at any radius is then depth × 2π × radius ÷ (time per revolution × wetted-band width), which is highest at the outer tower.
Why is the outer span's application rate so much higher?+
A center pivot lays the same depth everywhere on each pass, but a point near the end-tower is swept by the wetted band far faster than a point near the centre, because the outer band travels a much longer circle in the same revolution time. So the depth is delivered in a shorter time at the edge, giving a higher instantaneous rate in mm/hr — often three to five times the rate near the pivot point.
What is the soil basic intake rate and why does it matter?+
The basic (or steady) intake rate is how fast water soaks into the soil once infiltration has settled, in mm/hr. Sands take 40–50 mm/hr; loams around 10–13 mm/hr; clays only 2–5 mm/hr. When the pivot's instantaneous application rate exceeds this, water can't soak in fast enough, so it ponds and runs off — wasting water and eroding soil.
When does a center pivot cause runoff?+
Runoff begins when the instantaneous application rate at a point exceeds the soil's allowable intake rate (basic intake adjusted for slope). Because the outer span has the highest rate, runoff almost always starts at the field edge first. This tool flags that ring: if the peak rate is 50 mm/hr on a loam that takes 13 mm/hr, the outer ground is receiving water nearly four times faster than it can absorb it.
Is 50 mm/hr a high application rate for a pivot?+
Yes — 50 mm/hr is high for anything but coarse sand. Most loams and silt loams accept only 10–18 mm/hr, so a 50 mm/hr outer rate guarantees runoff unless you slow the pivot (smaller depth per pass), widen the wetted band with a low-pressure spray package, or improve intake with residue and tillage that holds water in place.
How do I reduce a pivot's application rate without under-watering?+
Apply a smaller gross depth more often: faster pivot speed means a shorter wetted-band dwell time, but it also means a smaller depth per pass, which lowers the instantaneous rate. Widening the wetted band (booms, low-pressure sprays close to the canopy, or LEPA bubblers) spreads the same depth over more time at each point, cutting the peak rate directly.
What is hours per revolution and how is it set?+
Hours per revolution is how long the pivot takes to make one full circle, set by the end-tower speed (the timer percent). A longer revolution applies a deeper pass; a shorter one applies a shallower pass at a higher instantaneous rate near the edge but a lower depth. The tool reports the revolution time for your target depth so you can match it to your speed timer.
What is system capacity in mm per day?+
System capacity is the depth the pivot can apply if it runs continuously for 24 hours over its whole circle: 1000 × flow × 24 ÷ area. It tells you the maximum daily depth the machine can deliver — if peak crop water use exceeds it, the pivot can't keep up in mid-season and the crop draws down stored soil moisture.
Does field slope change the runoff risk?+
Strongly. On slopes, water starts moving downhill before it can infiltrate, so the allowable rate is lower than the soil's flat-ground intake. This tool multiplies the basic intake by a slope factor — about 0.8 on 3–6% slopes, 0.6 on 6–10%, and 0.4 on steeper ground — so the same pivot that's safe on flat land may run off on a slope.
What's the difference between gross and net application depth?+
Gross depth is what leaves the sprinklers; net depth is what actually enters the root zone after evaporation, wind drift and any runoff. This calculator works in gross depth per pass. If your outer span runs off, the net depth there is less than the gross — so runoff doesn't just waste water, it under-irrigates the very edge it's flooding.
How wide should the wetted band be to avoid runoff?+
Wide enough that the peak instantaneous rate falls at or below the soil's allowable intake. Because the peak rate is inversely proportional to the wetted-band width, doubling the band roughly halves the peak rate. If your outer rate is twice the soil intake, you need roughly twice the wetted width — which is exactly what low-pressure spray and LEPA packages provide.
Can I use this for a linear-move (lateral) system too?+
The depth-per-pass and system-capacity logic carries over, but a linear move applies a uniform instantaneous rate across its whole length (it has no outer span sweeping faster). The runoff comparison against soil intake still applies — just use the single uniform rate. This tool is built for the radial geometry of a center pivot, where the outer-span peak is the governing case.