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Furrow Stream Size & Fast But Not Erosive

Sizes the stream

Max L/sSlope-basedCutbackNon-erosive

“This tool gets better every time someone says what confused them.

Enter the furrow slope to get the maximum non-erosive stream in litres per second, plus a cutback recommendation so water advances fast without washing out the furrow.

Size your furrow stream

Field slope

Drop in field level per 100 m of run.

Your result

1.2 L/s max

Max non-erosive furrow stream

0.5

% slope

recommendation

1.2

L/s

What this means

On a 0.5% slope the safe inflow per furrow is about 1.2 L/s. Above this maximum stream, the water moves fast enough to scour soil and erode the furrow — gully and rilling follow. If you need a bigger flow to advance the water, use a cutback (start big, then reduce) or simply run shorter furrows.

Next: keep the inflow at or below 1.2 L/s per furrow.

The 0.6/slope rule is an empirical guide; soil texture, furrow shape and length also affect the safe stream — verify in the field.

Furrow stream size — key facts

Rule of thumb: Max ≈ 0.6 ÷ slope% (L/s)
0.5% slope: ≈ 1.2 L/s
2% slope: ≈ 0.3 L/s
Steeper furrow: Smaller stream needed
Goal: Advance fast, don't erode
Cutback: Big advance, then reduce
Erodible soils: Use a lower stream
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The stream must race down without tearing the furrow

Furrow irrigation lives and dies by the size of the stream you let in. Too small and water creeps down the row, over-soaking the head while the tail stays dry; too big and the flow scours the furrow bed, carrying soil off the field and running to waste at the end. The sweet spot is the largest stream that still moves below the soil's erosion threshold — and on a furrow that threshold falls sharply as the grade gets steeper.

This tool applies the field rule of thumb that the maximum non-erosive stream is about 0.6 ÷ slope% in litres per second, then suggests a cutback so you can advance with a larger flow and reduce it once water reaches the tail. Use it with the Furrow Irrigation, Siphon Tube and Soil Infiltration tools to set up an even, low-loss surface system.

Protect the soil

Keep the stream below the erosion threshold.

Advance evenly

A right-sized stream wets head to tail alike.

Match the slope

Steeper furrows automatically get smaller streams.

Cut the waste

Cutback trims runoff once water reaches the end.

Frequently Asked Questions

What is the furrow stream size?+

It's the rate of water let into a single furrow, usually in litres per second. The stream must be big enough to push water quickly to the far end so the field wets evenly, yet small enough that it doesn't scour the soil and carve the furrow. Getting this balance right is the heart of good furrow irrigation.

How is the maximum non-erosive stream calculated?+

A widely used rule of thumb caps the non-erosive stream at about 0.6 ÷ slope% litres per second. So a furrow on a 0.5% slope can take roughly 0.6 ÷ 0.5 ≈ 1.2 L/s, while a steeper 2% furrow is limited to about 0.3 L/s before erosion starts.

Why does a steeper furrow need a smaller stream?+

Water runs faster down a steeper grade, and faster water carries more energy to detach and move soil. To keep flow velocity below the erosion threshold, you must reduce the inflow as the slope rises — which is exactly why the rule of thumb divides by the slope percentage.

What is a cutback stream?+

Cutback is a two-stage technique: you start with a larger advance stream to get water quickly down the furrow, then cut the flow back to a smaller stream once the water reaches the far end. This wets the furrow fast, then reduces runoff and deep percolation at the tail.

What happens if the stream is too big?+

Too large a stream scours the furrow bed, widens and deepens it unevenly, carries soil and nutrients off the field, and often runs off the tail end as wasted water. You see muddy tail water and a rutted furrow — clear signs to cut the inflow back.

What happens if the stream is too small?+

Too small a stream advances slowly, so the head of the furrow soaks for far longer than the tail. That over-irrigates and waterlogs the top, under-irrigates the bottom, and wastes water to deep percolation near the inlet. Increase the stream or shorten the furrow run.

How do I deliver the right stream to each furrow?+

Siphon tubes, gated pipe and spiles are sized to deliver a set flow per furrow. Match the tube or gate opening to the maximum stream this tool gives, and reduce the number of furrows watered at once if your supply can't deliver that rate to each.

Does soil type change the safe stream?+

Yes. Erodible silty and fine sandy soils scour at lower velocities, so they need a smaller stream than stable clays. The 0.6 ÷ slope rule is a general guide; on highly erodible soils use a lower coefficient and watch the furrow during the first irrigation.

How does furrow length interact with stream size?+

Longer furrows need a larger advance stream to reach the end before the head over-soaks — but slope still caps how large that stream can safely be. If the non-erosive stream can't advance a long furrow in time, shorten the run or use a cutback approach.

Related farming tools

Furrow Irrigation Calculator Siphon Tube Irrigation Calculator Soil Infiltration Rate Calculator Irrigation Efficiency Calculator Water & Irrigation Farming Hub

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Calculator

Furrow Stream Size & Fast But Not Erosive

Size your furrow stream

Furrow stream size — key facts

The stream must race down without tearing the furrow

Protect the soil

Advance evenly

Match the slope

Cut the waste

Frequently Asked Questions

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