“How far apart should the sprinklers go?” The answer is a percentage, and it is not read off the catalogue but measured in the field. The FAO irrigation manual sets the rule: spacing between sprinklers must not exceed 65 percent of the wetted diameter in nil to moderate wind in square or rectangular patterns, it can go up to 70 percent in a triangular pattern, and it drops to 50 percent in strong wind, with the lateral then set perpendicular to the wind. On a green bean bed in the Niayes, sown for the off season campaign, those few percentage points decide how even the stand is along the whole length of the row.
The overlap rule, and why it exists
A sprinkler does not wet its circle evenly. It puts down a lot close to the riser and less and less towards the edge. Uniformity therefore does not come from one device but from the overlap of several circles. The FAO irrigation water management manual on irrigation methods puts it in these terms: uniform coverage requires overlap over at least 65 percent of the wetted diameter.
The same document gives the orders of magnitude for a conventional network: laterals of 5 to 12,5 cm in diameter, in aluminium alloy or plastic, and sprinklers spaced 9 to 24 m apart. That wide range is normal, since spacing is derived from the wetted diameter, which itself depends on the nozzle and the pressure.
Measure the wetted diameter instead of reading it
The value printed in the catalogue is obtained under test conditions. The NC State Extension fact sheet on stationary sprinkler systems records that field measured diameters average 10 percent less than the values published by manufacturers. The same sheet draws a useful equivalence from this: spacing calculated on 70 percent of the field measured diameter comes out roughly the same as spacing calculated on 65 percent of the manufacturer’s value, and it keeps as a safeguard the rule of not exceeding 65 percent of the published diameter.
The measurement takes four steps, in the field, before the risers are pegged out.
- Pick a moment of light wind. The NC State sheet asks for measurement in wind below 5 miles per hour, roughly 8 km/h.
- Set a single sprinkler running at the intended operating pressure, and measure the pressure at the sprinkler, not at the pump. The sheet is explicit on this: pump pressure is not an acceptable substitute.
- Run it for ten minutes, then record the radius beyond which the ground is no longer wet, in four directions. Take the smallest of the four values.
- Multiply the resulting diameter by 0,65 in a square pattern, by 0,70 in a triangle, by 0,50 if your site is windy. That is your spacing.
Wind decides more than the catalogue does
The FAO irrigation manual classes wind conditions in four levels: 0 to 0,7 m/s nil wind, 0,7 to 2,5 m/s light, 2,5 to 3,5 m/s moderate to strong, above 3,5 m/s very strong. And it is firm: above 3,5 m/s, sprinkling is not recommended.
For a scheme in the coastal Niayes belt, that limit translates into an instruction about timing rather than spacing: irrigate early in the morning or late in the day, when the breeze drops, and keep the lateral perpendicular to the dominant wind direction. The FAO manual on irrigation methods also notes that even a light wind noticeably degrades uniformity, and that the only correction available is to tighten the spacing.
Pressure, nozzles and drop size
The agricultural sprinklers covered by the FAO manual work at an operating pressure of 2,0 to 3,5 bars, for a discharge of 1,1 to 3,0 m³/h, with two nozzles of 3,0 to 6,0 mm for the range nozzle and 2,5 to 4,2 mm for the spreader. The minimum to respect is 2,0 bars.
Dropping below that pressure does not just cost you diameter. The jet breaks into drops of 0,5 to 4,0 mm depending on pressure: too low, the drops grow larger and hammer the canopy and the soil, too high, the fine spray falls close to the sprinkler and no longer covers the planned spacing. The FAO manual recommends small diameter nozzles working at or slightly above the recommended operating pressure.
On green beans, with their soft foliage and a bed that is often sown close, this point is not a detail. FAO states explicitly that large sprinklers damage delicate vegetables such as lettuce, and advises light fine sprays on soils that form a crust.
The hourly rate must stay below the soil’s infiltration
This is the second constraint, independent of spacing. The network’s precipitation rate, that is the millimetres applied per hour, must stay below the basic infiltration rate of the soil, otherwise water ponds and then runs off. The FAO irrigation manual gives the limits by texture and states that precipitation rates of 8 to 14 mm/h give the best results. Its worked design example lands on 10,4 mm/h.
| Soil texture | Acceptable hourly rate |
|---|---|
| Light, sandy soils | Up to 25 mm/h |
| Loams | 8 to 16 mm/h |
| Clays | 2 to 8 mm/h |
| Range giving the best results | 8 to 14 mm/h |
The sandy soils of the Niayes sit at the top of that table, which leaves room on the hourly rate but shortens the interval between two sets, since the available water holding there is low.
How much water a green bean bed needs
The FAO manual on crop water needs gives beans a total requirement of 300 to 500 mm over the whole growing period, with drought sensitivity rated medium to high, and a total growing period of 75 to 90 days for beans harvested green. The sustainable green bean production guide published by COLEAD in 2023 gives a consistent range, 3 000 to 8 000 m³ per hectare depending on soil and climate, that is 300 to 800 mm, and puts the Senegalese cycle at 48 to 55 days after sowing.
Brought down to the day, the FAO benchmark gives a simple order of magnitude: 300 mm over 75 days is 4 mm a day, 500 mm over 90 days is 5,6. For finer management, the same manual points to the formula ET crop equals ETo multiplied by Kc, and gives beans crop coefficients of 0,35 at the initial stage, 0,70 during development and 1,10 at mid season. The late season coefficient depends on whether the crop is harvested green or dry.
The set time then follows directly. With a requirement of 5 mm a day and a network applying 10 mm an hour, the set lasts 30 minutes. If you irrigate every other day it becomes an hour, provided the soil reserve accepts 10 mm at once, which is not a given on sand.
COLEAD adds the conditions in which that water has to arrive: a temperature of 15 to 25 °C for growth and flowering, quality deterioration above 30 °C, soil pH of 6,0 to 7,5, and a minimum depth of 40 to 50 cm. It also insists on no water stress from emergence onward, and on germination and flowering as two phases where moisture must not slip. Those two windows are what justify setting the network up before sowing, not during.
What sprinkling costs in disease risk
Watering from above wets the canopy, and a canopy that stays wet for long is a way in. The FAO manual also calls for clean water, free of suspended sediment, both to avoid blocking the nozzles and to avoid contaminating the harvest.
Two instructions follow. Irrigate in the morning, so the canopy dries during the day, and never late in the evening if the night is humid. And accept that on a sensitive crop, drip remains preferable once the stand is established: the mechanism by which leaf wetness duration triggers an epidemic is set out in our article on late blight in a tropical climate. Many Niayes growers combine the two, sprinkling for establishment then drip for the rest of the cycle, which means planning both control heads from the start.
Checks before the first run
- Wetted diameter measured in the field, not read from the catalogue, with the smallest of the four radii retained.
- Spacing set at 65 percent of that diameter at most, 50 percent if the site is windy.
- Pressure read with a gauge at the sprinkler furthest from the pump, at least 2,0 bars.
- Hourly application rate calculated and compared with the soil texture.
- Lateral set perpendicular to the dominant wind.
- Filtration in place and water free of sediment.
- Irrigation timing set for the morning.
Sizing the pump that feeds this network follows the same logic of flow and total head as the one set out in our article on sizing the pumping for an onion plot. For the context of Senegalese horticultural value chains, see our overview of agriculture in Senegal.
Having the network sized
A sprinkler network is calculated once, from the area, the soil texture, the dominant wind speed and the flow available at the borehole. JOGOO carries out that sizing and supplies the matching equipment through its Irrigation and pumping offer. For a quote matched to your plot, go through the Contact page.
Sources and references
- FAO, Irrigation Manual, chapter 8, Hose-move sprinkler irrigation
- FAO, Irrigation Water Management, Training Manual 5, Irrigation Methods, chapter 5, Sprinkler irrigation
- FAO, Irrigation Water Management, Training Manual 3, Irrigation water needs, crop water needs and crop coefficients
- NC State Extension, Stationary Sprinkler Irrigation System
- COLEAD, 2023, Guide de production durable du haricot vert
Frequently asked questions
What spacing between two sprinklers?
At most 65 percent of the wetted diameter in nil to moderate wind in a square or rectangular pattern, up to 70 percent in a triangular pattern, and 50 percent in strong wind with the lateral perpendicular to the wind, according to the FAO irrigation manual.
Why is the catalogue diameter too optimistic?
Because it is measured under test conditions. NC State Extension records field measured diameters averaging 10 percent less than the values published by manufacturers.
At what wind speed should sprinkling stop?
Above 3,5 m/s sprinkling is not recommended, according to the FAO irrigation manual, which classes that range as very strong wind.
What hourly application rate should you aim for?
8 to 14 mm per hour gives the best results. The limit depends on the soil: up to 25 mm/h on light soils, 8 to 16 mm/h on loams and 2 to 8 mm/h on clays, with the hourly rate always staying below the basic infiltration rate.
How much water does a green bean crop use?
300 to 500 mm over the cycle according to the FAO manual on crop water needs, that is 4 to 5,6 mm a day on average. COLEAD gives 3 000 to 8 000 m³ per hectare depending on soil and climate.
Should drip be preferred on green beans?
Sprinkling suits establishment well, but it wets the canopy on every pass. Once the stand is established, drip cuts the leaf wetness duration and therefore the disease risk, which leads many growers to combine the two within the same cycle.
Further reading
- Sizing the pumping for an onion plot: flow, total head and buffer reservoir, the upstream side of a sprinkler network.
- Drip in market gardening: the mistakes on small schemes: filtration, water quality and maintenance, valid for any pressurised network.
- Late blight of tomato in a tropical climate: the link between leaf wetness duration and the start of an epidemic.
- Agriculture in Senegal: the place of horticulture and the Niayes in national value chains.
