Hydraulic calculation for an irrigation system
Hazen-Williams formulas, ready-made tables, step-by-step calculation of mainline diameter, hydraulic losses, and pump selection.
Why hydraulic calculation is critical
Without a calculation, the system will operate unevenly: one part is waterlogged, the other is dry. A common mistake is using 16 mm LayFlat as a mainline for 1,000 m² at 30 L/min flow. Result: 3 bar pressure drop over 50 m — plants at the end of the line receive no water.
The 20% rule: The pressure difference between the first and last emitter in a zone must NOT exceed 20% of the operating pressure. For 2 bar: no more than 0.4 bar difference; otherwise, flow at the end will be 10%+ lower.
Key formulas
1. Total flow Q (L/s)
Q = N × q / 3600
where N = number of emitters, q = flow per emitter (L/h)
2. Pipe diameter D (mm)
D = 35.7 × √(Q/v)
where Q = flow (L/s), v = velocity (m/s, ideally 0.8–1.5). Round UP to the nearest standard diameter
3. Hazen-Williams pressure losses (m/100 m)
hf = 10.67 × (Q/C)^1.852 / D^4.87 × 100
where Q = m³/s, C = roughness coefficient (HDPE: 140–150, PVC: 150, steel: 100–120), D = m
4. Total head H (m water column)
H = H_статичний + h_тертя + h_фітинги + h_висота + h_фільтр
Divide H by 10.2 to convert to bar: 1 bar = 10.2 m water column
Ready-made table: diameter vs. flow vs. losses
HDPE PN6, Hazen-Williams coefficient C = 140, losses per 100 m of pipe at ~1 m/s velocity.
| Outer diameter | Inner diameter | Max. flow | Losses at Qmax | Typical application |
|---|---|---|---|---|
| 16 мм | 13.6 | 0.5 м³/год | 4 м/100м | LayFlat, garden up to 300 m² |
| 20 мм | 17.0 | 1.0 м³/год | 3.5 м/100м | Garden 300–700 m² |
| 25 мм | 21.2 | 2.5 м³/год | 3 м/100м | Vegetable plots 700–1,500 m² |
| 32 мм | 27.2 | 4.5 м³/год | 2.5 м/100м | Farms 0.1–0.3 ha |
| 40 мм | 34.0 | 7 м³/год | 2 м/100м | Farms 0.3–0.7 ha |
| 50 мм | 42.6 | 12 м³/год | 1.8 м/100м | Farms 0.7–1.5 ha |
| 63 мм | 53.6 | 20 м³/год | 1.5 м/100м | Farms 1.5–3 ha |
| 90 мм | 76.6 | 45 м³/год | 1.2 м/100м | Farms 3+ ha |
Full calculation example (1 ha of tomatoes)
Input data
- Area: 1 ha (100 × 100 m)
- Crop: field tomatoes, beds 1.5 m (width + row spacing)
- Number of beds: 100 / 1.5 = 67 rows × 100 m = 6,700 m of tape
- Tape: 8 mil, 20 cm spacing, 1.6 L/h flow
- Number of emitters: 6,700 m × 5 pcs/m = 33,500 pcs
- Total flow Q: 33,500 × 1.6 = 53,600 L/h = 53.6 m³/h
Problem: Irrigating the entire hectare simultaneously requires 53.6 m³/h — a very large pump and a 90 mm mainline. Solution: divide into 4 zones of 0.25 ha each with Q ≈ 13.5 m³/h.
Calculation for 1 zone (0.25 ha)
- Zone Q = 13.4 m³/h = 3.72 L/s
- Velocity 1.2 m/s → diameter D = 35.7 × √(3.72 / 1.2) = 63 mm
- We select 63 mm HDPE PN6
- Mainline length from the unit to the zone: 50 m + 50 m distribution ≈ 100 m
Pressure losses
- 63 mm mainline, 100 m, Q = 13.4 m³/h: ~1.5 m/100 m × 100 m = 1.5 m (0.15 bar)
- Fittings and valves: +10% = 0.17 bar
- 120 mesh disc filter: 0.3 bar
- Elevation difference (flat site): 0 bar
- Emitter operating pressure: 1.0 bar
- Pressure drop in 100 m of tape: 0.2 bar (standard non-PC tape)
- Total head: 1.0 + 0.15 + 0.17 + 0.3 + 0.2 = 1.82 bar
Margin: Add 20% for future expansion and fertigation: a pump with 2.2 bar head at 13.5 m³/h flow is needed.
Pump selection
A pump rated at 13.5 m³/h @ 2.2 bar (22 m head) is needed. From the catalog:
- Grundfos CM 15-2 — 15 m³/h @ 2.5 bar, 2.2 kW (suitable)
- Wilo MultiVert MVI 1606 — 16 m³/h @ 2.5 bar, 2.2 kW (suitable with slight margin)
- Pedrollo 4CPm 100-C — 12–14 m³/h @ 2.5–3 bar, 2.2 kW (optimal by operating point)
Tip: Choose a pump 10–20% above the calculation — filter contamination increases resistance over time. A pump with a VFD allows smooth pressure adjustment.
Common mistakes
- Excessive savings on mainline: choosing 32 mm instead of 40 mm to save UAH 1,000. Result: 3 bar loss instead of 1 — now you need a more powerful pump for UAH 5,000
- Ignoring elevation changes: a plot with a 10 m slope means +1 bar load on the pump. Without accounting for it — weak pressure in the upper section
- Irrigating all zones simultaneously: calculating for full flow requires a pump 3–5× more expensive. Zoning saves tens of thousands of hryvnias
- Not accounting for the filter in the pressure balance: a disc filter with a fouled element can cause 0.8 bar loss instead of 0.3 — your 20% limit is already exceeded
- Long tape runs without PC: 100+ meters without pressure-compensating emitters — the end will receive 30% less water, causing uneven crop growth
For commercial projects from 1 ha
- Software: AutoCAD + IrriCAD, Netafim IrriExpert, Rain Bird Landscape Design Plus for precise CAD design
- Flow sensor + Master valve — automatic shutoff on overconsumption (mainline burst). A MANDATORY node for large areas
- Pressure regulators per zone (Senninger, Bermad) — stabilize pressure regardless of mainline fluctuations
- Standards: ASABE EP405 (microirrigation design), ASABE EP458 (field evaluation of microirrigation systems), ISO 9912 (filters for microirrigation)
- Water intake: a geological assessment of the borehole (yield, static/dynamic level) is required BEFORE design
- VFD on the pump — 30–40% electricity savings + soft start without water hammer
Need help with the calculation?
For projects from 1 ha, our engineering department provides CAD design with full hydraulic calculations.