Drip irrigation design connects two calculations: how much water the crop needs, and whether the pipework can deliver it uniformly. A daily volume and run time are useful starting points; they do not establish a suitable lateral length or emitter pressure.
Estimate demand for the crop and site
The FAO single crop coefficient approach estimates standard crop evapotranspiration as ETc = Kc × ETo. Select the coefficient for the actual crop, growth stage and conditions. The method and its standard, non-stressed conditions are explained in FAO Irrigation and Drainage Paper 56, chapter 6.
A field water balance must also consider effective rainfall, soil water storage and other contributions. Root depth, allowable depletion and soil infiltration help determine when and how to apply the water. Salinity and leaching requirements need a separate assessment where relevant.
Worked example: one hectare of drip irrigation
Assume a hypothetical one-hectare crop with ETo = 5 mm/day, Kc = 0.80 and application efficiency = 90%. Assume no effective rainfall, stored-soil-water credit, capillary contribution, water stress or additional leaching requirement for this daily example.
| Step | Calculation | Result |
|---|---|---|
| Crop evapotranspiration | 5 × 0.80 | 4.00 mm/day |
| Net irrigation depth | No other water contribution assumed | 4.00 mm/day |
| Gross applied depth | 4.00 / 0.90 | 4.44 mm/day |
| Gross daily volume | 4.444… × 1 ha × 10 | 44.44 m³/day |
One millimetre over one hectare is 10 m³. Dividing net depth by application efficiency as a fraction follows the gross-depth relationship described in the FAO Irrigation Manual, module 7, section 3.2.2. The assumed 90% is not a guaranteed performance for a drip installation.
Now assume 5,000 emitters, all operating together at their rated discharge of 2 L/h each. Zone flow = 5,000 × 2 = 10,000 L/h = 10 m³/h.
Daily run time = required volume / zone flow
44.44 / 10 = 4.44 hours per day, approximately 4 hours 27 minutes.
The figure excludes any additional flushing water or time. It assumes the emitters actually deliver their rated flow. Changing the active zone area or number of simultaneous emitters requires recalculating the corresponding volume and flow.
Check the lateral before choosing a length
- Emitter data: use the exact model's discharge-pressure relationship, operating range, spacing and filtration requirements.
- Pressure distribution: include lateral inlet pressure, internal bore, emitter spacing, slope and reducing flow along the lateral.
- Uniformity: assess the project's discharge variation and distribution criteria; volume alone cannot demonstrate even application.
- Operation: check the available irrigation window, flushing arrangement, filtration losses, water quality and maintenance plan.
Pressure-compensating emitters still need to operate within their specified range. A manufacturer's maximum-length table is usable only when its bore, spacing, discharge, slope and inlet-pressure assumptions match the proposed installation.
Common drip-design questions
How long can a drip lateral be?
There is no defensible universal length. Use the selected product's data and the actual pressure and elevation profile. A daily crop-demand calculation cannot answer this on its own.
Should every crop run for 4 hours 27 minutes?
No. That result belongs only to the hypothetical inputs above. Weather, crop stage, soil moisture, active area and measured system delivery can change the schedule.
Does a pressure-compensating emitter fix all uneven watering?
No. Blockage, operation outside the pressure range, installation variation and unsuitable scheduling still matter. Check field performance as well as the design calculation.
Continue the design
Browse Tenselo's field and crop irrigation calculators. Connect the zone flow to mainline pipe sizing and pump duty and head. The public examples explain the method; project calculator execution remains within the account-based application.