Irrigation Water Requirement
Enter crop water need ETc, effective rainfall, irrigation efficiency and field area to compute net and gross irrigation depth and the actual supply volume (m³ and litres) for scheduling and water planning.
Input Data
Results
At a glance:Irrigation water requirement is the actual volume that must be supplied to the field to meet the crop. Compute in three steps: first net irrigation depth = crop water need ETc minus effective rainfall (rain truly used by the root zone), the root-zone deficit; then divide by irrigation efficiency to get gross irrigation depth, because conveyance leakage, evaporation and uneven application cause losses so the drawn water always exceeds what the crop absorbs; finally convert gross depth (mm) to volume: 1 mm over 1 m² equals 1 litre, so volume(m³) = gross depth(mm) ÷ 1000 × area(m²). Crop water need ETc comes from reference evapotranspiration ET₀ times crop coefficient Kc (ETc = Kc × ET₀). Efficiency varies by system: drip is most water-saving (~85–95%), sprinkler next (70–85%), traditional surface/flood lowest (50–65%); lower efficiency means more gross water. Accurate estimation avoids over-irrigation waste and prevents yield loss from deficit.
Formula
Net irrigation depth: net = max(0, ETc − effective rainfall) (mm).
Gross irrigation depth: gross = net ÷ (efficiency ÷ 100) (mm).
Irrigation volume: V(m³) = gross(mm) ÷ 1000 × area(m²) (1 mm/m² = 1 L).
$$I_{net} = \max(0,\ ET_c - P_{eff})$$$$I_{gross} = \frac{I_{net}}{E_a/100}$$$$V_{(m^3)} = \frac{I_{gross}}{1000} \times A_{(m^2)}$$How to Use
- Enter crop water need ETc (mm) and effective rainfall (mm).
- Enter irrigation system efficiency (%) and field area (m²).
- The tool returns net and gross irrigation depth and supply volume (m³ / litres); lower efficiency or larger area means more supply.
Approximate overall efficiency by irrigation method
| Method | Overall efficiency | Note |
|---|---|---|
| Drip / micro | 85–95% | Point supply, least loss, most saving |
| Sprinkler | 70–85% | Even coverage, affected by wind/evaporation |
| Furrow | 60–75% | Along furrows, moderate percolation |
| Surface/flood | 50–65% | Full flooding, largest loss and unevenness |
Ranges are approximate and vary with management, terrain and soil; higher efficiency needs less gross water for the same net need.
Case Studies
Water per irrigation for a vegetable field
A 1000 m² vegetable field: ETc = 6 mm, effective rainfall 2 mm, sprinkler efficiency 75%.
Net depth = 6 − 2 = 4 mm; gross depth = 4 ÷ 0.75 ≈ 5.33 mm.
Volume = 5.33 ÷ 1000 × 1000 ≈ 5.33 m³ (~5333 L) — about 5.3 t of water for this field this time.
Water saving by switching to drip
Same field, same net need 4 mm, but drip efficiency 90%.
Gross depth = 4 ÷ 0.90 ≈ 4.44 mm, volume ≈ 4.44 m³ (~4444 L).
About 890 L less than sprinkler (~17%), substantial over time — showing efficient irrigation's saving value.
FAQ
How do I get crop water need ETc?
ETc = crop coefficient Kc × reference evapotranspiration ET₀. Estimate ET₀ by FAO-56 Penman-Monteith (most accurate with full data) or Hargreaves (only max/min temperature) — this site has both; Kc from FAO tables by crop and stage, mostly 0.3–1.2. Multiply to get the period ETc, then enter here.
What is the difference between effective and total rainfall?
Total rainfall is all precipitation measured; part runs off the surface or percolates below the root zone and is unavailable. Effective rainfall counts only what the root zone actually absorbs. High-intensity rain on slow-infiltrating soil has low effective fraction; small steady rain is more effective. Practice often estimates it as total rainfall times a reduction factor; when unsure, use a conservative lower value.
What efficiency should I enter?
Depends on method and management: drip/micro ~85–95%, sprinkler 70–85%, furrow 60–75%, surface/flood 50–65%. It covers conveyance leakage, evaporation/drift and field unevenness. If unsure, take the mid-range for your system type; repairing pipes and avoiding midday high-evaporation irrigation raise efficiency and cut gross water.
Is the computed volume for one time or one day?
It follows the period of the ETc you entered. If you enter daily ETc, the volume is that day's; if you enter a cycle's accumulated net need (e.g. 3 days), it is that event's volume. The key is that ETc, effective rainfall and the result share the same period. In practice, deficit is accumulated over several days then irrigated once to match soil holding capacity.
Why is 1 mm of water equal to 1 litre/m²?
A 1 mm depth over 1 m² has volume = 0.001 m × 1 m² = 0.001 m³ = 1 litre. So gross depth(mm) × area(m²) is the litre count, ÷1000 gives m³. This conversion links a depth in mm directly to volume by area — the most common unit bridge in agricultural hydrology.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.