Inputs
Example Data Table
| Area (m²) | Need (mm/day) | Rain (mm/week) | Days/week | Emitters | Emitter (L/h) | Runtime (min) | Eff (%) | DU (%) | Required (L/event) | Applied (L/event) | Status |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 20 | 4 | 0 | 3 | 40 | 4 | 30 | 85 | 80 | ? | ? | Balanced target |
| 35 | 5 | 6 | 4 | 70 | 2 | 25 | 82 | 75 | ? | ? | Check surplus/deficit |
| 12 | 3 | 10 | 2 | 24 | 4 | 20 | 90 | 85 | ? | ? | Rain offsets demand |
Formula Used
- Weekly net depth: Dn = (Need × 7) − Rain
- Net weekly volume: Vn = Dn × Area (1 mm·m² = 1 L)
- Gross target volume: Vg = (Vn ÷ Eff ÷ DU) × (1 + Leach)
- Required per event: Ve,req = Vg ÷ Days
- Emitter flow to system flow: Q = (EmitterFlow × Count) ÷ 60
- Applied per event: Ve,app = Q × Runtime
- Balance ratio: BR = (Ve,app ÷ Ve,req) × 100%
- Runtime to balance: Runtime* = Ve,req ÷ Q
How to Use This Calculator
- Enter your irrigated area and an estimated daily water need.
- Add effective rainfall for the week if rain reached roots.
- Choose watering days per week and your planned runtime.
- Pick a flow method: emitter-based or measured total flow.
- Set efficiency and uniformity to reflect real field performance.
- Calculate, then adjust runtime until the status is balanced.
Field Notes
Understanding system balance
A balanced irrigation event applies enough water to refill the effective root zone without wasting water below roots or running off the surface. This calculator compares the required volume per event (net plant need adjusted for rainfall) against the applied volume from your emitters or measured flow.
Estimating daily demand
Daily demand is a practical planning value derived from crop type, canopy size, heat, wind, and soil texture. Use a conservative baseline, then refine it with observation: leaf turgor at midday, soil feel at 5–15 cm depth, and any stress patterns across the zone. Small changes in demand compound quickly over a week.
Turning net need into a target application
Plants do not receive every liter you apply. Losses occur from evaporation, leaks, and non-wetted areas. The calculator uses gross = required / efficiency and then accounts for distribution uniformity to show how “weak spots” reduce effective delivery. Improving efficiency and uniformity often saves more water than reducing runtime alone.
Diagnosing surplus and deficit
When applied water is lower than required, you may see dry patches, uneven growth, or higher soil salinity. When applied water is higher, symptoms include algae, fungus pressure, nutrient leaching, and soft soil. If the balance is high and your soil infiltrates slowly, shorten runtime and increase watering days.
Example scenario data
| Area | Need | Days/week | Emitters × L/h | Runtime | Result |
|---|---|---|---|---|---|
| 40 m² | 4.5 mm/day | 4 | 60 × 2.0 | 35 min | Slight deficit, increase 5–10% |
| 80 m² | 3.5 mm/day | 3 | 90 × 2.0 | 50 min | Balanced, monitor uniformity |
| 25 m² | 6.0 mm/day | 5 | 40 × 4.0 | 25 min | Surplus risk, reduce runtime |
Use these examples as starting points, then validate with a catch-can test for sprinklers or a bucket test for drip zones.
FAQs
1) What does “balanced” mean here?
Balanced means the applied liters per event are close to the required liters per event after accounting for rainfall, efficiency, and uniformity. Small positive balance can be acceptable for hot weeks.
2) Should I use emitter flow or measured total flow?
Use emitter flow when emitter count and rating are reliable. Use measured flow when the system has mixed emitters, unknown ratings, or pressure issues. Measured flow reflects real performance.
3) How do I choose irrigation efficiency?
Start with 85–95% for drip, 70–85% for micro-sprays, and 60–75% for overhead sprinklers. Lower the value if you see leaks, wind drift, overspray, or frequent puddling.
4) What is distribution uniformity (DU) used for?
DU represents how evenly water is delivered across the area. Low DU means some plants get less water even if totals look fine. Improving DU often fixes dry spots without increasing water use.
5) How should I enter rainfall?
Enter only the rainfall that actually reaches the root zone. Heavy storms can run off or sit on compacted soil. If you are unsure, use a smaller “effective” value and confirm with soil checks.
6) Why does increasing watering days help?
Splitting the same weekly volume into more events reduces runoff and improves infiltration, especially on clay soils or slopes. It can also reduce plant stress on hot, windy days.
7) What if the calculator shows a large surplus?
First confirm inputs and units. Then reduce runtime or increase days/week with shorter runs. Check for clogged emitters, broken heads, and pressure mismatches so you do not “overwater to cover” weak areas.