| Scenario | Area (m²) | ET0 (mm/day) | Kc | Rain (mm/day) | Eff. | Gross Daily (L/day) |
|---|---|---|---|---|---|---|
| Small lawn, warm week | 80 | 5.2 | 0.85 | 0.0 | 75% | 471.47 |
| Vegetable bed, light rain | 30 | 4.8 | 1.05 | 1.0 | 85% | 143.82 |
| Shrubs, cooler season | 60 | 3.0 | 0.70 | 0.5 | 70% | 150.00 |
Ra is extraterrestrial radiation based on latitude and day of year.
- Enter the planted area and select the correct area unit.
- Choose how you will provide ET0: enter it directly or estimate it.
- Select a crop type to auto-fill Kc, or set a custom Kc.
- Add average rainfall and adjust the effective rainfall factor if needed.
- Set irrigation efficiency and your watering frequency in days.
- Pick a planning period to view totals, then calculate.
- Download your results as CSV or PDF for records.
1) Why water demand varies daily
Landscape demand changes with weather, canopy size, and growth stage. Reference evapotranspiration (ET0) is a climate signal that typically rises with higher temperature, stronger sunlight, lower humidity, and wind. A crop coefficient (Kc) scales ET0 to your lawn, vegetables, flowers, shrubs, or trees. Larger leaf area and active growth generally increase Kc, while dormant periods reduce it.
2) Net irrigation requirement
The net depth needed is the plant water use minus effective rainfall. Not every millimeter of rain benefits roots; some runs off or evaporates. The effective rainfall factor in this calculator lets you apply a realistic portion of rainfall to the soil-water balance, producing a more reliable net irrigation estimate for planning.
3) Efficiency and real-world losses
Field application losses can be significant. Sprinklers may lose water to wind drift and evaporation, while drip systems are usually more targeted but still suffer from leaks, pressure issues, and distribution non-uniformity. By dividing the net requirement by irrigation efficiency, the calculator estimates the gross water you must supply from a tank, well, or municipal line.
4) Scheduling and watering frequency
Daily demand is useful for budgeting, but irrigation is often applied every few days. The watering frequency converts daily totals into per-event volumes so you can size run-times, storage capacity, and pump cycles. For sandy soils and shallow roots, shorter intervals are typically safer; heavier soils may tolerate longer intervals.
5) Using results for system sizing
Use the gross daily and period totals to check supply limits. Compare required flow to your pump or tap capacity, and verify that zone flow rates can deliver the per-event volume within practical run times. Re-run scenarios with different Kc, rainfall, and efficiency to stress-test your design and seasonal plan.
1) What is ET0 and why is it important?
ET0 represents the water use of a reference grass surface under standard conditions. It tracks weather-driven demand, so multiplying ET0 by Kc helps estimate how much water your specific planting needs.
2) How do I choose a crop coefficient (Kc)?
Select a preset that matches your planting type, then adjust if your canopy is sparse or dense. For mixed beds, use a weighted average based on area or run separate calculations per zone.
3) Why does rainfall not fully reduce irrigation?
Some rainfall is lost to runoff, evaporation, or deep percolation beyond roots. The effective rainfall factor accounts for these losses so your irrigation plan remains realistic and avoids under-watering.
4) What irrigation efficiency should I use?
Use higher values for well-maintained drip systems and lower values for windy sprinkler sites. If you are unsure, start conservative, then refine after observing uniformity, runoff, and soil moisture response.
5) Should I use daily or per-event volumes?
Daily values help compare seasons and estimate average supply. Per-event volume helps set runtimes and storage. If you irrigate every 3 days, the per-event target equals 3 times the gross daily demand.
6) How do I handle multiple zones?
Run the calculator for each zone using its area, Kc, and method. Sum daily totals to estimate whole-garden supply, and keep zone-specific per-event volumes for scheduling runtimes.
7) Can I use this for container plants?
Yes, but treat the “area” as the total surface area of containers or benches and expect lower effective rainfall. Containers dry faster, so consider higher frequency and verify with actual moisture checks.