Calculator Inputs
Example Data Table
| Input scenario | Area | Plant | Climate | Season | Method | Rain | Output (weekly) |
|---|---|---|---|---|---|---|---|
| Vegetable bed, warm season | 40 m² | Vegetables | Warm | Summer | Drip | 5 mm/week | ~ 1,050 L/week |
| Flower border, partial shade | 25 m² | Flower beds | Mild | Spring | Soaker hose | 10 mm/week | ~ 370 L/week |
| Lawn, sprinkler, low rain | 80 m² | Turf grass (lawn) | Hot | Summer | Sprinkler | 0 mm/week | ~ 4,250 L/week |
Formula Used
- Daily crop demand: ETc = ET0 × SeasonFactor × Kc
- Effective rainfall credit: Re = RainWeekly × (Days/7) × RainEfficiency
- Net depth needed: NIR = max(0, ETc × Days × SoilF × ShadeF × MulchF − Re)
- Gross depth applied: GIR = NIR ÷ Efficiency
- Volume: Liters = GIR(mm) × Area(m²) (since 1 mm over 1 m² = 1 L)
- Per event: VolumeEvent = TotalVolume ÷ (EventsPerWeek × Days/7)
- Efficiency accounts for spray drift, runoff, and uneven coverage.
- Mulch and shade reduce evaporation from soil and leaves.
- Sandy soil often needs more frequent watering, not deeper watering.
- Use custom ET0 when you have local irrigation data.
How to Use This Calculator
- Enter the garden area you plan to water.
- Select plant type, climate preset, and season for demand.
- Choose soil, shade, and mulch to reflect site conditions.
- Select an irrigation method, or override efficiency if known.
- Add weekly rainfall and effectiveness to credit natural water.
- Set watering events per week to estimate per‑event volume.
- Press Calculate to view results above the form.
- Download CSV or PDF to share, record, or compare scenarios.
Water Use Planning for Efficient Gardens
Start with weather-driven demand
Watering by habit often leads to stress or waste. A better approach estimates plant water demand from reference evapotranspiration (ET0) and crop coefficient (Kc). Typical summer ET0 values range from 3–7 mm/day depending on climate. Vegetables often use Kc around 0.7–1.1, while many ornamentals run 0.5–0.8. This calculator blends ET0, season factor, and Kc to estimate daily demand.
Convert depth to practical volume
Once you have a net irrigation depth, volume is straightforward: 1 mm over 1 m² equals 1 liter. For example, 15 mm over a 20 m² bed equals about 300 L before efficiency. Converting to gallons helps homeowners compare results with hose flow rates and timer settings. A typical garden hose delivers roughly 10–20 L/min, so volume targets become actionable.
Account for soil, shade, and mulch
Soil type changes how water is stored and how often you should irrigate. Sandy soil drains quickly, so it benefits from smaller, more frequent events. Clay holds more water but can shed runoff if applied too fast; slow application and cycle‑soak scheduling helps. Shade and mulch reduce surface evaporation; a 5–8 cm mulch layer can noticeably cut summer water loss and stabilize soil temperature, improving root performance.
Improve delivery efficiency and rainfall credit
Irrigation method matters. Drip and soaker systems typically deliver water more efficiently than overhead sprinklers because they reduce wind drift and leaf evaporation. Use the efficiency setting to represent losses and to see the benefit of upgrades. Splitting weekly water into 2–4 events can reduce runoff on slopes and improve root-zone wetting. For containers, increase frequency because potting mixes dry faster than ground soil.
Rainfall is free water, but not all rain is effective. Light showers may evaporate quickly, while heavy storms can run off compacted soil. Enter weekly rainfall and a realistic effectiveness factor (often 0.5–0.9) to credit what actually infiltrates. Recheck results monthly and adjust as plants mature and temperatures change.
FAQs
How accurate is the estimate?
It is a planning estimate based on ET0, plant coefficient, efficiency, and your inputs. Use it to compare scenarios. Fine‑tune by observing soil moisture, plant stress, and actual water meter readings.
What ET0 value should I use?
If you don’t have local data, start with 4–5 mm/day for warm seasons and 2–3 mm/day for mild seasons. Replace with station or smart‑controller ET0 when available for better accuracy.
Should I set Kc differently for lawns and beds?
Yes. Turf often uses a higher coefficient than many shrubs. Vegetable beds can be high during peak growth. If unsure, keep Kc near 0.8 and adjust upward for dense, fast‑growing plants.
How do I choose irrigation efficiency?
Use higher efficiency for drip/soaker and lower for sprinklers, especially in wind. If you notice runoff or overspray, reduce the value. Field uniformity, pressure, and maintenance strongly influence efficiency.
Why does rainfall not reduce the result much?
Only effective rainfall counts. Some rain evaporates, runs off, or stays on leaves. Lower the rain‑effectiveness factor for compacted soil, steep slopes, or intense storms to reflect limited infiltration.
How many watering events per week is best?
For most in‑ground beds, 2–4 events per week balances deep wetting and runoff control. Sandy soils and containers need more frequent events. Clay soils may need fewer events with slower application rates.
Can I use this for containers or raised beds?
Yes. Enter the surface area you water, then select soil and factors that match the mix. Containers typically dry faster, so use more frequent events and consider a lower mulch/shade reduction if exposed.