Calculator Inputs
Example Data Table
| Scenario | Area (m2) | Ks | Kmc | Kd | ETo (mm/day) | KL | ETL (mm/day) | IE | DU | Gross (mm/day) | Water (L/day) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Mixed shrubs, typical microclimate | 120 | 0.55 | 1.00 | 1.00 | 5.00 | 0.55 | 2.75 | 0.75 | 0.80 | 4.58 | 550 |
| Drought-tolerant, shaded, sparse | 90 | 0.35 | 0.90 | 0.85 | 6.00 | 0.27 | 1.61 | 0.85 | 0.85 | 2.23 | 201 |
| Annuals, sunny and windy, dense | 60 | 0.70 | 1.10 | 1.15 | 5.50 | 0.89 | 4.90 | 0.70 | 0.75 | 9.33 | 560 |
Formula Used
- Landscape coefficient: KL = Ks × Kmc × Kd
- Landscape evapotranspiration: ETL = KL × ETo (mm/day)
- Net irrigation depth: Net = max(0, ETL − Rain) (mm/day)
- Gross irrigation depth: Gross = Net ÷ (IE × DU) (mm/day)
- Water volume: Liters/day = Gross(mm/day) × Area(m2)
- Runtime per event: Minutes = (Depth per event ÷ PrecipRate) × 60
How to Use This Calculator
- Enter the landscaped area and choose the correct unit.
- Select plant type, microclimate, and density, or enter custom factors.
- Enter local ETo and optional effective rainfall.
- Set irrigation efficiency and distribution uniformity for your system.
- Choose irrigation days per week and your precipitation rate.
- Click Calculate to see KL, ETL, volumes, and runtime.
- Download CSV or PDF for sharing and record keeping.
Professional Notes
Understanding KL in water budgeting
The landscape coefficient (KL) scales reference evapotranspiration (ETo) to match a specific planting. A KL of 0.55 means the zone typically uses about 55% of ETo under similar weather. Using KL makes water planning consistent across zones. It also supports quick comparisons when seasons change fast.
Choosing Ks, Kmc, and Kd
Ks reflects plant type demand, Kmc adjusts exposure, and Kd represents canopy density. Shrubs often sit near Ks 0.50–0.60, while drought-tolerant beds may be near 0.30–0.40. Windy or reflective sites can raise Kmc toward 1.10, and dense canopies may push Kd above 1.00.
Turning ETo into daily depth
ETL is calculated as KL multiplied by ETo in mm/day. If ETo is 5.0 mm/day and KL is 0.55, ETL is 2.75 mm/day. Subtract effective rainfall to estimate net irrigation depth and avoid overwatering after useful rain.
From depth to volume and runtime
Net depth is converted to gross depth by dividing by irrigation efficiency (IE) and distribution uniformity (DU). Lower IE or DU increases gross demand because extra water is needed to cover losses and dry spots. Since 1 mm over 1 m2 equals 1 liter, daily depth converts directly to liters per day, then to per-event volume. Runtime per event is estimated from precipitation rate (mm/hr).
Interpreting outputs for scheduling
Use liters per event and runtime per event to set controller programs. When watering three days per week, the calculator spreads weekly demand across three applications. Increase frequency for sandy soils, reduce it for heavier soils, and verify results with soil checks and plant response.
| Input | Value | Unit |
|---|---|---|
| Area | 120 | m2 |
| Ks / Kmc / Kd | 0.55 / 1.00 / 1.00 | — |
| ETo | 5.0 | mm/day |
| Effective rainfall | 0.0 | mm/day |
| IE / DU | 0.75 / 0.80 | fraction |
| Precipitation rate | 15.0 | mm/hr |
FAQs
1) What does a higher KL indicate?
A higher KL means the zone uses a larger fraction of ETo. It usually reflects thirstier plants, harsher exposure, or denser planting, so irrigation demand and runtime increase.
2) Should I use effective rainfall or measured rainfall?
Use effective rainfall: the portion that infiltrates and benefits roots. Runoff, canopy interception, and evaporation reduce effectiveness, so measured rainfall often overestimates usable moisture.
3) Why are IE and DU both needed?
IE covers losses like evaporation and overspray. DU covers uneven distribution across the zone. Using both estimates the gross depth needed so the driest areas still meet net demand.
4) How do I choose a precipitation rate?
Use manufacturer precipitation rate for the installed nozzle type, or run a catch-can test and average results. Enter mm/hr. The runtime estimate assumes the rate applies evenly across the irrigated area.
5) Can I use this for drip irrigation zones?
Yes. Use a drip-appropriate IE and DU, and compute precipitation rate from emitter flow and spacing. The depth-to-volume conversion remains valid because it is area based.
6) Why does gross depth sometimes look high?
Gross depth rises when IE or DU are low, because more water is required to overcome losses and variability. Improving pressure regulation, fixing leaks, and correcting head spacing can lower the required gross amount.
7) How often should I update inputs?
Update ETo weekly during peak season and after unusual heat or wind. Revisit coefficients when plants mature, pruning changes density, or shade and hardscape alter microclimate.