What is a landscape coefficient?
Most crop-water calculations use a single crop coefficient (Kc) for one uniform planting, following the FAO-56 approach. A home or commercial landscape is usually a mix of different species planted at different densities, in a site with its own sun, wind and reflected heat, so Costello and Jones developed the landscape coefficient (KL) for the Water Use Classification of Landscape Species (WUCOLS) method to handle that mix.
KL combines three separate factors: how thirsty the species themselves typically are, how dense and layered the planting is, and how the specific site's microclimate compares with the open conditions used to measure reference evapotranspiration.
| Factor | Low | Average / Moderate | High |
|---|---|---|---|
| Species (Ks) | 0.1-0.3 | 0.4-0.6 | 0.7-0.9 |
| Density (Kd) | 0.5-0.9 | 1.0 | 1.1-1.3 |
| Microclimate (Kmc) | 0.5-0.9 | 1.0 | 1.1-1.4 |
If you're after a simpler single-species estimate, or a specific plant type, the shrub watering calculator and tree watering calculator use the related plant-factor approach behind California's landscape water ordinance instead.
How this landscape coefficient calculator works
1. Landscape coefficient
KL
KL = Ks × Kd × Kmc
This calculator uses the middle of each range you choose, or your own value.
2. Landscape evapotranspiration
ETL
ETL = KL × ETo
If your ETo is reported per day rather than per week, the calculator multiplies it by 7 first.
3. Net water use
US units
Gallons = ETo × KL × Area (sq ft) × 0.623
Metric units
Liters = ETo × KL × Area (m²)
This is the water the planting itself uses (net), not a gross irrigation volume; it doesn't include system losses or leaching.
When this calculator isn't the right tool
Use a different approach for:
- A gross depth, run time or full schedule, once you have ETL. See the irrigation water requirement calculator or the irrigation scheduling calculator.
- A single crop, shrub or tree rather than a mixed landscape. See the vegetable garden water calculator, the shrub watering calculator or the tree watering calculator.
- A single reference ET value, if you don't have one yet. See the evapotranspiration calculator.
Landscape coefficient calculation example
A mature planting of a single moderate-water-use groundcover, with full, average-density canopy cover, in an open, unshaded site with typical wind: Ks 0.5 (moderate), Kd 1.0 (average), Kmc 1.0 (average), with ETo of 1.5 in this week over 500 sq ft.
- KL: 0.5 × 1.0 × 1.0 = 0.5.
- ETL: 0.5 × 1.5 = 0.75 in.
- Net water use: 1.5 × 0.5 × 500 × 0.623 ≈ about 234 gallons.
A mixed planting in metric
A mixed planting of several low-water-use shrubs (Ks 0.2), full and layered enough to count as high density (Kd 1.2), in an average microclimate (Kmc 1.0), with ETo of 35 mm this week over 50 m²:
- KL: 0.2 × 1.2 × 1.0 = 0.24.
- ETL: 0.24 × 35 = 8.4 mm.
- Net water use: 35 × 0.24 × 50 ≈ about 420 liters.
Both examples use the same factor values as worked examples published for the landscape coefficient method; enter your own planting's numbers above for your result.
Practical guidance
Choose Ks from the plant's classification in the WUCOLS list where you can find it, rather than guessing; many water agencies and university extension programs link to the current WUCOLS plant list. For a mixed planting, use a single representative Ks for the dominant or highest-water-use species present, since that's the common practice the worked examples above follow, rather than trying to average many different species by hand.
For density, judge canopy cover and layering as you actually see it on site: a newly planted bed with visible bare soil between plants is lower density than the same bed once it has filled in. For microclimate, compare your site with the open, unobstructed conditions used for reference ET: a south- or west-facing bed against a sunny wall, or an exposed windy corner, both call for a higher Kmc than a sheltered, shaded spot.
Treat KL as a planning estimate. WUCOLS species factors come from expert horticultural judgment rather than controlled field trials for every plant, so adjust based on how your own landscape actually responds.
What changes each factor
| Factor | Pushes it lower | Pushes it higher |
|---|---|---|
| Species (Ks) | Drought-tolerant, native or Mediterranean-climate species | Lush, tropical or moisture-loving species |
| Density (Kd) | Sparse, single-species, low canopy cover | Full, multi-layered, several vegetation types together |
| Microclimate (Kmc) | Shade, shelter from wind, cooler microclimate | Reflected heat from pavement or walls, exposed wind |
Frequently asked questions
What is a landscape coefficient (KL)?
A landscape coefficient is a multiplier, similar to a crop coefficient, that adjusts reference evapotranspiration (ETo) for a mixed landscape planting rather than a single agricultural crop. Developed by Costello and Jones for the Water Use Classification of Landscape Species (WUCOLS) method, it's calculated as KL = Ks × Kd × Kmc, combining a species factor, a density factor and a microclimate factor.
What is the species factor (Ks)?
The species factor accounts for how much water a particular plant type typically needs relative to reference evapotranspiration. WUCOLS groups species into low (0.1-0.3), moderate (0.4-0.6) and high (0.7-0.9) water use, based on the plant's classification in the WUCOLS list.
What is the density factor (Kd)?
The density factor accounts for how full and layered the planting's canopy is. A single, sparse species with low canopy cover is assigned a low density factor (about 0.5-0.9); an average, single-layer planting with full cover uses about 1.0; a dense, multi-layered mixed planting uses a high factor (about 1.1-1.3).
What is the microclimate factor (Kmc)?
The microclimate factor accounts for how the specific site's conditions differ from the open, unobstructed conditions used to measure reference evapotranspiration. A sheltered, shaded site with little wind uses a low factor (about 0.5-0.9); typical open conditions use about 1.0; a site with reflected heat from pavement or walls, or unusually exposed to wind, uses a high factor (about 1.1-1.4).
How is landscape ET (ETL) calculated from KL?
Landscape evapotranspiration is reference evapotranspiration (ETo) multiplied by the landscape coefficient: ETL = KL × ETo. This is the WUCOLS method's equivalent of the FAO-56 ETc = ETo × Kc formula, adapted for mixed ornamental landscape plantings rather than a single crop.
Does this calculator tell me how much to irrigate?
It gives landscape ET and the net water use that represents, which is what the planting itself uses, not what an irrigation system needs to apply. To carry that through application efficiency, effective rainfall and a full gross depth or run time, use the irrigation water requirement calculator or the irrigation scheduling calculator.
See the irrigation water requirement calculator or the irrigation scheduling calculator.
Is the WUCOLS / landscape coefficient method scientifically validated?
WUCOLS species factors are based on the collective field experience and expert judgment of a panel of California landscape horticulturists rather than controlled research trials for every species, and some published critiques note the values haven't been confirmed by dedicated scientific field research for many plants. Treat KL and ETL as a widely used planning estimate, and adjust based on how your own plantings actually respond.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The KL, ETL and net water use math were verified by hand, and both worked examples reproduce published landscape-coefficient worked examples exactly.
- Every factor range is checked against the WUCOLS sources below.
- The page states plainly, per the sources themselves, that WUCOLS values reflect expert judgment rather than universal research validation.
Sources
- Costello, L.R., Matsuda, K.N., Pittenger, D.R. et al. A Guide to Estimating Irrigation Water Needs of Landscape Plantings in California (KL = Ks × Kd × Kmc; ETL = KL × ETo).
- California Water Efficiency Partnership. WUCOLS Toolbox (landscape coefficient worked examples).
- Alazba, A.A. et al. Water Demand Determination for Landscape Using WUCOLS and LIMP Mathematical Models, Water 2025, 17, 1429 (tabulated Ks, Kd, Kmc ranges).
- Barton Springs/Edwards Aquifer Conservation District. Methodology for Estimating Landscape Irrigation Demand (Gallons = ETo × KL × Area × 0.623; also notes on the method's limitations).
- Food and Agriculture Organization of the United Nations. Allen, R.G. et al. FAO Irrigation and Drainage Paper No. 56: Crop Evapotranspiration (reference ET and coefficient-based ET methodology).