Under identical weather conditions, different plant species can have different evapotranspiration (ET) rates and thus different water requirements because of their physiology and dissimilar response to weather. Since ETo estimates weather’s influence on the water use of the reference crop (tall fescue in good health) growing in reference conditions (full sun and unlimited water), ETo must be adjusted to estimate the water required by crops and landscape plants growing in the same general area where ETo is calculated.
Plant Type | Plant Factor |
Tree, Shrubs, Vines, Groundcovers (woody plants) | 0.5 |
Herbaceous Perennial Plants | 0.5 |
Desert Adapted Plants | 0.3 |
Annual Flowers & Bedding Plants | 0.8 |
General Turfgrass Lawns, cool-season (tall fescue, Ky. bluegrass, rye, bent) | 0.8 2, 3 |
General Turfgrass Lawns, warm-season (bermuda, zoysia, St, Augustine, buffalo) | 0.6 2, 3 |
Home Fruit Crops, Deciduous | 0.8 2 |
Home Fruit Crops, Evergreen | 1.0 |
Home Vegetable Crops | 1.0 2 |
Mixed Plantings | PF of the planting is that of the plant type present with the highest PF |
1 Values do not apply to any plant production operations, such as nurseries, greenhouses, sod farms, or commercial farms.
2 Plant Factor shown is the annual average Kc value; monthly or seasonal Kc's are available if more precision is desired.
3 Plant Factor does not apply to sports fields, golf greens or tees.
Data adapted from: ANSI/ASABE Standard S623, Determining Landscape Plant Water Requirements. 2015.; Snyder, R.L. 2014. Irrigation scheduling: Water balance method. http://biomet.ucdavis.edu/irrigation_scheduling/bis/ISWBM.pdf.; Meyer, J.L., V.A. Gibeault, and V.B. Youngner. 1985. Irrigation of turfgrass below replacement of evapotranspiration as a means of water conservation: Determining crop coefficient of turfgrasses. Proc. 5th Intl. Turfgrass Research Conf.
An ETo adjustment factor is known as a crop coefficient (Kc) when plants are expected to produce high performance, optimum growth, and/or maximum yield, while it is termed a plant factor (PF) when plants are simply expected to provide acceptable appearance and function. Whereas ETo accounts for variation in the effects of weather on plant water use, a Kc or PF accounts for differences in plant morphology, crop physiology, and irrigation management between the plant of interest and the reference crop. Normally, a Kc or PF is the only adjustment to ETo that is needed to accurately estimate plant water requirements. Both Kc and PF are dimensionless numbers usually ranging from 0.2 to 1.2. The appropriate Kc or PF is used with ETo, the irrigation system’s precipitation rate and distribution uniformity, plant root depth, and soil water holding capacity to establish an irrigation schedule for a landscape area.
Although ETo can vary from one climate zone or geographic area to another, the percentage of ETo (Kc or PF) required by a given crop (Kc) or landscape plant (PF) does not change. An exception can exist if the performance expectation for a plant is different than the performance level applied when the ETo adjustment factor was obtained. For example, to perform acceptably in a lawn, bermudagrass needs about 60% of ETo. If ETo for a day is 0.20 inches in location A and 0.3 inches in location B, then bermudagrass would need 0.12 inches (0.20 in. × 0.6) in location A and 0.18 inches (0.3 × 0.6) in location B.
The concept of using ETo modified by an adjustment factor to estimate a plant’s water needs was developed by agricultural crop scientists to estimate the water required by large tracts of field and orchard crops. The model assumes a planting meets the following standard specifications:
- Plants are growing vigorously with the goal of near optimum growth or yield.
- An extensive, nearly uniform canopy is formed and it transpires as a single big leaf.
- Plant water use is tightly synchronized with ETo.
- Soil is well-watered and soil moisture is adequate in the planting at all times.
To establish scientifically a Kc or PF, plant water use and plant performance are simultaneous measured. The quantity of water used by a given crop or plant to attain a desired level of performance is then compared to ETo during the same period and it is expressed as a percentage or fraction of ETo known as a Kc or PF. There are several methods used to measure plant water use to establish a PF or Kc. They include measuring:
- The quantity of water applied to a plant and estimating application losses or assuming they are zero.
- The weight of water lost from the plant using lysimeters or other weighing devices.
- Water flux from the crop canopy to the atmosphere (aerodynamics).
- Water flow through plant stems or tree trunks (sap flow technique).
- Water lost from a plant using a combination of these methods.