Using ET Data to Estimate Turgrass and Landscape Plants' Water Needs
Using ET Data to Estimate Turfgrass and Landscape Plants’ Water Needs
Water needs of plants vary by individual species and their location in the landscape. Most landscape plants, including turfgrass, groundcovers, shrubs and trees, need less water than ETo (reference ET) which is the estimated water use of a standard pasture grass. Thus, their water needs are expressed as a percentage of ETo.
The percentage of ETo need by a plant is called a crop coefficient (KC), plant factor (PF), or landscape coefficient (KL). The relationship is:
ETo x Kc = Plant ET,
where Kc can be substituted with PF or KL.
For example, to perform optimally, bermudagrass needs about 60% of the water that the standard pasture needs. Thus, if we knew that ETo for a day was 0.20 inches, then bermudagrass would need 0.20 inches times 0.60, or 0.12 inches of water. Although ETo varies from one climate zone to another, the percentage of it used for a given species (the crop coefficient or plant factor) does not change. Crop coefficients and PFs are dimensionless numbers usually ranging from 0.1 to 1.2.
The concept of using the ETo standard to estimate a crop’s water needs through a crop coefficient was initially derived by agricultural crop scientists to estimate the water requirements of large tracts of field and orchard crops. Thus, the scientific application of ETo to calculate crop coefficients assumes the plant material of interest is:
♦ well-watered with soil moisture unlimited at all times.
♦ growing vigorously.
♦ forming a uniform, nearly continuous canopy that functions as a single big leaf.
♦ grown with the goal of optimum growth and development and yield.
♦ using water in direct proportion to the rate of ETo.
Crop coefficients (Kc values) and PFs are developed by determining the water use of a given species or crop and comparing it to ETo over the same time period. There are several methods used to estimate crop water use by measuring:
♦ applied water to the crop and estimating application losses.
♦ the weight of water lost from the crop using lysimeters or weighing devices.
♦ water flux from the crop canopy to the atmosphere (aerodynamics).
♦ water flow through plant stems or tree trunks.
♦ water lost through a combination of these methods.
In addition, the crop condition and performance are evaluated in terms of yield, growth, appearance, or other parameters. Then a relationship between water use, ETo, and the performance of the crop is developed and the Kc or PF value is determined for the crop.