Estimating Water Needs of Landscape Plants and Entire Landscapes
Estimating Water Needs of Landscape Plants and Entire Landscapes
Reliable research-based data on landscape water needs is extremely limited. There simply are hundreds of plant species to evaluate and the scientific process requires a great deal of resources to identify water requirements of an individual species.
Few information sources offer quantitative estimates of landscape plants’ water requirements. Most of those that do, including the widely-referenced publication, WUCOLS (Water Use Classification of Landscape Plants), are not confirmed by scientific field research. However, we know from the limited scientific data available that most of the commonly used landscape plants need significantly less than 100% ETo to provide acceptable performance during most of the year. The Landscape Plant Water Needs Table is a good reference.
Landscape plantings usually violate one or more assumptions of the relationship between ETo and a plant’s Kc, which presume the plants are:
♦ 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.
Mixed plantings of groundcover, shrub, and tree species create variations in the plant canopy and shading that prevent the overall planting from functioning as a single big leaf, soil water content is not always at optimum levels, and the plants are not usually grown with optimum growth, development, and yield as the goal. Expectations of landscape plant performance are simply acceptable appearance and function, which are much less stringent than optimum growth, development, and yield, as expected in high-quality turfgrass areas or agricultural commodities.
Also, research in plant physiology has revealed that water use of some woody landscape plants does not increase proportionally as ETo increases throughout the day especially when site conditions are harsh, such as when trees are planted within paved parking lots. Some species actually use less water in harsh situations because their stomata close naturally when water is limited.
Altogether, these factors severely limit the ability of the ETo equation to accurately reflect a landscape’s water requirement and make it impossible to determine a precise crop coefficient (Kc) for landscape plant species.
Landscape Plant Factors (PF)
Since landscape plants do not conform to the scientifically accepted assumptions of calculating Kc’s, water needs of landscape plants are more appropriately defined as the percentage of ETo needed to maintain their appearance and intended function. The ETo standard has been used to determine ranges in percentage of ETo and plant factors (PF) for several species that quantify the amount of water they need to provide minimally acceptable performance and function, not necessarily optimum growth.
Research findings show that many widely used tree, shrub, and groundcover species maintain their aesthetic and functional value in a landscape when irrigated within a range of about 20% to 80% of ETo (see Table 2). A majority of plants evaluated have water needs in the middle to lower end of this range. These numbers are useful in estimating water budgets and irrigation schedules for landscapes even though the precise water use of the plants has not been quantified. Keep in mind the findings are for plants that are established in the landscape. Newly planted plants will have different water needs.
For the landscape species with unknown water needs, it is recommended to set initial irrigation schedules at 50% ETo for established non-turf landscape plantings. It may be necessary to adjust PF values slightly depending on site conditions and the amount of water stress desired.
For example, the PF may need to be adjusted for plantings subjected to partial shade from a building (lower PF), excessive wind (higher PF), or heat from nearby pavement (higher Kc). Plant performance must be evaluated and irrigation increased or decreased in increments of about 10% ETo until the desired level of performance is attained with the least amount of water. Intervals between irrigation of woody landscape plant materials can usually be greatly extended from fall through winter.
Landscape Coefficient (KL)
Calculating a landscape coefficient (KL) is another method of estimating an entire landscape’s water requirement. It is a formula used to develop and adjust PF values for local conditions that involves assigning additional microclimate, density, and species factors to a landscape.
While useful in generating numbers needed by computerized irrigation scheduling programs, some of the smart irrigation controllers, and irrigation designers and managers in need of numbers to calculate water budgets, the KL formula may provide a false sense of precision. It is based on theory with little supporting research. The KL does not address or account for plant stress or minimum irrigation requirements which greatly influence a landscape’s water need. The exact relationship of the factors included in the formula remains unproven as well.
While the KL methodology can be used for irrigation scheduling, a user may find that the additional estimations and necessary calculations do not result in a more accurate or more meaningful estimate of the landscape’s water needs than using the general PF values discussed above.
Summary
To summarize the overall process of estimating a landscape’s irrigation need, we use ETo information adjusted by a plant factor to determine the approximate amount of water that specific plants require for desired quality. With this information and the irrigation system precipitation rate and uniformity, we determine how long to run the irrigation system. The next step is to decide the irrigation frequency, or how often to apply water.