Nursery Crop Science

College of Agriculture and Life Sciences

The Feasibility of Using Reference Evapotranspiration (ETo) to Estimate the Irrigation Needs of Landscapes in the Southeast
Patricia Lindsey
North Carolina State University

Meeting today’s urban water demands is increasingly challenging in the face of water shortages and periodic drought here in the southeast. As municipalities strive to conserve often limited supplies, measures are adopted that reduce or frequently restrict landscape irrigation altogether. As we continue to look for water efficient options, it is very useful to examine approaches developed over the last decade in California. Two initiatives are especially noteworthy and very applicable to the southeast:

1. As of January 1, 1993, all cities and counties in California were required to administer the State Model Water Efficient Landscape Ordinance, which required landscape professionals to:
* Group plants with similar water needs in zones with a separate irrigation valve.
* Schedule irrigations based on normal evapotranspiration (ET) adjusted by plant coefficients.
* Maintain efficient irrigation systems.
* Abide by set limits on total allowable water use.
This ordinance was supported by the Green Industry as a way to stay proactive and be an equal partner in the discussion to regulate yet stretch water supplies within the context of maintaining and preserving healthy and beautiful commercial and residential landscapes.
The ordinance resulted in the organization of an active statewide workgroup of landscape specialists and University of California researchers, whose purpose was fill the gap in the knowledge that would allow for successful implementation of the ordinance. Specifically, they were able to identify and classify over 1800 plants based on their estimated water usage and have refined a water budget formula to accurately predict landscape water use- the Landscape Coefficient Method.

The Water Budget Approach
The basis of water budgeting is evapotranspiration-based landscape irrigation scheduling. Reference evapotranspiration (ETo) data is estimated from a Class A evaporation pan or from specialized weather stations. Normal year (historical) monthly averages for four cities in NC are found in Table 2. Using ETo promotes conservation because it ensures that the landscape receives only the amount of water actually needed (or lost from the soil by evaporation and from plants by transpiration).

Water Budget Formula
= ETo x Landscape Area x Conversion Factor

Adjusted Water Budget Formula
= ETo x Crop Coefficient x Landscape Area x Conversion Factor

Where the crop coefficient is a multiplier that adjusts the reference ET value so that it more realistically represents the water use of a specific crop.
Landscape Evapotranspiration Formula

LCM- Landscape Coefficient Method (KL). The LCM utilizes the above water budget approach but substitutes a landscape coefficient for a crop coefficient. It is the equivalent of the crop coefficients used in agriculture and has been developed specifically for estimating the water requirements of landscape plantings. It accounts for differences in species, planting density and microclimate. This formula is covered in UC Cooperative Leaflet 21493 (Costello, Matheny, and Clark, 1991) and the formula is:
= ETo x Crop Coefficient x Landscape Area x Conversion Factor
Where the Crop Coefficient or adjustment factor = ks x kd x kmc

See Table 1 for the estimated values for each factor. This is an area where obviously much more research is needed. Not much is known empirically about the actual water needs of plants and how the coefficients may vary from region to region, over the course of a year, and over the lifespan of the landscape. How transferable is the California plant water use database to the southeast? Likewise, there will be considerable variation in density and microclimatic factors relative to the complexity of landscape plantings and a greater understanding of site design features and its impact on evapotranspiration will need to be studied in much greater detail.

Case Study: Using the Landscape Evapotranspiration Formula to Estimate Landscape Water Use for a Planting in Raleigh, NC.

Step 1.
Calculating the Landscape Coefficient (KL)
A well established 200ft2 bed of flowering cherry trees on the south side of a 3 story building in Raleigh that reflects light and heat throughout the afternoon and a planting of the same on the shady, cooler, north side of the building. There is no underplanting, just mulch.
KL = ks x kd x kmc
(south side) KL = 0.8 x 1.0 x 1.4 = 1.12
(north side) KL = 0.8 x 1.0 x 0.5 = 0.4

Step 2.
Calculating Landscape Evapotranspiration (ETL)
ETL = KL x ETo
(south side) ETL = 1.12 x 0.22" = 0.25"(north side) ETL = 0.4 x 0.22" = 0.08"

Step 3.
Calculating the Total Water to Apply (TWA) for one day
TWA = ETL ETL = landscape coefficient
IE IE = irrigation efficiency, assume .70, sprinkler
(south side) ETL = 1.12 x 0.22 = 0.36"
TWA = 0.25"/.70 = 0.24"
(north side) ETL = 0.4 x 0.22 = 0.08"
TWA = 0.08"/.70 = 0.11"
Converting to gallons of water needed to maintain area to be irrigated (gallons = inches x area x 0.62).
(south side) TWA = 0.25"/.70 = 0.36"
0.36" x 200ft2 x 0.62 = 44.6 gallons
(north side) TWA = 0.06"/.70 = 0.08"
0.11" x 200ft2 x 0.62 = 13.6 gallons
contrast the LCM derived water usage with conventional PET or potential evapotranspiration rates for mature shade trees (.80 in a warm, humid climate (.30 PET inches/day):
TWA for entire site:
= .623 x plant area x plant factor x P.E.T (taken from regional chart)
= .623 x 200ft2 x .80 x 0.30"
= 29.9 gallons
= 29.9/.70 = 42.7 gallons or 85 gallons for both sites
LCM:
South side= 44.6 gallons (4% more than PET)North side = 13.6 gallons (73% less than PET)
58 gallons for both sites (31% less overall than PET)

Summary
The Landscape Evapotranspiration Formula ultimately has the potential to help refine and more accurately predict landscape water use requirements. This in turn may provide the Green Industry in NC and the southeast with greater ability to influence policies on water planning and management as conservation and regulatory approaches are developed and implemented.

Species Factor (ks):Orchard values range from 0.56 to 1.12 in summer
Assume landscape trees can lose less than potential maximum and be healthy
Density Factor (kd):
60-100% canopy cover is considered average, kd =1.0
If surface evaporation is not controlled, increase kd by 10-20%
Microclimatic Factor (kmc):
An "average" microclimate is one in which site features such as buildings, structures, pavements, slopes, surface reflectivity do not influence the microclimate, kmc =1.0
In "high", site features greatly increase evaporative conditions, kmc = 1.0-1.4
In "low", site is shaded, protected from winds, north facing, kmc = 0.5-1.0

Table 2. NORMAL EVAPOTRANSPIRATION DATA FOR NORTH CAROLINA (INCHES)

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