Don’t Overdo It
By Edward C. Martin and Kai Umeda
People often talk about balance—a balanced diet, balance between work and home, and balancing checkbooks for some of us Baby Boomers, who do things “old school.” Similarly, when irrigating landscapes—especially during times of drought like some people are experiencing in the West—there is talk about balancing water applications. Applying the right amount of water to keep plants alive but not too much to cause drainage or excessive water loss is a challenge all irrigators face. For example, bermudagrass in Arizona thrives in the summer at high temperatures well above 100 F on almost a daily basis. To make turf healthy, safe to use, and aesthetically pleasing, it must be irrigated regularly and adequately. But how much is enough, and how much is too much? The Arizona Meteorological Network, consisting of over 20 weather stations, collects daily data for temperature, wind speed, solar radiation, and other climate parameters to enable calculating turf water use. Visit http://ag.arizona.edu/azmet/az-data.htm and click the tab for turf to see a link for turfgrass water use in Phoenix, Tucson, and northern Arizona. Check the Web to see if a similar service is offered in other states. Turfgrass, however, is just one type of plant in park landscapes, so what about trees, shrubs, flower gardens, etc.? How does one ensure that various types of plants are adequately watered?
One approach is to categorize different plants into hydrozones by water-use requirements and frequency of water applications. For example, most trees require less frequent and deep irrigation to thoroughly wet the soil around the deeper extensive roots. Shrubs, on the other hand, may require more frequent irrigation and usually less water per irrigation. Meanwhile, flowers and bedding plants require more frequent and shallow irrigation on nearly a daily basis for a comparatively short duration. This method has been used for years in home landscaping (Figure 1) but is more difficult to apply when dealing with larger areas found in parks. However, the concept is the same: The logistics of adapting a current irrigation system to address the zones may be the greatest challenge.
Reality Versus Theory
Splitting landscapes into hydrozones isn’t as easy as it seems, especially for existing landscapes. The balance here is cost. The technology exists to control multiple hydrozones with only one irrigation controller (up to 48 valves or more). Also, the wide variety of emitters from drippers to bubblers to microsprays—some pressure-compensating, others with adjustable flow—can be used on almost any type of plant. The challenge is finding the money to run pipes and controller wires under sidewalks and around playgrounds, connecting thousands of feet of poly tubing, and installing multiple-valve boxes. It doesn’t take too long before more money is being spent to save water than the cost of paying for the water. Many of these systems pay for themselves over the years, but the initial layout of cash is often hard to find.
Use Available Resources
But suppose there is funding available to purchase and install such a system. How much is needed? How much is enough? These questions require even more balancing—the plant with the soil, that is. First, one must meet plant water requirements. In Arizona, there are several sources that provide information about the plants’ water requirements. One source is the Landscape Watering by the Numbers (http://wateruseitwisely.com/100-ways-to-conserve/landscape-watering-guide/), a free online publication by the Water--Use it Wisely campaign. Guidelines on the amount of water used daily by various landscape plants and recommendations are given for frequency of irrigation. The guide describes how to adjust irrigation for the seasons, how to develop an irrigation schedule using a controller, and even how to deal with maintenance. In California, many people use WUCOLS (Water Use Classification of Landscape Species). Version IV was released in 2014. Other references provide a gallons-per-day water use for trees, shrubs, or ground cover. Many of these numbers are reported in terms of canopy diameter—the bigger the tree or shrub and the bigger the canopy, the more water required. Much of that is based on empirical data—not necessarily measured but observed. On a recent visit to the horticulture department at Walt Disney World in Florida, the question was asked how the attraction schedules irrigation and what is used to determine plant water use. The reply was that, over the years, schedules were developed through trial and error, and now there is a system that works.
Soil Water Capacity
If one knows that a tree or shrub requires a certain number of gallons of water per day, what about the soil? Soil can hold a certain amount of water; using too much will either drain out the bottom of the rootzone (water loss), run off the landscape surface into the street or sidewalk (water loss), or simply pond to expose the water to evaporation (water loss), and in some cases, cause injury by waterlogging plants. So how much water can soil hold?
Soil water-holding capacity is based on texture. Sand has large soil particles and holds very little water. If sandy soil is over-irrigated, the water will drain out the bottom. Sand holds about 10 percent water, which means for every foot (12 inches) in depth, about 1.2 inches of water is held. In general, as the clay content of soil increases, so does the water-holding capacity. Figure 2 shows a generalized graph of water content for different soil types. The top line is the water-holding capacity; the bottom line is the permanent wilting point (PWP). The PWP marks the point at which most plants can no longer obtain water from the soil because it is being held too tightly by the soil particles. It’s approximately 50 percent of the total water-holding capacity of the soil. So the sand that holds 1.2 inches of water only has about 0.6 inches available for the plants to use. Luckily, plant roots grow into the soil, and as they grow deeper, more water is available for uptake. Figure 3 shows available water over depth. The sand in Figure 3 shows the result if the sand is all of the way down; there is no change in soil texture whatsoever. The sandy loam is probably more realistic in that soil water changes with depth as most soils change texture with depth. The gallons per foot listed in the figure are based on a 6-foot diameter rootzone. Many of the references found will have water-use data based on canopy size. Sandy soil should be irrigated less (the soil holds less) but should be irrigated more often. In soils with more clay, it should be irrigated more but less frequently. Check out a short 3-minute video about water-holding capacity of sand, loam, and clay soils at https://www.youtube.com/watch?v=Ond_-SsiWE8.
It is difficult to put a price on water other than the water bill. But normally, that cost is only for the operation and maintenance of the system to deliver the water. The West has been struggling with this for many years. The goal of any irrigation-management scheme should be to conserve water—period.
Edward C. Martin, Ph.D., is the County Extension Director for the Maricopa County Cooperative Extension in Phoenix, Ariz. Reach him at (602) 827-8200 or firstname.lastname@example.org.
Kai Umeda is the Area Agent for the Maricopa County Cooperative Extension in Phoenix, Ariz.