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Estimating Landscape Tree Water Requirements in the Western U.S.
Understanding How to Estimate Tree Water Requirements

By Dennis R. Pittenger, University of California Riverside


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Evapotranspiration, or ET, is the process of water moving from the soil to the atmosphere through evaporation from the soil surface, and transpiration through plant uptake. To help estimate a tree's water requirements, a standard value known as reference ET (ETo), calculated from weather data, is then adjusted by a plant factor or PF, which is a set value depending on the plant species. Photo: Leslie Nistico, Green Concepts


Editor's note: Portions of the following article address the effects of drought on tress and though El Nino-spawned rain and snow storms are having an impact on the drought in the West, scientists have cast doubts that it will be corrected by them. For instance, NOAA meteorologist Tom Di Liberto points out that based on just one measure of what the U.S. Drought Monitor team considers drought conditions, this year's precipitation (Oct. 2015 - Sept. 2016) would have to be 191 percent above normal on California's Central Coast, and 198 percent above normal on its southern coast and the San Joaquin Valley just to inch those areas above that measure's threshold of drought.

Tree water demand may be met by precipitation, irrigation, or a combination of the two depending on the local climate and season of the year. In most areas of the Western U.S., a portion of trees' summer water requirements will need to be provided by irrigation. Estimating tree water requirements can be done simply and scientifically, but first let's look at some basics of plant water.

In a landscape, water from the soil moves to the atmosphere through evaporation from the soil surface and plant uptake by transpiration. This process is called "evapotranspiration", or ET. ET is usually expressed as the inches (depth) or volume (gallons) of water used by a planted area each day, week, month or year. The physiology and structure of plants and weather conditions are the primary factors affecting ET.

To gauge the influence weather has on plant water use, a standard value known as reference ET (ETo) is calculated from weather data at numerous locations. These ETo values must be adjusted by a plant factor (PF) to estimate the water required by particular landscape plants. Research on tree water demand shows using a single PF of 0.5, or 50 percent, to adjust ETo numbers produces an accurate estimate of the amount of water that landscape trees require to provide acceptable landscape function and performance in most of the western half of the U.S. with normally low humidity and precipitation. Trees of all types have relatively good drought resistance mechanisms so this single PF applies to all established, climatically-adapted trees, including traditionally used species, so-called drought tolerant, low-water use, and native trees. Consequently, all trees require just modest amounts of water to perform acceptably in landscapes, although the common perception is trees labeled "drought tolerant" require much less water than others.

Using one PF for all tree species simplifies the process for estimating their water requirements. This may seem imprecise since many plant lists and databases provide plant-specific PF values. However, there is no scientific basis for the numbers provided in these lists and databases, including those listed in the popular Water Use Classification of Landscape Species (WUCOLS). Also, ETo only marginally represents the water demand of landscape trees, so going to great lengths to fine-tune a PF for each tree species would add little if any accuracy to the estimate. Likewise, there is no science-based reason to apply additional ETo adjustment factors such as one for variation in planting density.

Trees are often located in turf-covered street medians, lawns, and other turfgrass plantings. The water required by these trees is accounted for in the water provided to the turf so long as the turf receives enough water to provide good performance. However, turfgrass irrigation may be stopped or greatly reduced in street medians and other general lawn settings to conserve water during severe drought. If irrigation is stopped or reduced during warm months of the year, trees located within the turf will likely become water stressed within several weeks if there is no significant precipitation.


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Surface drip irrigation systems for trees should wet at least 50 percent of the soil surface over the tree's estimated existing root zone. The drip lines should be covered with 2"-4" of organic mulch to reduce weed growth and evaporation. Photo: Netafim


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When having to reduce or eliminate irrigation to a tree, many factors must be taken into account to gauge how long it can last before becoming water stressed, including the amount of water reduction, the season, the soil moisture content at the time irrigation was reduced or eliminated, the soil's water holding capacity, the extent of tree root development, depth of root zone, root system health, and the tree species' ability to resist drought.


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Trees located in turfgrass that has had the irrigation turned off will face competition from the turf for soil water until the turf goes dormant or dies. In Southern California and similar arid climates, established trees can usually survive at least several weeks without irrigation provided the soil is fully wet at the start. To prevent trees planted in turfgrass from having to compete for scant water supplies and extend the time before the trees become severely water stressed, the turf can be eliminated with a registered herbicide. Photo: Dennis R. Pittenger, University of california riverside


Water stress will develop more slowly if watering is stopped or reduced in the cooler months of the year. Depending on the species and the severity of water stress, trees running out of water will show wilting, thinning canopy, marginal browning (burn) on leaves, and/or complete loss of foliage.

How quickly trees begin to suffer and the degree to which they lose their ability to provide the desired landscape function and performance once watering is reduced or eliminated depends on the:
  • Amount of irrigation reduction that is imposed;
  • Season of the year;
  • Soil moisture content at the time irrigation was reduced or eliminated and the soil's water holding capacity;
  • Extent of tree root development, depth of root zone, root system health;
  • Tree species' ability to resist drought.
Trees located in turfgrass that has had the irrigation eliminated will face competition from the turf for available soil water until soil water becomes so limited that the turf goes dormant or dies. Turfgrass usually turns brown and goes dormant within 2 to 4 weeks after irrigation and precipitation stop in the growing season. The trees will likely begin to experience moderate to severe water stress within a few weeks after this. When the decision is made to allow turf to die from drought, immediately killing it using a registered herbicide will eliminate its competition with the trees for the remaining soil water and extend the time before the trees become severely water stressed.

The water requirements of the remaining trees, in gallons, can be estimated by multiplying ETo x 0.5 x 0.623 x sq. ft. of soil covered by tree canopy. The square feet of soil area covered by tree canopy is calculated by squaring the radius of the canopy and multiplying that product by 3.14. If the tree canopy covers at least 80% of the ground, irrigation can be applied using the existing overhead irrigation system used to maintain the turf. The overhead irrigation system can be retrofitted with a surface drip irrigation system using tubing with inline emitters. Organic mulch, 2-4 inches deep and 6-inches from tree bases, should be placed over the drip lines to reduce weed growth and evaporation.

Drip irrigation systems are often designed so that only a portion of the soil surface around plants and over their roots receives water. As a rule of thumb, the drip system should be designed to wet at least 50 percent of the soil surface over the estimated existing root zones of trees. The less of the tree root system covered by emitters the more frequently the area will need to be irrigated since only a portion of the former root system will be receiving water. A professional irrigation designer or irrigation consultant can be helpful in designing a retrofit irrigation system tailored to a specific landscape situation.

Details on applying this scientific approach to estimate tree water requirements, as well as additional science-based information on ET and landscape plant water requirements, can be found at http://ucanr.edu/cluh.






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