Irrigation scheduling using evapotranspiration (ET)
Irrigation can be scheduled by monitoring ET. Evapotranspiration is the combination of evaporation of water from soil surfaces and transpiration from plants, commonly expressed in inches per day. Monitoring soil moisture and observing plants for symptoms of water stress are other methods of deciding when to irrigate to avoid plant damage from a deficient or excess of soil moisture.
Reference ET (ETo) provides a standard estimate of a plant’s water demand based on climatic conditions over a given period of time, such as a day or month. It is the amount of water used by well-watered, cool-season turfgrass, which typically requires more water than trees and shrubs. After they are established, most woody plants perform well in landscapes by irrigating them with about 50 to 60% of ETo.
A standard recommendation for landscapes is to irrigate established, woody plants when evapotranspiration monitoring indicates that about 50% of available water in soil has been used. To irrigate based on ET, learn what type of soil is present; water availability ranges from about 1/2 to 2-1/2 inches per foot of depth depending on the soil type.
For example, assume most tree and shrub roots are in the upper 2 feet of a loam soil. At field capacity, after irrigation and initial drainage, uncompacted loam contains about 1-1/2 inches of available water per foot or a total of 3 inches of available water in the upper 2 feet. Thus, irrigation would occur when ET accumulates to 1-1/2 inch (50% of available water).
Water budget. If a typical summer ETo is 1/5 inch per day, many woody plants would need 1/10 inch of water per day and use up the 1-1/2 inches of available water in 15 days:
1.5 inches of water ÷ 0.1 inches of water/day = 15 days
Therefore, 1-1/2 inches of water should be applied after 15 days. This method of irrigation scheduling is called a water budget. Irrigation and rainfall represent deposits and daily ET rates represent withdrawals.
Controllers and sources of ET information. Some irrigation controllers can access historical ETo or current ET gathered on-site or from the Internet and use these data to schedule irrigation automatically and adjust for seasonal changes. Historical and current (real-time) reference ET for many sites are available from the California Irrigation Management Information System (CIMIS) and the University of California Statewide Integrated Pest Management Program (UC IPM). Some irrigation controllers use private, commercial sources of ETo data.
Evaporation outdoors can be monitored on-site by regularly measuring water loss from a shallow pan or an atmometer (a porous, water-filled tube from which water evaporates at a rate similar to that from soil around plants). Automated evaporation pans and atmometers use a sensor to monitor water level and send that information to a data logger or automated, irrigation-control system.
For more information, consult ETo Zones Map on the CIMIS website and "Water Management" in the California Master Gardener Handbook. See also Conserve Water in Landscapes, Estimating Irrigation Needs, Irrigation Methods, Irrigation of Trees and Shrubs, Soil Properties and Water Availability to Roots, and Water Deficit and Excess. Irrigating Fruit and Shade Trees and Shrubs provides an index to more resources.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).