NUTS & BOLTS
Reference Evapotranspiration-based Irrigation
Reference Evapotranspiration-based Irrigation
Evapotranspiration or ET is a measure of all the water lost to the atmosphere from the combined processes of evaporation from the soil and plant surfaces and transpiration from plants.
Reference evapotranspiration (ETo) is simply the ET of a reference plant, in California this is a standardized planted surface of cool-season turf. We use CIMIS weather stations located at UC Davis and the South Coast Research & Extension Center to provide real-time local ETo as the basis of our weather-based irrigation protocols.
Field Layout
Plants and treatments are organized in a randomized block pattern. Researchers evaluate 8 individuals, or replicates, in each of 3 treatments evaluating a total of 24 plants per species. Evaluating this relatively large quantity of plants is unusual for most plant trials, which provide mostly anecdotal information from their sites. The method we use yields a robust data set for statistical analysis allowing us to confidently place plants in water use categories.
Plants are grown in beds covered with chipped wood mulch in full sun or under 50% shade cloth. Spacing of plants in and between rows is 2 meters (6' 7") apart on center in the main fields, and 3 meters (9' 10") apart in the large-plant field.
Irrigation Methods
Plants are evaluated on three irrigation treatments corresponding to the High, Moderate, and Low categories of water need as described in the Water Use Classification of Landscape Species (WUCOLS). At each irrigation, we apply enough water to refill 50% of plant available water in our targeted root zone volume around each plant that is defined as a 1 m-wide, 0.5 m-deep cylinder. Using CIMIS data, we can adjust our irrigation schedule in response to weather, irrigating when 50% of plant available water has been removed via evapotranspiration. As a result, the amount of water applied at each irrigation stays constant, while the frequency of irrigation varies between treatments. As a result, the High treatment is irrigated more frequently than the Moderate, which is irrigate more frequently than the Low treatment.
Data Collection
Researchers collect growth measurements and quality ratings to assess the effects of the different irrigation treatments on plant growth, health, and appearance. Plants are measured from spring to fall during the deficit season.