On the irrigation requirements of cottonwood (Populus fremontii and Populus deltoides var. wislizenii) and willow (Salix gooddingii) grown in a desert environment

Native tree plots have been established in river irrigation districts in the western U.S. to provide habitat for threatened and endangered birds. Information is needed on the effective irrigation requirements of the target species. Cottonwood (Populus spp.) and willow (Salix gooddingii) trees were grown for seven years in an outdoor plot in a desert environment in Tucson, Arizona. Plants were allowed to achieve a nearly complete canopy cover over the first four years, then were subjected to three daily summer irrigation schedules of 6.20 mm d^-1; 8.26 mm d^-1 and 15.7 mm d^-1. The lowest irrigation rate was sufficient to maintain growth and high leaf area index for cottonwoods over three years, while willows suffered considerable die-back on this rate in years six and seven. These irrigation rates were applied April 15-September 15, but only 0.88 mm d^-1 was applied during the dormant period of the year. Expressed as a fraction of reference crop evapotranspiration (ET_o), recommended annual water applications plus precipitation (and including some deep drainage) were 0.83 ET_o for cottonwood and 1.01 ET_o for willow. Current practices tend to over-irrigate restoration plots, and this study can provide guidelines for more efficient water use.