Evaluation of a Pan Evaporation Model for Estimating Post-planting Street Tree Irrigation Requirements

"Budget restraints force many cities to rely on volunteers for street tree irrigation. Reliable, easy to follow recommendations are needed. Using a pan evaporation model created by others, we developed a method for predicting irrigation amount and frequency for street trees and tested it on mulched, 3-in (7.5 cm) caliper, balled and burlapped Pyrus calleryana Redspire (pear) and Betula nigra Heritage (birch) trees five months after planting. The model predicted that root balls should be saturated every 3 days with 10 gal (38 l) of water. When tested against control trees which were irrigated on an "as needed" basis according to root ball moisture sensors, model trees required a total 130 gal (494 l) each of water, while pear and birch controls required an average of 108 and 122 gal (410 and 464 l) each, respectively, over the experimental period of two months. However, pear and birch controls required 25 and 24 site visits respectively, whereas model trees required only 13 visits. Refitting the model assumptions with actual tree measurements and adjusting the root ball soil tension point at which root balls were to be irrigated to well above the permanent wilting point, resulted in a 5 gal (19 l), every 3 day regime."