For the third installment of our Summertime Watering Series (click on these links to check our parts one and two), we’re going beyond tree care tips and digging deeper to look into the role of urban trees in the water cycle. We understand that some residents feel a little guilty about watering their trees when Central Texas is in a drought, and we’re here to remind you that it’s okay (and actually critically important) to do so! There are several good reasons that our City watering restrictions always allow for efficient watering methods for our urban trees.
Trees help regulate our hottest and driest days by providing shade. We all know shade feels good, but did you know that in the shade you may experience temperatures 15 degrees cooler than in a nearby sunny spot? Trees also shade some of the plentiful concrete found in our urban areas. Unshaded concrete bakes in the sun all day, storing and releasing the absorbed heat all night. This phenomenon is called the urban heat island effect, and it is the reason urban centers can have daytime temperatures 5 degrees higher than more rural surroundings. The heat island effect is even more striking at night, with urban centers having temperatures up to to 22 degrees higher than nearby areas.
When rain does fall, it’s typically only a matter of hours before the sun peeks back out and gets straight to work evaporating all the precious water collected in our puddles and creeks. Trees slow this evaporative process by shading the ground and our waterways, and also by absorbing their share of rainfall, estimated for a mature tree to be anywhere from 30 to 50 gallons a day. This water is absorbed by trees’ roots, and then is slowly released back into the atmosphere through a process called transpiration. Dry climates like Central Texas benefit from transpiration, which cools the air and holds vital moisture close to the earth’s surface, creating a more hospitable environment for humans and many other species.
For more information about the effect of trees on the water cycle, check out this useful USGS resource here.