4 Ways Trees Help When It Rains

Our urban forest is working hard for San Francisco when it rains! Trees reduce the amount of rainwater that ends up flooding our streets and filling our combined sewer system during storms. Trees do this in a number of ways:

#1 Street tree basins (and sidewalk gardens) reduce impervious surface cover.

Even before a tree or garden is planted, the mere removal of concrete from the sidewalk gives rainwater a chance to seep into the soil before it enters the storm drain.

#2 Trees intercept water with their leaves.

Leafy tree canopies, especially large evergreen trees, intercept a significant portion of rainwater! Much of this water will evaporate back into the atmosphere, never touching soil or pavement.

#3 Trees uptake water into their roots & increase the infiltration of water into the soil.

When the remaining rainwater reaches ground level, trees will increase the amount of water that sinks into the soil. This is especially true in compacted urban soils where tree roots can more than double the amount of water that infiltrates the soil. Some of this water will be wicked up by the tree roots and some will recharge the groundwater.

#4 Trees reduce the overall volume and rate of water runoff.

Inevitably, some rainwater is still going to end up as stormwater runoff and enter the storm drain, but at this point, the stormwater has been reduced and slowed so that it doesn’t overwhelm the combined sewer system and lead to unhealthy sewage discharges into the Pacific Ocean and the San Francisco Bay.