Urban tree rainfall interception measurements and modeling in WinSLAMM

Recently, the role of urban trees in stormwater management has received increasing interest. The interception of rainfall by urban trees has been proposed to provide substantial benefits by reducing runoff rates and quantities. However, few data are available for rainfall interception of trees in typical urban settings, in contrast to research from natural forests having dense standings of trees. Lacking data includes how interception changes for different seasonal changes in urban tree canopies for different types of trees, how these interception values vary for different rains (and during rains), and the fate of rainfall under trees that is not intercepted.

There is a possibility for double counting some of these benefits. For example, calibrated stormwater models rely on monitored outfall flow measurements of existing areas. These areas have varying amounts of trees through their landscapes. Adding additional interception to these calibrated models can result in improper estimates of runoff. However, if new trees are planted in an area, interception benefits may increase.

This paper describes a series of direct interception measurements under urban trees to quantify some of these hydrologic benefits for inclusion in WinSLAMM. This study includes a standard rain gage located in an open area and rain gages under deciduous oak and evergreen pine trees. About 100 rain events monitored from early December 2018 through January 2020 have been statistically evaluated. These results have been used to add urban tree interception benefits to WinSLAMM for appropriate conditions (tree overstory above directly connected paved areas). Only direct interception is considered, as trunk flow is assumed to infiltrate near the base of an urban tree in the surrounding landscaped or tree planter box area. These tests are part of the Ph.D. research being conducted by Ryan Bean at the University of Alabama which will incorporate similar interception measurements from other locations.

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