Sacramento, Calif. (July 1, 1998)- A healthy urban forest can mitigate stormwater impacts of urban development. Trees intercept and store rainfall on leaves and branch surfaces, thereby reducing runoff volumes and delaying the onset of peak flows. Root growth and decomposition increase the capacity and rate of soils to infiltrate rainfall and reduce overland flow. Urban forest canopy cover reduces soil erosion by diminishing the impact of raindrops on barren surfaces.
This study focuses on interception of rainfall by Sacramento’s urban forest. The objectives are to 1) quantify annual rainfall interception, 2) describe relations between interception and rainfall seasonality, duration, and volume for typical storm events, and 3) identify important structural traits of urban forests that can be manipulated to increase rainfall interception.
A one-dimensional mass and energy balance model was developed to simulate rainfall interception in Sacramento County, California. The model describes tree interception processes: gross precipitation, leaf drip, stem flow, and evaporation. Kriging was used to extend existing meteorological point data over the region. Regional land use/land cover and tree canopy cover were parameterized with data obtained by remote sensing and ground sampling. Annual interception was 1.1% for the entire county and 11.1% of precipitation falling on the urban forest canopy. Summer interception at the urban forest canopy level was 36% for an urban forest stand dominated by large, broadleaf evergreens and conifers, and 18% for a stand dominated by medium-sized conifers and broadleaf deciduous trees. For 5 precipitation events with return frequencies ranging from 2 to 200 years, interception was greatest for small storms and east for large storms. Because small storms are responsible for most pollutant washout, urban forests are likely to produce greater benefits through water quality protection than through flood control.
Rainfall Interception by Sacramento’s Urban Forest
Green Solutions to Stormwater Management