LB 41-348 - Plant species generate a potentially large impact on green roof runoff moderated by seasonality

Green roofs are an important tool to decrease and delay peak storm flows, however, the impact of different plant communities on stormwater retention is not well understood, despite researchers identifying plant selection as a determining factor for the degree to which green roof benefits are achieved. This work aims to quantify the relationship between rainfall and temperature seasonality, plant community, and runoff in the green roof setting. With previous work suggesting differences of 30-55% reduction in green roof runoff across various plant communities, systematically exploring the cause of these differences could yield increased stormwater retention through modifications to green roof plant communities.Here, the Photo3 model, a soil-plant-atmosphere continuum model estimating net carbon assimilation and transpiration, is adjusted to simulate existing experimental green roof trays by changing parameters for soil depth, soil porosity, soil water holding capacity, and plant type. The Photo3 model is based on the Farquhar model for carbon assimilation with modifications for the C4 and Crassulacean Acid Metabolism (CAM) photosynthetic pathways- important for accurately modeling hydraulic fluxes for different photosynthetic strategies on a daily timescale. The model is parameterized for Bouteloua gracilis (C4) and Sedum album (CAM), which have a difference in water use efficiency of one-to-five times, generating potentially large differences in runoff volume.To compare the impacts of temperature and rainfall seasonality on greenroof runoff rates between these two photosynthetic strategies, we model three scenarios: 1) constant rainfall and constant daily temperature; 2) seasonal rainfall and constant daily temperature; 3) seasonal rainfall and seasonal temperature. Annual and seasonal temperature and precipitation from Portland, Oregon are used as the base case for modeling. Model results suggest that C4 species decrease runoff compared to CAM species by 55% in the absence of seasonality (1), 59% under seasonal precipitation (2), and up to only 41% under seasonal precipitation and temperature (3).Understanding the relationship between seasonality, photosynthetic pathway, and runoff rates contributes to functional implementation of green roofs, and better storm-water management by maximizing runoff reduction based on climate and plant community. Future work will compare model results from this work to experimental greenroof runoff and soil moisture data from existing experimental greenroof trays. This will allow calibration of species-specific photosynthetic and hydraulic parameters to improve Photo3 accuracy for runoff prediction on green roofs based on existing and future seasonal climate patterns.