Improving Evapotranspiration Mechanisms in the US Environmental Protection Agency's Storm Water Management Model

Most stormwater models oversimplify evapotranspiration (ET) processes, which restricts their applications in water budget analyses, ecosystem services assessments, and coupled modeling studies such as with urban climate models to investigate benefits of green infrastructure (GI) implementation. To address this need, the Penman-Monteith scheme was incorporated into the United States Environmental Protection Agency's storm water management model (SWMM) to improve the latter's ET routine. The modified SWMM allows heterogeneous and subdaily potential ET (PET) inputs, and includes the effects of water stress on actual ET (AET). The updated ET routine was validated for GI, including bioretention (R2=0.57) and green roof (R2=0.78), by comparing with direct measurements. A case study showed the upgraded SWMM model generates improved daily ET estimates and more accurate temporal patterns than previous SWMM versions. The results also suggest that water cycle fluxes could be miscalculated if a homogeneous potential ET is used in SWMM and other stormwater models. For example, the annual percolation and AET amounts could be overestimated and underestimated by 406% and 9% respectively for landscapes, and the annual AET and runoff amounts could be overestimated and underestimated by 12-19% and 14-19% respectively for GI, if the mismatched PET time series is used.