Abstract
Urban hydrology has so far lacked a suitable model for a precise long-term determination of evapotranspiration (ET) addressing shading and vegetation-specific dynamics. The proposed model “SWMM-UrbanEVA” is fully integrated into US EPA’s Stormwater Management Model (SWMM) and consists of two submodules. Submodule 1, “Shading”, considers the reduction in potential ET due to shading effects. Local variabilities of shading impacts can be addressed for both pervious and impervious catchments. Submodule 2, “Evapotranspiration”, allows the spatio-temporal differentiated ET simulation of vegetation and maps dependencies on vegetation, soil, and moisture conditions which are necessary for realistically modeling vegetation’s water balance. The model is tested for parameter sensitivities, validity, and plausibility of model behaviour and shows good model performance for both submodules. Depending on location and vegetation, remarkable improvements in total volume errors Vol (from Vol = 0.59 to −0.04% for coniferous) and modeling long-term dynamics, measured by the Nash–Sutcliffe model efficiency (NSE) (from NSE = 0.47 to 0.87 for coniferous) can be observed. The most sensitive model inputs to total ET are the shading factor KS and the crop factor KC. Both must be derived very carefully to minimize volume errors. Another focus must be set on the soil parameters since they define the soil volume available for ET. Process-oriented differentiation between ET fluxes interception evaporation, transpiration, and soil evaporation, using the leaf area index, behaves realistically but shows a lack in volume errors. Further investigations on process dynamics, validation, and parametrization are recommended.
Highlights
The megatrends of urbanization, global warming, and demographic change increasingly confront cities with new challenges
In order to evaluate validity of Submodule 1 (SM1), first ET0,loc and ET0,Ks are compared on a daily basis for 2017
For SM1, the equivalence of the calculation approaches for ET0,loc and ET0,Ks can be concluded for both locations on daily values which confirms very good model performance for the period under consideration (Section 3.1)
Summary
The megatrends of urbanization, global warming, and demographic change increasingly confront cities with new challenges. The water and energy balance is affected by major changes: A reduction in groundwater recharge and ET can be observed, while runoff volume and runoff peaks are increasing and occur strongly accelerated [1]. Due to the direct relationship between water and energy balance, the lack of cooling ET leads to higher temperatures in urban areas (urban heat island, [3]). Sustainable adaptation strategies such as decentralized blue-green infrastructures (BGI, [4]) should address these problems. Numerous studies have already been carried out on water quality and quantity issues (e.g., [5,6,7]) and cooling effects (e.g., [8,9,10])
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