Urban watershed modeling in Seattle, Washington using VELMA: a spatially explicit ecohydrological watershed model

Abstract

Urban watersheds are notoriously difficult to model due to their complex, small-scale combinations of landscape and land use characteristics including impervious surfaces that ultimately affect the hydrologic system. We utilized EPA’s Visualizing Ecosystem Land Management Assessments (VELMA) model, which is a spatially explicit (i.e., gridded) ecohydrological watershed model, to simulate watershed-scale hydrologic discharge and nutrient concentrations for several urban stream systems in Seattle, Washington, including Thornton Creek, Piper’s Creek, Longfellow Creek, and Taylor Creek. A 1-meter land use classification is used to distinguish four cover types, including roads, buildings, trees, and grass. After model calibration and validation, we construct scenarios of hypothetical green roof implementations and simulate their impacts on watershed-scale discharge. Results show that VELMA is capable of simulating the impacts of targeted green infrastructure management practices to reduce peak stream flow events. These results suggest that VELMA can facilitate the prioritization of urban water infrastructure to improve water quality in urban streams leading to Puget Sound