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Modeling Ephemeral Gully Erosion from Unpaved Urban Roads: Equifinality and Implications for Scenario Analysis

Ronald L. Bingner, Napoleon Gudino-Elizondo, Trent W. Biggs, Yongping Yuan, Eddy J. Langendoen, Kristine T. Taniguchi, Thomas Kretzschmar, Encarnacion V. Taguas, Douglas Liden
Geosciences 2018 v.8 no.4 pp. 137-154
AGNPS model, gully erosion, infiltration rate, model uncertainty, roads, sediment yield, shear stress, soil mechanical properties, statistical analysis, stormwater, urban areas, watersheds, Mexico
Modeling gully erosion in urban areas is challenging due to difficulties with equifinality and parameter identification, which complicates quantification of management impacts on runoff and sediment production. We calibrated a model (AnnAGNPS) of an ephemeral gully network that formed on unpaved roads following a storm event in an urban watershed (0.2 km2) in Tijuana, Mexico. Latin hypercube sampling was used to create 500 parameter ensembles. Modelled sediment load was most sensitive to the SCS curve number, tillage depth (TD), and critical shear stress (tc). Twenty-one parameter ensembles gave acceptable error (behavioral models), though changes in parameters governing runoff generation (SCS curve number, Manning’s n) were compensated by changes in parameters describing soil properties (TD, tc), resulting in uncertainty in the optimal parameter values. The behavioral models were used to evaluate uncertainty under management scenarios. Paving the roads increased runoff by 146-227%, increased peak discharge by 178-575%, and decreased sediment load by 90-94% depending on the ensemble. Additional field data on the infiltration rates of unpaved roads will be critical to reduce uncertainty in management impacts on runoff. The method can be used in other watersheds to simulate runoff and gully erosion, to quantify the uncertainty of model-estimated impacts of management activities on runoff and erosion, and to suggest critical field measurements to reduce uncertainties in complex urban environments.