EHSMu: a new conceptual model for hourly discharge simulation under ecohydrological framework in urban area

A parsimonious conceptual lumped model is presented here with the aim of simulating hourly discharge in urbanareas. The EHSMu (EcoHydrological Streamflow Model for urban areas) is able to reproduce the discharge at theoutlet of an urban drainage system and, at the same time, soil moisture dynamics and evapotranspirative fluxesover vegetated areas within an urban catchment.In urban areas, rain falling over impervious surfaces is directly transferred towards the drainage system in a timedepending on the catchment characteristics, and drainage network geometry. If the rain falls over pervious and vegetatedareas the runoff generation is driven by soil moisture content, which in turn is linked to evapotranspirationand leakage. While on one side soil water content determines if rainfall produces saturation excess or a leakageloss, on the other side it constrains the evapotranspirative fluxes, so that, when it approaches to saturation, theactual evapotranspiration tends to the potential one.The hydrological scheme of the urban catchment follows these premises and consists of three interconnected elements:a soil bucket and two linear reservoirs. The soil bucket epitomizes in two distinct classes different conditionswithin a catchment: the first interprets impervious areas while the second describes pervious and vegetated soils.The soil bucket is linked to the two linear reservoirs: one is responsible for the runoff within the drainage system,while the other is used to delay the entry of subsurface runoff component into the drainage system.The surface reservoir is fed by the rain falling on imperviuos areas, by the saturation excess generated over perviousareas and by the delayed contribution arising from the subsurface reservoir, which is solely supplied byleakage pulses. Soil moisture dynamics in the pervious part of the basin, are simulated by a simple bucket modelfeed by rainfall and depleted by evapotranspiration. The latter component is calculated as a linear function of soilmoisture.The model has been calibrated using Montecarlo simulations on an urban catchment in the United States. Thismethod allows to adapt the conceptual model framework to the catchment characteristics and at the same time toobtain the set of parameters with the higher efficiency in reproducing historical discharge at the outlet.The proposed model gives reliable estimate of runoff, soil moisture traces and evapotranspiration fluxes. Modeloutputs could be very useful for urban ecohydrology, because they allow for the simulation of vegetation waterstress and consequently the design of sustainable urban green spaces. At the same time the model structure allowsto simulate the effects of stormwater management best practices for achieving the hydraulic invariance.