The influence of rainfall time resolution for urban water quality modelling Gabriele Freni, Giorgio Mannina and Gaspare Viviani ABSTRACT Gabriele Freni (corresponding author) Giorgio Mannina Gaspare Viviani Dipartimento di Ingegneria Idraulica ed Applicazioni Ambientali, Universita ` di Palermo, Viale delle Scienze, 90128 Palermo, Italy E-mail: freni@idra.unipa.it; mannina@idra.unipa.it; gviv@idra.unipa.it The objective of this paper is the definition of a methodology to evaluate the impact of the temporal resolution of rainfall measurements in urban drainage modelling applications. More specifically the effect of the temporal resolution on urban water quality modelling is detected analysing the uncertainty of the response of rainfall– runoff modelling. Analyses have been carried out using historical rainfall –discharge data collected for the Fossolo catchment (Bologna, Italy). According to the methodology, the historical rainfall data are taken as a reference, and resampled data have been obtained through a rescaling procedure with variable temporal windows. The shape comparison between ‘true’ and rescaled rainfall data has been carried out using a non-dimensional accuracy index. Monte Carlo simulations have been carried out applying a parsimonious urban water quality model, using the recorded data and the resampled events. The results of the simulations were used to derive the cumulative probabilities of quantity and quality model outputs (peak discharges, flow volume, peak concentrations and pollutant mass) conditioned on the observation according to the GLUE (Generalized Likelihood Uncertainty Estimation) methodology. The results showed that when coarser rainfall information is available, the model calibration process is still efficient even if modelling uncertainty progressively increases especially with regards to water quality aspects. Key words | GLUE, rainfall temporal resolution, uncertainty assessment, urban stormwater quality modelling INTRODUCTION In urban drainage modelling, rainfall temporal variability can be considered as one of the most critical knowledge elements when dealing with rainfall – runoff model input data. The temporal resolution of rainfall data, usually available for practical applications, is often lower than the one requested for the rainfall-runoff simulation in urban areas, thus compromising model accuracy (Aronica et al. 2005). The impact is more relevant on water quality models because the uncertain measurement of rainfall intensity affects both the rainfall-runoff transformation and pollution wash-off modelling thus compounding the level of uncertainty. Literature studies show that, in urban catchments, where concentration times are often short, the shape, timing and peak of hydrographs are significantly influ- enced by the time resolution of the rainfall: it has been proved that a too coarse temporal resolution of rainfall data causes a systematic underestimation of peak runoff (Gujer & Krejci 1998). The recommended time resolution of rain data should be such that the rising limb of the resulting runoff hydrograph is divided into three or more time steps (Schilling 1991). Similar considerations can be made for the estimation of the pollutograph derived from wet weather runoff. The temporal resolution of the doi: 10.2166/wst.2010.162 2381 Q IWA Publishing 2010 Water Science & Technology—WST | 61.9 | 2010