Recent results in the field of rainfall modelling have proved the benefits that can be gained by combining the competence and efforts of hydrologists, meteorologists and engineers. The main objective of this Special Issue is to integrate these different points of view so as to obtain a more comprehensive understanding of present research in urban environment hydrology and to identify a few issues that should be addressed by future research in this field.
Accurate modelling of the main hydrological processes involved in the rainfall runoff transformation is a key issue in many applications of urban environment hydrology. Engineering solutions are indeed required to solve stormwater control problems in a reliable, yet cost-effective way. Modelling is needed at different levels of complexity: in integrated urban drainage modelling, adoption of detailed models leads to unacceptably long calculation times and implies large memory needs. Therefore, simplified models are appropriate to simulate very quickly long rainfall time series, thus allowing rapid assessment of the environmental impact on the receiving water bodies.
However, as matter of fact, in recent decades many research efforts have addressed improving the reliability of modelling rainfall-runoff processes in the urban environment, since extremely simplified approaches, such as the rational formula and lumped conceptual models, now appear to be out of date.
Taking advantage of the increasingly available computational capabilities, complex approaches to rainfall-runoff modelling become affordable and 'integration' and 'reliability' are the new keywords for addressing research developments. Integrated tools have been developed for linking different models concerning rainfall-runoff transformation and runoff propagation, regarding both water quantity and quality aspects, in order to analyse the system in its wider complexity. In the past, the main limitation to integration was determined by the large computational effort needed to simulate several physical phenomena contemporaneously. A typical example of integration can be found in urban flooding propagation modelling, where the underground drainage hydraulics is linked to flooding propagation at the ground surface.
Hydrological processes in the urban environment must be considered at much finer temporal and spatial scales than in natural areas. As a matter of fact, owing to the usual lack of rainfall data representative for temporal and spatial variations of the natural rainfall processes, the rainfall input is often a weak point. Rainfall is the ultimate driving force of all hydrological processes, and its accurate modelling and monitoring constitutes a key issue in many problems of urban hydrology. Especially in this context, the problem of data reliability is more pressing, since it affects the uncertainty and the 'rate of belief' connected with the model output. Many efforts have been devoted to evaluating the effects of incomplete or inconsistent input data on the reliability of models.
This Special Issue collects research and application papers dealing with the modelling of hydrological processes in an urban environment, especially in the framework of a physically based and distributed approach, both for single and/or continuous event-based modelling. A few contributions deal with the quality and