Weather radar for urban hydrological applications: lessons learnt and research needs identified from 4 pilot catchments in North-West Europe

International audienceThis study investigates the impact of rainfall estimates of different spatial resolutions on the hydraulic outputs of the models of four of the EU RainGain project’s pilot locations (the Cranbrook catchment (UK), the Herent catchment (Belgium), the Morée-Sausset catchment (France) and the Kralingen District (The Netherlands)). Two storm events, one convective and one stratiform, measured by a polarimetric X-band radar located in Cabauw (The Netherlands) were selected for analysis. The original radar estimates, at 100 m and 1 min resolutions, were aggregated to a spatial resolution of 1000 m. These estimates were then applied to the high-resolution semi-distributed hydraulic models of the four urban catchments, all of which have similar size (between 5 and 8 km2), but different morphological, hydrological and hydraulic characteristics. When doing so, methodologies for standardising rainfall inputs and making results comparable were implemented. The response of the different catchments to rainfall inputs of varying spatial resolution is analysed in the light of model configuration, catchment and storm characteristics. Rather surprisingly, the results show that for the two events under consideration the spatial resolution (i.e. 100 m vs 1000 m) of rainfall inputs does not have a significant influence on the outputs of urban drainage models. The present study will soon be extended to more storms as well as model structures and resolutions, with the final aim of identifying critical spatial-temporal resolutions for urban catchment modelling in relation to catchment and storm event characteristics

Multi-storm, multi-catchment investigation of rainfall spatial resolution requirements for urban hydrological applications

Rainfall estimates of the highest possible resolution are required for urban hydrological applications, given the small size and fast response which characterise urban catchments. While significant progress has been made over the last few decades in high resolution measurement of rainfall at urban scales and in the modelling of urban runoff processes, a number of questions as to the actual resolution requirements for input data and models remain to be answered. With the aim of answering some of these questions, this work investigates the impact of rainfall estimates of different spatial resolutions and structures on the hydraulic outputs of models of several urban catchments with different characteristics. For this purpose multiple storm events, including convective and stratiform ones, measured by a polarimetric X-band radar located in Cabauw (NL) were selected for analysis. The original radar estimates, at 100 m and 1 min resolutions, were aggregated to coarser spatial resolutions of up to 1000 m. These estimates were then applied to the high-resolution semi distributed hydraulic models of four urban catchments of similar size (approx. 7 km2), but different morphological and land use characteristics; these are: the Herent catchment (Belgium), the Cranbrook catchment (UK), the Morée Sausset catchment (France) and the Kralingen District of Rotterdam (The Netherlands). When doing so, methodologies for standardising rainfall inputs and making results comparable were implemented. Moreover, the results were analysed considering different points at each catchment, while also taking into account the particular storm and catchment characteristics. The results obtained for the storms used in this study show that flat and less compact catchments (e.g. polder areas) may be more sensitive to the spatial resolution of rainfall estimates, as compared to catchments with higher slopes and compactness, which in general show little sensitivity to changes in spatial resolution. While this study provides interesting insights, further investigation is still required in order to obtain a more complete answer regarding rainfall resolution requirements for urban hydrological applications. Future work should include testing on higher resolution fully distributed hydro models, as well as the analysis of many more storm events

Sensitivity of urban drainage models to the spatial-temporal resolution of rainfall inputs: A multi-storm, multi-catchment investigation

Urban hydrological applications require high resolution precipitation and catchment information in order to well represent the spatial variability, fast runoff processes and short response times of urban catchments (Berne et al., 2004). Although fast progress has been made over the last few decades in high resolution measurement of rainfall at urban scales, including increasing use of weather radars, recent studies suggest that the resolution of the currently available rainfall estimates (typically 1 x 1 km2 in space and 5 min in time) may still be too coarse to meet the stringent requirements of urban hydrology (Gires et al., 2012). What is more, current evidence is still insufficient to provide a concrete answer regarding the added value of higher resolution rainfall estimates and actual rainfall input resolution requirements for urban hydrological applications. With the aim of providing further evidence in this regard, a collaborative study was conducted which investigated the impact of rainfall input resolutions on the outputs of the operational urban drainage models of four urban catchments in the UK and Belgium (Figure 1)

RainGain – Radar for high resolution urban rainfall estimation and flood prediction

Hydrological analysis of urban catchments requires high resolution rainfall and catchment information because of the small size of these catchments, their fast runoff processes and related short response times. Over the last three decades, analysis of the performance of urban drainage systems has been done mainly through hydrodynamic model simulations. Rainfall input into these models has often been restricted to a single or a few rain gauge(s) in or near the catchment, rendering rainfall input into one of the main sources of uncertainty in model calculations. In recent years, rainfall data from weather radars that provide space-time estimates of rainfall is becoming increasingly available. Still, the scale of available radar rainfall information, with pixels of 1 to 2.5 km2, does no not meet the relevant scales of urban hydrology (e.g. Berne et al. 2004; Emmanuel et al., 2011). In addition, studies comparing rainfall data from radar and rain gauges show a deviation of 10 to even 50%, with larger differences for increasing temporal and spatial resolutions (e.g. Overeem and Holleman, 2010). A new type of high resolution (X-band) weather radars promises to provide more accurate rainfall estimates at the spatial and temporal scales that are required for urban hydrological analysis (Willems et al., 2012). Recently, the RAINGAIN project was started to analyse the applicability of this new type of radars in the context of urban hydrological modelling. In this project, rainfall data from C-band and X-band radars and a network of rain gauges will be analysed in four highly urbanised catchments: Leuven (Belgium), two boroughs of London (UK), two catchments in Paris (France) and two polder catchments in Rotterdam (the Netherlands). High resolution rainfall data will be used as input into high resolution urban hydrological and hydrodynamic models to simulate and predict urban flood flooding using hybrid 1D-2D approaches (Simões et al., 2010). Details of the radar equipments, characteristics of the four urban catchments and hydrological and hydrodynamic simulation models will be provided; results of the project stage and of a specialist workshop on radar rainfall estimation will be reported.status: publishe

The need for high resolution precipitation data to improve urban drainage modelling

In this study high resolution precipitation data are used, derived from polarimetric X-band radar at 100 m, 1 min resolution. The data are used to study the impact of different space-time resolutions of rainfall input on urban hydrodynamic modelling response for 9 storms, in 7 urban catchments. The results show that hydrodynamic response behaviour was highly sensitive to variations in rainfall space-time resolution, more strongly so for changes in temporal than in spatial resolution. Under- and overestimations of flow peaks amounted to up to 100% with respect to the original 100 m, 1 minute rainfall input.status: publishe

Spatial-temporal rainfall input resolution requirements for urban drainage modelling: a multi-storm, multi-catchment investigation

International audienc