Lessivage des polluants urbains et changement climatique : l’influence des modèles

Rainfall for the next century will be influenced by global warming. These modified rainfalls will have influences on rainfall runoff. In order to analyze these impacts, this study compared different buildup and washoff functions with several future rainfall series. These series of rain were first simulated with the Bartlett-Lewis method to disrupt the distribution of rainfall events in winter and summer. Then, the delta factor method has been applied to modify the intensity of rainfall under different scenarios of emissions. Three emissions scenarios were tested (A1B, A2 and RCP3PD) and for each of these scenarios, three estimates of the results (2.5th, 50th and 97.5th percentiles) were considered depending on the chain of General Circulation Models (GCM) and Regional Climate Models (RCMs) used. It was found that concentrations of leached pollutants are particularly susceptible to washoff functions and empirical parameters included in these functions. Disruption of rainfall also influences the leached concentrations in summer and winter. The emissions scenario has very little impact on levels instead of percentiles that play an important role in their distribution. The type of washoff function and the choice of GCMs and RCMs are important elements to evaluate the influence of climate change on pollutants washed into urban areas.

Impact of climate change on runoff pollution in urban environments

David Andrew Barry, Sylvain Coutu, Sébastien Kramer

Runoff from urban environments is generally contaminated. These contaminants mostly originate from road traffic and building envelopes. Facade envelopes generate lead, zinc and even biocides, which are used for facade protection. Road traffic produces particles from tires and brakes. The transport of these pollutants to the environment is controlled by rainfall. The interval, duration and intensity of rainfall events are important as the dynamics of the pollutants are often modeled with non-linear buildup/washoff functions. Buildup occurs during dry weather when pollution accumulates, and is subsequently washed-off at the time of the following rainfall, contaminating surface runoff. Climate predictions include modified rainfall distributions, with changes in both number and intensity of events, even if the expected annual rainfall varies little. Consequently, pollutant concentrations in urban runoff driven by buildup/washoff processes will be affected by these changes in rainfall distributions. We investigated to what extent modifications in future rainfall distributions will impact the concentrations of pollutants present in urban surface runoff. The study used the example of Lausanne, Switzerland (temperate climate zone). Three emission scenarios (time horizon 2090), multiple combinations of RCM/GCM and modifications in rain event frequency were used to simulate future rainfall distributions with various characteristics. Simulated rainfall events were used as inputs for four pairs of buildup/washoff models, in order to compare future pollution concentrations in surface runoff. In this way, uncertainty in model structure was also investigated. Future concentrations were estimated to be between ±40% of today’s concentrations depending on the season and, importantly, on the choice of the RCM/GCM model. Overall, however, the dominant factor was the uncertainty inherent in buildup/washoff models, which dominated over the uncertainty in future rainfall distributions. Consequently, the choice of a proper buildup/washoff model, with calibrated site-specific coefficients, is a major factor in modeling future runoff concentrations from contaminated urban surfaces.

2012

A Generic Framework to Produce Probabilistic Rainfall Scenarios Incorporating Climate Change

Xavier Beuchat

The present work addresses the problematic of forecasting impacts of climate change on future rainfall regimes and their consequences on urban stormwater infrastructures. Researches carried out allowed to develop an integrated framework for producing high resolution probabilistic rainfall projections suitable for studying hydrological processes at the scale of urban drainage. Downscaling is at the core of the methodology as the predictions of the numerical General Circulation Models (GCMs) employed by the climate scientific community to model climate evolution are too coarse for hydrological impact studies. The proposed downscaling approach respects the scales of the physical processes characterizing precipitations and consists in three steps: i) Daily rainfall series at the location of interest are downscaled from coarse-gridded monthly GCMs projections (scale of weather events); ii) The generated daily series are further downscaled to the hourly time-step (scale of storms dynamics); iii) Finally, hourly series are disaggregated to sub-hourly level (scale of raincells). Daily downscaling is achieved by a statistical procedure, based on Generalized Linear Models (GLMs), seeking to relate large-scale atmospheric variables, corresponding to the scale of GCMs, to local daily rainfall series. The proposed methodology is assessed using three contrasted situations in Switzerland (Geneva, Sion and Säntis) and is shown to perform well in reproducing historical rainfall statistics (including extremes and inter-annual variability) in the present-day climate; furthermore, projections were shown to be consistent with the simulations of physically-based dynamical models (i.e. Regional Climate Models). Projections for the second part of the 21th century indicate considerably drier summers, but no significant tendency toward more extreme events was detected except for Säntis. Finally, extensions of the methodology were presented allowing to downscale other atmospheric variables than rainfall. Sub-daily rainfall downscaling is achieved using a stochastic hourly rainfall generator based on Poisson clusters model which aims at conceptualizing storm dynamics in a simple way. To provide sensible results such generators have to be fitted on historical rainfall statistics computed at different levels of temporal aggregation. In the present context, this raises a fundamental problem as the required fitting statistics at the sub-daily time-scale are not available for the future. Shortcomings of existing methods led us to develop a novel approach based on Multivariate Adaptive Regression Splines (MARS) which were so far seldom used in hydrology. The proposed MARS models are conditioned on climate and fit thus particularly well in the general downscaling framework. In addition, atmospheric predictors allow to account naturally for seasonal variations meaning that a single MARS model holds for the whole year, whereas existing models are specific to each month of the year and are therefore not robust against the seasonal changes that might induce global warming. The methodology was applied to generate hourly rainfall series from daily data simulated by the GLMs at Geneva for the end of the 21th century. Climate change was found to impact significantly summer storm dynamics: raincells are predicted to be shorter but more intense, and storms are projected to be less frequent. A frequency analysis made on the simulated hourly rainfall series revealed a significant increase of hourly rainfall return levels. Hourly rainfall series are further disaggregated to the 10-minute level using a cascade-based model. Using case studies in the Geneva area, the performances of this sub-hourly rainfall disagreggator (in particular the reproduction of extreme values) were shown to be equivalent when fitted on statistics derived from the temporal levels 10-minute to 1-hour (sub-hourly fitting set), or from the 1-hour to 3-hour levels (supra-hourly fitting set). In consequence, the supra-hourly statistics of the hourly rainfall series generated by the stochastic Poisson clusters model can be used to fit the disagreggator model in order to simulate 10-minute rainfall series. Projections of 10-minute rainfall at Geneva for the end of the 21th century indicate an increase of extreme events intensities. Uncertainties in the proposed downscaling procedure are dealt with using, whenever feasible, probabilistic models (i.e. GLMs, hourly rainfall generator, MARS model and sub-hourly disagreggator), and relying on a large number of General Circulation Models projections conditioned on various greenhouse gases emissions scenarios. The present work concludes with a case study illustrating how the developed downscaling methodology may be used to evaluate different strategies of sustainable stormwater management. The Industrial Zone of Plan-les-Ouates (ZIPLO) taken as example is a small urbanized area of Geneva. Urban drainage was characterized using a semi-distributed rainfall-runoff model, and climate change (under the higher greenhouse gas emissions scenario) was shown to increase significantly the peak discharge flows at the ZIPLO outlet. Different sustainable stormwater options were then evaluated in order to limit the peak discharge flows under the joint scenario of climate change and projected urbanization increase.

EPFL2011

A Generic Framework to Produce Probabilistic Rainfall Scenarios Incorporating Climate Change

Xavier Beuchat

EPFL2011

Impact of climate change on runoff pollution in urban environments

David Andrew Barry, Sylvain Coutu, Sébastien Kramer

2012