Hydrological Consequences of Climate Change in Urban Environments

Abstract

The possible intensification of the hydrological cycle due to climate change and its effects on hydrology should be taken into account in order to prevent future problems in urban drainage systems. The intensity-duration-frequency (IDF) curves, an important tool used in the design and construction of different hydrological structures for water management, could be altered by a supposed increase in heavy rainfall due to climate change (Willems et al. 2011, Rodríguez et al. 2014). Whereas in other latitudes an increase in total precipitation due to global warming has been reported (IPCC, 2007), in the Mediterranean area several studies point to a decreasing dominant trend in recent decades (Pérez and Boscolo 2010). This is not the case with extreme or torrential precipitation, for which an increase is expected, due to a greater occurrence of shorter and more intense rainy episodes (Christensen and Christensen 2003). The evaluation of these changes is especially interesting in urban areas with high population density, due to the vulnerability of their complex sewage systems to torrential rain (Casas et al. 2010, Rodríguez et al. 2013b).

To describe mathematically the IDF curves of a place, it is possible to use a scaling relationship related to the fractal property of scale invariance of rainfall intensity (Menabde et al. 1999, Rodríguez-Solà et al. 2017). In the monofractal case or simple scale, this scaling relationship is determined by a single parameter or scaling exponent β, whose value seems to be related to the rainfall pattern of the place, or more particularly, to the irregularity of the rain (Casas-Castillo et al. 2018b). Thus, this parameter takes values close to -1 for areas where rain is usually very irregular, with sudden maximum isolated values, while it takes higher values for areas with a more regular rainfall pattern. It is to be expected that the supposed tendency of the annual precipitation to decrease and the relative increase in extreme precipitation due to climate change will be reflected in changes in the rainfall scaling behavior, with values of parameter β that tend to decrease. In order to highlight these changes, simulated daily precipitation series for the 21st century have been analyzed for two densely populated cities: Barcelona and Madrid. These future series were obtained from several general circulation models and for different climate change scenarios: A1B, A2, B1 and B2 for Barcelona (114 series in total), and RCP2.6, RCP4.5, RCP6.0 and RCP8.5 for Madrid (213 series).

In spite of the great variability of results obtained from the climatic models, it is possible to draw some conclusions with the sufficient degree of certainty. For the total annual precipitation calculated from the projected future series, it has been observed that, in Barcelona, this magnitude will suffer an approximate decrease of 5% in the last third of the 21st century. In Madrid, on the other hand, there seems to be a little significant increase in annual rainfall, being the uncertainty very high in this case.

Regarding the simple scaling relationships obtained for the intensity of future rainfall in both cities, a significant decrease in the scaling exponent β has been obtained from the second third of the 21st century, which is consistent with the forecasts of a possible increase of heavy rains in the near future due to climate change. These results indicate a probable increase in the frequency of torrential rains and the consequent increase in the risk of flooding in both cities, so it should be taken into consideration in the design of its hydraulic infrastructure, being recommended an according improvement of its networks of sewage system and its drainage systems.