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CHAPTER ONE

INTRODUCTION

Background to the Study

In March 2012 the global population exceeded 7 billion people for the first time, representing a doubling of the global population in less than 50 years (United States Census Bureau, 2012). It is estimated that more than 55% of the global population live in cities and that 394 of the world’s cities have a population that exceeds 1 million inhabitants (UN 2011). Furthermore, it is anticipated that 83% of the developed world and 53% of the developing world will live in urban areas by 2030 (Cohen 2004). As the global population continues to grow at a rapid rate, the expansion of urban areas continues to pose a significant threat to natural dynamics, resource availability and environmental quality, and advancing our knowledge of urban hydrological processes remains a priority within the field of hydrological science (Niemczynowicz 1999, Vörösmarty et al. 2000). The discipline of urban hydrology remains relatively young and has become increasingly relevant in a world that has experienced a marked, rapid growth in population in the past few decades, with varying dynamics of urban growth across the world (Jacobson 2011). Traditional research sought to assess a catchment scale response to urban development, seeking to identify the impacts of upstream urban development on downstream hydrological and water quality dynamics. In developing countries, urban growth continues to occur across large spatial scales, often with entire cities being constructed in short times (e.g. Binhai New Area, China; Li et al. 2015). By contrast, urban development in developed countries occurs at local scales, with individual buildings or small housing estates being typical, aided in part by advances in monitoring technologies (such as highresolution remote-sensing platforms) that provide insight into changing dynamics within the urban environment (Ragab et al. 2003, Blocken et al. 2013). A universal metric for measuring urban expansion remains elusive and the terminology of “urban” remains frustratingly disparate, impacting our scope for comparative analysis (MacGregor-Fors 2011). Multiple studies have addressed urban expansion using several metrics ranging from total population and population density to total or effective impervious area as a driver for hydrological dynamics, though a comprehensive metric of urban space remains elusive. The urban landscape has a demonstrable impact on meteorological and hydrological dynamics alike. The artificial thermal properties and increased particulate matter from urban areas impact the way rainfall is generated and enhance downwind precipitation and may enhance the generation of convective summer thunderstorms (Jin and Shepherd 2005). Expansion of urban space results in an increase of impervious landscape and expansion of artificial drainage networks that can facilitate dramatic changes to the magnitude, pathways and timing of runoff at a range of scales, from individual buildings to larger developments (Walsh et al. 2005, Fox et al. 2012, Dams et al. 2013). The fabric of individual buildings can alter the way rainfall is translated into runoff and the interconnected nature of pervious and impervious surfaces impact the effectiveness of surface drainage during rainfall events (Yang et al. 1999). Furthermore, the presence of water supply and sewage treatment infrastructure help to transfer vast quantities of water and wastewater across urban areas. Traditionally, urban hydrology has sought to separate infrastructure flows from natural hydrological analysis, though an increasing awareness that inefficient and defective networks can lead to an additional influx of water and contaminants to natural systems has resulted in a step toward integration of the two cycles (Grimmond et al. 1986, Leung and Jiao 2006).