Urbanization Impacts on Houston Rainstorms
During urbanization the natural landscape is altered by removal of indigenous vegetation, stripping of topsoil, modification of the soil profile, importation of fill material, compaction of soil layers, and the introduction of impervious surfaces. These actions alter the hydrologic response at the regional and catchment scales, decreasing travel time of overland flow, infiltration, soil water content, and groundwater recharge, and increasing surface runoff volumes, discharge rates, and pollutant loadings. These are direct and quantifiable impacts to the hydrologic cycle manifested at and below the land surface, but there are other impacts not as well understood or quantifiable that occur through the land surface-atmosphere interface. One such impact is the influence of urban development on mesoscale circulations and resulting convection. Hypothesized mechanisms for urban enhancement of convective rainfall include enhanced convergence caused by the urban heat island, drag effects of the built-up surface, and modified microphysical and dynamical processes caused by the introduction of water and cloud condensation nuclei from automobiles and industry. Decades of accumulated observational and modeling evidence has shown that major cities may indeed be influencing convective activity causing modified precipitation patterns.
This chapter seeks to corroborate these findings by summarizing evidence that the urbanization of a major coastal city in the United States has resulted in modified rainstorm characteristics within the urbanized area and in the seasonally variant downwind urban-affected region compared to an upwind control region. It also includes a short summary of the coupling of a meteorological model and SWMM to study in greater detail the potential impact of modified rainstorm characteristics on hydrologic response.
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