Hydrological Impacts of Urbanization and Urban Roof Water Harvesting in Water-limited Catchments: A Review

Abstract

Roof water harvesting is a potential source of water for domestic and livelihood uses in water-scarce urban areas of the world such as sub-Saharan Africa (SSA). However, little is known about the hydrological impacts of incorporating roof water harvesting on on-site and downstream hydrology of urbanized catchments. Therefore, the current review investigates the effects of urbanization and urban roof water harvesting on hydrological processes, rainfall-runoff relationships, groundwater recharge and water contamination, and highlights future research directions. The review showed that the urban heat island effect increases the frequency and magnitude of convective storms. The high proportion and connectivity of impervious surfaces reduce infiltration, thereby increasing the runoff coefficient and Hortonian runoff. Urbanization reduces the minimum threshold rainfall for runoff generation, resulting in multi-peak hydrographs reflecting the contribution of both pervious and impervious surfaces. Urban roof water harvesting increases catchment lag time, but reduces downstream peak and total discharge, baseflow and flow velocity. Utility trenches, tunnels and buried structures form a complex network resembling a shallow urban karst system, which provides preferential flow pathways for groundwater recharge by imported water via leakages. Contrary to the widely held notion that urbanization reduces groundwater recharge by increasing impervious surfaces, empirical evidence shows significant urban-enhanced recharge in water-limited urban catchments. However, we contend that excessive groundwater abstraction for multiple uses in water-scarce regions offsets the urban-enhanced recharge, resulting in groundwater depletion. Due to the overriding collective effects of reduced soil moisture and vegetation cover on evapotranspiration in water-limited environments, we conclude that urbanization lowers evapotranspiration. Urban roof water harvesting short-cuts the urban water cycle, thereby minimizing the risk of runoff contamination that could occur during its extended flow over contaminated land surfaces. Contaminated sources of recharge, such as wastewater leakages coupled with the urban karst system, promote groundwater pollution. Overall, urban roof water harvesting imparts additional complexity to urban catchments, and has potentially adverse effects on ecohydrology. Understanding these impacts is critical for planning, designing and operation of urban roof water harvesting systems. Future research may provide a comprehensive understanding of these impacts by combining hydrological measurements and process modelling in urbanized catchments incorporating roof water harvesting.