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 Zimbabwe 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.

Reliability analysis of roof rainwater harvesting systems in a semi-arid region of sub-Saharan Africa: case study of Mekelle, Ethiopia

“…This leads to inefficient rainwater harvesting. In agreement to our finding, a water balance analysis in sub-Saharan Africa (SSA) identified the under-utilization RWH schemes (Gwenzi and Nyamadzawo 2014). This gives rise to a call for water policies in SSA like developed countries, so that people will recognize the role of RWH and use it effectively.…”

Economic Analysis and Feasibility of Rainwater Harvesting Systems in Urban and Peri-Urban Environments: A Review of the Global Situation with a Special Focus on Australia and Kenya

“…Ishida et al [61] recognized the potential benefit of RWH systems in stormwater management and combined sewer overflow control. Gwenzi et al [83] found that urban RWH systems reduced downstream peak and total discharge, baseflow, and flow velocity. DeBusk et al [84] in a comprehensive review found very little research on the stormwater management benefits of RWH systems.…”

“…Using a RWH system for irrigation also has the potential to improve food security [85][86][87][88], and particularly nutrition among women and children via small-scale domestic gardens [1,83]. Ngigi et al [89] investigated 50m 3 of water storage used in conjunction with drip irrigation kits over 0.2 ha of cropped land, where cash flow was calculated from the improved yield of 4000-5000 kg/ha, showing an increase of 1000 kg/ha.…”

“…For example, a review of Queensland's mandate showed that many areas have reached their water saving targets and that adopting a RWH system beyond the mandate may significantly exceed the target [123]. The potential of RWH systems to also reduce water and food crises in developing countries and water-scarce urban and peri-urban areas like Sub-Saharan Africa is recognized by several authors [1,83,[86][87][88]124]. Some governments are offering financial assistance in an effort to achieve Millennium Development Goals (MDG) [125].…”

Estimating the Effect of Urban Growth on Annual Runoff Volume Using GIS in the Erbil Sub-Basin of the Kurdistan Region of Iraq

“…An assessment of the impact of land use changes on water resources is one of the recent thrusts in hydrological modelling [1][2][3]. It is expected that approximately 60% of the world's population will be living in urban areas by 2030.…”