Urban development in all forms impacts the natural landscape, changing vegetation cover, infiltration rates and hydrological (surface and sub-surface) flows. With increasing urbanization urban areas lose a host of natural infrastructure and ecosystem services as ecosystems are modified, degraded and/ or shrunk. Benefits such as flood control, aquifer replenishment, microclimate control, improved air quality are diminished, increasing the risks faced by urban dwellers.
For the Indian context there has been limited study of the interlinked impacts of urbanization on natural infrastructure. In this new report ‘Urban Blue-Green Conundrum’ by WRI India the authors trace the changes to the built footprint (built-up area) and impacts on blue cover (surface water), green cover changes (vegetation) and the groundwater recharge potential.
For the 10 most populated cities in India they analyse satellite imagery to study the core city (0-20 km) and peripheral city (20-50 km) regions and the changes between 2000 and 2015. In addition, the impact on groundwater recharge is estimated from the conversion of natural spaces with higher recharge potential to concretized surfaces with lower recharge potential. Groundwater recharge potential is estimated using the method described in the report titled “The Impact of Climate Change on Groundwater Availability” (Mvandaba et al. 2019).
The potential is derived from the NDVI (Chen 1996) and slope of the land surface (taken from the digital elevation model [DEM]). The method used in this study is limited to the use of satellite imagery and other open-source remote sensing information. It does not use soil maps with detailed infiltration coefficients or evapotranspiration rates derived from on-ground studies.
The cities studied are Ahmedabad, Bengaluru, Chennai, Delhi, Hyderabad, Jaipur, Kolkata, Mumbai, Pune, and Surat. Satellite imagery enables the actual spatial extents of urban areas vis-à-vis built-up areas, blue cover, and green cover change trend to be studied.
The study uses existing spatial assessment and analysis methods to estimate the spatial extents of various built and natural features in 10 Indian cities. It examines changes in extent beyond administrative boundaries, as urbanization processes are not bounded by these jurisdictions. The study considers a 50 km region around each study city to estimate the changes.
This study is limited to establishing correlations between increases in built-up area and impacts on natural infrastructure as observed from satellite imagery only. Changes to natural infrastructure in urban areas are driven by various interconnected factors including urbanization, climate change driven weather extremes, and other human activities (such as agriculture and quarrying).
The use of spatial intervals (0–20 km and 20–50 km) helps assess how the built-up area, blue space, and vegetation change trend and recharge zones are distributed between core-city and peripheral areas, and whether blue-green spaces in core-city and peripheral areas are differentially impacted by the growing urban footprint.
Results and discussion
The study cities experienced (on aggregate) an increase in built-up area and net vegetation change trend in the 20–50 km study regions between 2000 and 2015. A net aggregate 10,000 sq. km of low-level gain has occurred across all the 10 cities. This low-level gain is not uniformly distributed within each city, with about 84 percent of it occurring in the 20–50 km (peripheral) region. Overall, an area of 7,400 sq.km is estimated to have a decreasing vegetation trend, with 33 percent of this decline occurring in the 0–20 km region. In six of the cities (Bengaluru, Chennai, Delhi, Hyderabad, Jaipur, and Pune), losses exceed gains in the 0–20 km region.
Simultaneously, blue cover decreased by average 15 percent in all cities except Ahmedabad (0–50 km region). A greater change is seen in the 20–50 km region. Chennai and Jaipur experienced minor increases in the 0–20 km region, and Pune experienced a significant increase in the 20–50 km region The minimum volumetric loss assuming a 3 m depth for all these water bodies is 900 billion liters across the 10 cities, a volume that could potentially meet about 60 percent of the annual demand added to the 0–50 km region around these cities. Thus, although overall Ahmedabad performs better than the other study cities, the co-benefits accruing from the increase (groundwater recharge, microclimate benefits) are not likely to be distributed across the entire city.
About 44 percent of this new development is located in zones with high and very high recharge potential, and an estimated 300 billion litres of water per year is now diverted away from underground aquifers.
The current form of urbanization is disconnected from the natural environment and is associated with various negative outcomes for cities, such as water scarcity, increased groundwater stress, and increasing incidence of urban flooding. The choices made by public and private development agencies regarding where and how to develop can mitigate or exacerbate biophysical disruptions.
Using scientific evidence to accurately identify the correlations between urbanization, loss of natural infrastructure, and increasing climate shocks and stresses can enable the state and municipal authorities to strengthen urban planning and development in the future.
Measures such as identifying areas of high recharge potential (as done in this study) and retaining high permeability in these areas, restricting building activity in high-risk areas such as floodplains and (ephemeral and permanent) lake beds, and incorporating blue-green infrastructure solutions can be steps toward a resource-sufficient and resilient urban future.
State and municipal authorities can use spatial and on-ground evidence to embark on a paradigm shift in planning in which an integrated urban blue-green approach is undertaken to conserve and restore natural spaces, water bodies, aquifers, and other ecosystems to increase urban resilience. Diversion and/or regulation of development to areas with fewer natural infrastructure resources and close replication of the functions of these natural systems through nature-based solutions (restored urban wetlands, rain gardens, green roofs) can serve as alternatives to the present trend of unmanaged urban expansion.
The report recommends that state and city authorities adopt an integrated urban blue-green approach to urban planning and development regulation using spatial and on-ground evidence to conserve and restore natural spaces, water bodies, aquifers, and other ecosystems to increase urban resilience. Natural infrastructure in urban areas can also function as a buffer against climate change driven extreme weather events and therefore is an important adaptation and mitigation measure to increase urban resilience.
Valuing natural infrastructure as a key component of the urban (water) infrastructure can expand the range of solutions available to urban water managers and enable city regions to reduce risks and become more resilient and liveable. Later research can build on this study to identify the specific causes of natural infrastructure degradation and explore solutions and strategies to prevent the loss of blue-green spaces during urbanization.
The full report can be viewed here
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