Briefly explain the effects of urbanization on the natural water cycle.
Answer:-
The natural water cycle or hydrological cycle is a continuous process through which water circulates within the Earth’s atmosphere, surface, and subsurface. It involves key processes such as precipitation, infiltration, evaporation, transpiration, runoff, and groundwater recharge. In its undisturbed form, this cycle maintains a balance between various water sources and sinks.
However, urbanization—the transformation of natural land into towns and cities—profoundly disrupts this balance. The replacement of forests, grasslands, and wetlands with concrete structures, roads, and buildings drastically changes the way water moves through the environment. This has significant implications not only for water availability and quality but also for the frequency of floods, droughts, and ecosystem health.
1. Reduced Infiltration and Groundwater Recharge:
- In natural landscapes, a significant portion of rainfall infiltrates into the soil, replenishing groundwater aquifers.
- Urbanization replaces soil with impermeable surfaces such as asphalt and concrete.
- This prevents infiltration, causing reduced groundwater recharge.
- As a result, water tables decline, wells dry up, and dependency on external water sources increases.
- It also causes drying of wetlands, springs, and base flow reduction in rivers and streams during dry seasons.
2. Increased Surface Runoff and Flooding:
- With reduced infiltration, more rainwater becomes surface runoff.
- Impervious surfaces accelerate the speed and volume of runoff.
- This leads to frequent urban floods, especially during intense rainfall.
- Drainage systems may be overwhelmed, and stormwater quickly carries pollutants into rivers and lakes.
- The natural storage function of the land is lost, making cities more vulnerable to flood disasters.
3. Altered Stream Flow and River Morphology:
- Natural river systems experience seasonal flow variations. Urban runoff creates unnatural, flashy discharges into rivers.
- This results in:
- Erosion of riverbanks
- Sedimentation downstream
- Loss of aquatic habitats
- Small streams and tributaries may be channelized, culverted, or encroached, further disrupting the river’s flow regime and ecology.
4. Decline in Evapotranspiration:
- Evapotranspiration includes water lost to the atmosphere through evaporation from soil and water bodies and transpiration from plants.
- Urban areas have limited vegetation and open soil, leading to a sharp reduction in evapotranspiration.
- This affects the local humidity, cloud formation, and rainfall patterns, often leading to hotter and drier microclimates within cities.
5. Urban Heat Island Effect and Accelerated Evaporation:
- Cities absorb and retain heat due to dark, non-reflective surfaces.
- This increases local temperatures—known as the Urban Heat Island (UHI) effect.
- Higher temperatures cause rapid evaporation from urban water bodies, contributing to increased water loss.
- This intensifies water stress, especially during dry months.
6. Water Pollution and Degraded Water Quality:
- Runoff from urban surfaces carries:
- Oil, grease, and heavy metals from roads
- Detergents and sewage from households
- Pesticides and fertilizers from urban landscaping
- These pollutants enter rivers and lakes, leading to:
- Eutrophication
- Oxygen depletion
- Destruction of aquatic life
- Groundwater may also become contaminated, making it unsafe for drinking and irrigation.
7. Stress on Water Resources and Over-Extraction:
- Urban populations require large volumes of water for domestic, industrial, and recreational use.
- Cities often depend on distant rivers, dams, or groundwater reserves to meet this demand.
- This leads to:
- Over-extraction of groundwater
- Reduced flow in rivers
- Conflicts between urban and rural users
- Over time, aquifers may collapse, leading to land subsidence in some regions.
8. Loss of Ecosystem Services:
- Natural landscapes provide several services such as:
- Water filtration
- Flood control
- Climate regulation
- Urbanization leads to the loss of wetlands, forests, and green belts, thereby reducing these ecosystem services.
- The biodiversity that supports the water cycle—such as wetlands species, amphibians, and riparian flora—is lost.
9. Social and Economic Impacts:
- Urban flooding, water pollution, and scarcity lead to economic losses and health hazards.
- Poor urban communities, often living in low-lying or flood-prone areas, are most affected.
- There is an increasing need for engineered solutions like artificial recharge, rainwater harvesting, and wastewater treatment, adding to urban management costs.
10. Climate Change Amplification:
- Urban water cycle changes also exacerbate the effects of climate change.
- For example, heat islands and reduced vegetation reduce carbon sequestration and intensify local climate extremes.
- Reduced resilience in urban water systems makes cities more vulnerable to droughts and extreme rainfall events.