Impermeable Surfaces: The replacement of natural soil with concrete and asphalt prevents infiltration, the process by which water enters the soil, leading to a significant increase in surface runoff.
Hydrograph Alteration: Urban drainage systems are designed to remove water quickly, which reduces the 'lag time' between precipitation and peak river discharge, often resulting in flash flooding.
Groundwater Depletion: Reduced infiltration in urban areas means that underground aquifers are not recharged at the same rate they are depleted by human consumption, leading to a falling water table.
Deforestation Effects: Removing trees eliminates the interception layer, allowing rain to hit the soil directly with more force, which increases erosion and reduces the amount of water returned to the atmosphere via transpiration.
Farming Practices: The use of drainage ditches accelerates the removal of water from fields, while over-abstraction for irrigation can lead to salinisation (salt buildup) or seawater ingress in coastal aquifers.
Soil Compaction: Heavy machinery and livestock can compact soil, reducing its porosity and making it harder for water to percolate into deeper stores.
Enhanced Greenhouse Effect: The burning of fossil fuels increases atmospheric , leading to global warming which accelerates evaporation rates and alters precipitation patterns.
Glacial Retreat: Rising temperatures cause mountain glaciers to melt, providing a temporary increase in runoff but ultimately depleting a critical long-term freshwater store for many communities.
Acid Rain: Industrial pollutants can lead to acid rain, which damages forest canopies, thereby reducing interception and transpiration rates across large regions.
| Feature | Natural Landscape | Urbanized Landscape |
|---|---|---|
| Primary Transfer | Infiltration & Throughflow | Surface Runoff |
| Lag Time | Long (slow response) | Short (flashy response) |
| Evapotranspiration | High (due to vegetation) | Low (lack of plants) |
| Groundwater Recharge | High | Low |
Infiltration vs. Percolation: Infiltration is the entry of water into the soil surface, whereas percolation is the downward movement of water through the soil and rock layers to the water table.
Evaporation vs. Transpiration: Evaporation is the physical change of liquid water to vapor from surfaces, while transpiration is the biological release of water vapor from plant leaves.
Mass Balance Approach: When analyzing an impact, always ask: 'Where did the water go?' If infiltration decreases, runoff MUST increase unless evaporation compensates.
Identify the Variable: In exam questions, identify which specific part of the cycle is being targeted (e.g., a 'store' like an aquifer or a 'flow' like throughflow).
Common Mistake: Do not assume water is 'destroyed' by human activity; it is simply transferred to a different store or flow, often making it less accessible or causing hazards like flooding.
Check for Feedback: Consider how one change leads to another, such as how deforestation leads to less local rainfall, further drying out the ecosystem.
