How landscapes’ ‘memories’ shape the way Indian cities flood, PgII

Key Highlights:

• Indian cities experience prolonged waterlogging due to the landscape's memory of past rainfall, a phenomenon called hydrological hysteresis.
• Hydrological hysteresis means a landscape's response to rainfall depends on current rainfall and past events, affecting water movement through soils, wetlands, rivers, and cities.
• Rivers behave differently during rising and receding floods due to the evolving interaction between water flow and land, impacting water storage and resistance.
• In October 2024, Bengaluru's Kogilu and Doddabommasandra lakes overflowed, demonstrating how saturated soils and submerged drains can prolong urban flooding even after rainfall decreases.

Detailed Insights:

• Landscapes retain a memory of past rainfall, influencing how water moves; a catchment saturated from weeks of monsoon rain responds differently than a dry one to the same rainfall amount.
• Water is stored in soils, aquifers, wetlands, and floodplains, releasing it at varying rates, causing a non-linear relationship between rainfall and river flow that shifts as the land moistens and dries.
• During monsoons, dry soils and wetlands initially absorb water, but as they near saturation, infiltration decreases, leading to increased runoff and flooding even without intensified rainfall.
• As monsoon rain intensifies, river channels fill rapidly, but once water exceeds the banks, it spreads laterally into floodplains and wetlands, slowing flow and causing sediment to settle.
• Floodplains don't drain instantly; stored water seeps slowly through soils, re-entering channels or remaining ponded, delaying drainage as groundwater levels rise, altering the river's physical state.
• Bengaluru's interconnected lakes, established in the 16th century, were linked by natural streams and wetlands, but these connections were straightened into concrete channels, causing quick filling, abrupt spilling, and slow emptying.
• Recognizing the land’s hydrological memory is crucial for policymakers, emphasizing basin-scale planning and the importance of urban lakes, wetlands, and floodplains as critical infrastructure for water storage and release.

Key Concepts Involved:

• Hydrological Hysteresis: The phenomenon where a landscape's response to rainfall depends on both current rainfall and past events.
• Floodplain: A flat area of land near a river or stream that is subject to flooding.
• Catchment: The area from which rainfall flows into a river or reservoir.