Local Water Budget summary L4

Local Water Budget Overview

• Learning Objective: Understand how the hydrological cycle impacts local water budgets and river systems.

• Water Balance Equation: $P = Q + E \pm S$

  • $P$ = Precipitation

  • $Q$ = Channel discharge

  • $E$ = Evapotranspiration (combined evaporation and transpiration)

  • $S$ = Change in storage (soil moisture, groundwater)

• Key Inputs: Precipitation, water/groundwater/surface flow into the area.

• Key Outputs: Evapotranspiration, water/groundwater/surface flow out of the area, industrial/residential uses.

Seasonal Water Budget Dynamics

• Summer: Potential evapotranspiration (PE) exceeds precipitation; soil moisture declines; possible water deficit.

• Winter: Precipitation exceeds evaporation; storage areas recharge, leading to surplus and increased flood risk.

• Field Capacity: Maximum moisture capacity a soil can hold.

• Effective Rainfall: Precipitation remaining after evaporation.

• Recharge: Increase in soil moisture due to precipitation after a dry period.

River Regimes

• Definition: The annual pattern of flow within a river.

• Influences: Climatic conditions, drainage basin characteristics (physical factors, human interventions).

• Simple Regimes: Seasonally high discharge followed by low discharge (e.g., mountainous areas with summer snowmelt, like the Rhone).

• Complex Regimes: Occur when a river spans diverse relief and climatic zones; influenced by human factors and river length (e.g., Amazon, Yukon, Murray-Darling).

• Discharge: Volume of water passing a point over time, measured in cumecs ($m^3/s$).

Sustainable Drainage Systems (SuDS)

• Purpose: Reduce runoff from rainfall.

• Examples: Rain gardens, green roofs, permeable paving, detention basins, wetlands.

• Planning: In the UK, planners must assess a proposed development's impact on flood risk.