Water Cycle, Hydrology and Water Insecurity
Water Cycle, Hydrology and Water Insecurity
This domain examines stores and flows in the hydrological cycle, drainage-basin response, water scarcity and management. Exam questions often use hydrographs and demand evaluation of physical and human causes.
Core rules
- Inputs include precipitation. Outputs include evapotranspiration, runoff and groundwater discharge. Stores include interception, soil moisture, groundwater and channel storage.
- Flows include infiltration, percolation, throughflow, groundwater flow, overland flow and channel flow.
- A storm hydrograph shows rainfall and discharge. Lag time is the delay between peak rainfall and peak discharge. Urbanisation, impermeable geology, steep slopes, bare soil and drainage increase peak discharge and shorten lag.
- Water scarcity can be physical (supply low relative to demand) or economic (supply exists but infrastructure or cost prevents access).
- Management includes dams, reservoirs, groundwater abstraction, desalination, rainwater harvesting, recycling, pricing and integrated water resource management. Evaluate environmental and social costs.
Worked example
Two catchments receive the same storm. Catchment A is urbanised; catchment B is forested. Explain the hydrographs. Reasoning: urban surfaces and drains reduce infiltration and increase overland flow, so A has a higher, earlier peak and shorter lag. Forest interception and infiltration delay and reduce runoff, so B has a lower, later peak. The reasoning must refer to flows, not just more concrete.
Traps
Do not confuse infiltration (into soil) with percolation (to groundwater). Do not equate drought with water scarcity: scarcity includes access and quality. Do not ignore groundwater recharge time. Do not state that dams remove all flood risk.