The Water Cycle
The Water Cycle
The hydrological cycle is a closed global system linking stores and flows. Stores include oceans (about 97% of water), ice caps and glaciers, groundwater, lakes, rivers, soil moisture, the atmosphere and living things. Flows (fluxes) include evaporation, transpiration, evapotranspiration, condensation, precipitation, interception, infiltration, percolation, surface runoff, throughflow and groundwater flow. Candidates must explain how water moves between stores, and how human activity alters the cycle.
Understand the drainage basin as a subsystem: inputs (precipitation), stores (interception, soil, groundwater, channel) and outputs (evaporation, transpiration, runoff). Learn the storm hydrograph - lag time, rising limb, peak discharge, recession limb - and how basin characteristics (size, shape, slope, rock type, vegetation, urbanisation) affect it.
Traps: confusing infiltration (into soil) with percolation (into rock); treating groundwater as a flowing river; forgetting that transpiration returns water to the atmosphere; and assuming the global water total changes (it does not - only its distribution and quality).
Rule of thumb: impermeable surfaces, steep slopes and little vegetation shorten lag time and raise peak discharge; permeable rock, gentle slopes and dense forest lengthen lag and lower the peak.
Worked example: Two catchments, one forested and one urbanised, receive the same storm. The urban catchment has a shorter lag time and higher peak discharge because tarmac and drains reduce infiltration and speed runoff; the forested catchment intercepts rain, infiltrates more and releases water slowly.