5.2 - Drainage basins as open systems

Inputs, pathways, stores and outputs

A drainage basin receives and loses water across a defined boundary, so it is an open system within the global water cycle.

A drainage basin, or catchment, is the area drained by a river and its tributaries. High ground commonly forms its surface watershed. Precipitation enters as rain, snow or hail; totals, intensity, seasonality and spatial distribution all influence the subsequent response. Groundwater boundaries may differ from the surface watershed. A careful budget includes cross-boundary groundwater or water transfers where they are significant.

Canopy water, snow, soil moisture, groundwater and channel water are stores, not separate outputs. Evaporation and transpiration return water to the atmosphere. Channel flow is an output where the river crosses the basin outlet; upstream channel movement is an internal transfer. Water diverted outside the basin is another output. A reservoir inside the basin changes storage; filling it does not itself create new water.

For a period with no external transfers or significant cross-boundary groundwater, the basin budget is $P=ET+Q+\Delta S$, where precipitation $P$, actual evapotranspiration $ET$, outlet discharge $Q$ and storage change share units and period. In an original month, $P=90$ mm, $ET=35$ mm and $Q=40$ mm give $\Delta S=15$ mm: storage increases. Discharge in millimetres is the volume expressed as an equivalent depth across the entire catchment.

Physical controls on the pathways

The same rainfall total can produce different flows because a basin's climate, soil, vegetation, geology and relief interact.

Climate controls precipitation amount, intensity and state, and the energy available for evapotranspiration. A short intense storm can exceed infiltration capacity on an otherwise unsaturated soil. Several wet weeks can fill the soil store, allowing saturated overland flow during a gentler storm. Snow delays the release of precipitation; thaw can concentrate it into a short melt period. High temperatures increase potential evaporative demand, but actual evapotranspiration cannot remain high once available water is exhausted.

Soils with large connected pores generally infiltrate water more rapidly than compacted soils. Clay-rich soils may restrict infiltration, although cracks and biological channels complicate a simple texture rule. Initial soil moisture matters: a wet soil has less storage space, while a dry water-repellent surface may also shed water. Geology controls deeper permeability and aquifer capacity. A permeable, well-connected rock system can store water and sustain baseflow after rainfall; an impermeable layer encourages lateral throughflow or surface runoff. Porosity is the amount of pore space; permeability is the ability of connected spaces to transmit water. A rock can be porous yet poorly permeable.

Vegetation intercepts rain, withdraws soil water through roots and adds surface roughness. Roots and soil organisms can increase connected pores; litter protects the surface from raindrop compaction. These effects commonly increase storage and slow surface transfer. Interception capacity is finite, and a saturated canopy cannot indefinitely prevent runoff. Relief influences travel time: steep slopes often transfer surface water rapidly, while gentler slopes offer more opportunity for infiltration and temporary storage. Steep relief does not automatically imply impermeable ground.

Consider two original catchments receiving 30 mm in one hour. Catchment A has compacted clay, steep slopes and already wet soil; catchment B has dry, permeable soil and extensive woodland. A is likely to generate a larger fast surface-flow component. B can initially absorb and store more water, with some later throughflow and baseflow. This is a conditional explanation: if B's rainfall exceeds its infiltration capacity or its stores fill, its response will change.

Forest change from plot to Amazonia

Forest clearance changes water pathways locally and can also affect atmospheric water recycling across a much larger region.