IB Geography A.5.1 Spatial Interactions Within a Drainage Basin Notes

A Change in One Part of a Basin Alters Water Elsewhere

1. System boundary: a drainage basin collects precipitation and transfers water, sediment, nutrients and pollutants toward a common outlet.
2. Stores: vegetation, soil moisture, groundwater, lakes, wetlands, snow and reservoirs hold water for different lengths of time.
3. Transfers: interception, infiltration, throughflow, percolation, groundwater flow and channel flow connect the stores.
4. Inputs and outputs: precipitation enters the basin while evapotranspiration, river discharge and abstraction remove water from particular stores.
5. Connectivity: the strength and speed of a downstream effect depend on distance, slope, permeability, channel links and the condition of intervening stores.

Land-Use Change Alters Several Transfers at Once

1. Urbanization: impermeable surfaces reduce infiltration and surface storage while drains route runoff rapidly into channels.
   1. A shorter lag time and a higher peak discharge can increase downstream flood risk even when the total storm rainfall is unchanged.
2. Deforestation: reduced interception and root uptake can increase overland flow, soil erosion and sediment delivery to rivers.
   1. The effect varies with rainfall intensity, soil damage, slope and the land cover that replaces the forest.
3. Wetland drainage: a lower water table removes storage and shortens the time water spends in the floodplain.
   1. Faster transfer can raise downstream peaks while peat oxidation releases carbon and weakens water-quality regulation.

River Engineering Redistributes Benefits and Risks

1. Dam storage: wet-season water is retained for power, supply or flood regulation, which changes the timing of downstream discharge.
2. Sediment interruption: reservoir deposition reduces the material reaching floodplains, channels and deltas.
3. Levees: higher banks can protect one reach while increasing flow depth or transferring flood risk downstream when water is confined.
4. Channelization: a straighter and smoother channel moves water faster, which can lower local flood duration and raise the speed of the flood wave elsewhere.
5. Abstraction: withdrawal for farms or cities lowers downstream flow and can concentrate pollutants during dry periods.

Effects Change with Scale, Timing and Thresholds

1. Spatial scale: a small intervention can be important locally but difficult to detect at the basin outlet.
2. Time lag: groundwater and soil stores delay some effects, while storm drains and connected channels transmit others within hours.
3. Cumulative effect: many small abstractions or drains can create a basin-wide change even when each permit appears minor.
4. Threshold: a store can absorb disturbance until its capacity is exceeded, after which runoff, erosion or ecological change rises sharply.
5. Season: the same reservoir release or abstraction has a larger effect during low flow than during a wet-season surplus.

A System Diagram Should Make the Causal Chain Testable

1. Boxes: use boxes for named stores such as soil moisture, groundwater and reservoir storage.
2. Arrows: use labelled arrows for transfers and show direction from the changed process to the affected store or output.
3. Scale: vary arrow width only when it represents a measured or clearly ranked flow.
4. Feedback: show a loop only when the final change acts back on an earlier process.
5. Annotation: add the place, time period and management intervention so the diagram explains a real basin rather than an abstract cycle.
6. Check: every claimed downstream effect should be traceable through connected stores and transfers without skipping a mechanism.