Water and Carbon Cycles
Drainage Basins as Open Systems
Introduction
A drainage basin is the area of land drained by a river and its tributaries. It functions as an open system within the global hydrological cycle, with energy and water entering and leaving it.
The characteristics of a drainage basin
The watershed marks the boundary between neighbouring basins. Drainage basins exist at multiple scales, from small hill slopes to large systems such as the Amazon and Nile Basins.
The movement of water through a drainage basin is shown by the drainage basin hydrological cycle. This is an open system that receives water from precipitation and loses it through runoff and evapotranspiration.
Drainage Basin (hillslope system) Hydrological Cycle
The main features include:
- Inputs: mainly precipitation.
- Outputs: include evapotranspiration and runoff (discharge).
- Within the basin, water is stored and transferred through a range of flows and stores, each operating over different timescales.
1. The Drainage Basin as a System
The drainage basin operates as an open system, constantly exchanging energy and matter with its surroundings.
- Inputs: precipitation and condensation.
- Stores: interception, surface, soil, groundwater, and channel.
- Flows: infiltration, percolation, throughflow, groundwater flow, overland flow, stemflow, and channel flow.
- Outputs: evapotranspiration and river discharge.
| System Component | Description | Example |
|---|---|---|
| Inputs | Water entering the system. | Precipitation (rain, snow, sleet, dew). |
| Outputs | Water leaving the system. | River discharge at the mouth, evapotranspiration. |
| Store (components) | Water held within the basin for varying durations. | Interception, surface water, soil water, groundwater, channel storage. |
| Flows | Movements of water between stores. | Infiltration, percolation, throughflow, groundwater flow, overland flow, stemflow, channel flow. |
2. Inputs to the Drainage Basin
Precipitation
The primary input to a drainage basin is rainfall, snow, sleet, or hail. Its type, intensity, duration, and frequency affect how much water enters surface, soil, and subsurface stores.
Factors affecting precipitation inputs:
- Type: snow temporarily stores water; rain generates quicker runoff.
- Intensity: heavy rainfall increases surface runoff.
- Duration: prolonged rainfall saturates soils, enhancing throughflow and overland flow.
- Seasonality: affects water balance and river regimes.
3. Outputs from the Drainage Basin
Evapotranspiration (ET)
The combined water loss through evaporation (from water bodies and soil) and transpiration (from vegetation).
| Type | Definition |
|---|---|
| Potential Evapotranspiration (PET) | The amount of water that could be lost if water were freely available. |
| Actual Evapotranspiration (AET) | The actual amount lost, limited by water availability. |
Controls: temperature, vegetation cover, soil moisture, and wind speed.
Runoff (River Discharge)
- The total flow of water leaving the drainage basin via the river channel, measured in cubic metres per second (cumecs).
- Represents the basin’s main output and reflects the balance between inputs, flows, and storage.
4. Stores within the Drainage Basin
| Store | Description | Timescale |
|---|---|---|
| Interception Storage | Water held temporarily on leaves and branches before dripping or evaporating. | Hours to days |
| Surface Storage | Water stored in depressions, puddles, ponds, or lakes. | Hours to months |
| Soil Water Storage | Water held in the unsaturated upper soil layer. | Days to months |
| Groundwater Storage | Water stored below the water table in permeable rocks (aquifers). | Years to millennia |
| Channel Storage | Water held within the river channel. | Hours to days |
5. Flows and Transfers within the Drainage Basin
1. Infiltration
- Downward movement of water from the surface into the soil.
- Controlled by soil texture, vegetation, antecedent moisture, and surface cover.
2. Percolation
- Downward transfer of water through soil and permeable rock layers to recharge groundwater.
3. Throughflow
- Lateral movement of water downslope within the soil layer towards a river channel.
- Faster in permeable soils and during wet conditions.
4. Groundwater Flow (Baseflow)
- Slow transfer of water through permeable rocks below the water table.
- Maintains river discharge during dry periods.
5. Overland Flow (Surface Runoff)
When rainfall intensity exceeds infiltration capacity.
- Saturation overland flow: soil becomes saturated.
- Infiltration-excess overland flow: rainfall exceeds infiltration rate.
6. Stemflow
- Water flows down the stems or trunks of vegetation after interception.
7. Channel Flow
- Movement of water through the river channel from source to mouth.
Micro-scale Flows (Additional Surface Transfers)
At smaller scales, water also moves via:
- Throughfall: precipitation that passes directly through vegetation canopies to the ground.
- Drip Flow: water that drips from leaves after interception.
These short-term flows occur alongside stemflow, especially in forested basins, and together help redistribute precipitation within the system.
6. The Concept of Water Balance
Definition
The water balance (or water budget) shows the relationship between inputs, outputs, and changes in storage within a drainage basin:
P=Q+E±ΔS
Where:
- P = Precipitation
- Q = Runoff (streamflow)
- E = Evapotranspiration
- ΔS = Change in storage
Seasonal Variations
- Winter: precipitation > evapotranspiration → water surplus and high discharge.
- Summer: evapotranspiration > precipitation → water deficit and low discharge.
- Autumn/Winter: soil moisture recharge as precipitation refills soil and groundwater stores.
Example (UK):
In January, mean precipitation (~100 mm) exceeds potential evapotranspiration (~25 mm), resulting in a surplus and groundwater recharge. In August, PET often exceeds 100 mm, while rainfall averages 50 mm, resulting in a deficit and low baseflow.
Soil Moisture Budget
The soil moisture budget tracks changes in soil water availability through the year by comparing precipitation (P) and potential evapotranspiration (PET).
The soil moisture budget for a location in the east of England.
| Phase | Description | Typical Season (UK) |
|---|---|---|
| Recharge | Rainfall exceeds PET; soil stores refill. | Autumn–Winter |
| Surplus | Soil and ground stores are full; excess contributes to runoff. | Winter |
| Deficit | PET exceeds precipitation; soils dry out. | Late Summer |
| Utilisation | Plants and evaporation draw on stored moisture. | Spring–Summer |
7. Changes in the drainage basin
Changes in one component, such as deforestation reducing interception or urbanisation increasing runoff, can alter other components, demonstrating feedback and interdependence within the system.
Summary
- A drainage basin is an open system within the global water cycle.
- Inputs: precipitation; outputs: evapotranspiration and runoff.
- Stores: interception, surface, soil, groundwater, and channel storage.
- Flows: infiltration, percolation, throughflow, groundwater flow, overland flow, stemflow, and channel flow.
- Water balance and soil moisture budget show how inputs, outputs, and storage vary seasonally.
Exam Tip
When answering A Level questions on systems:
- Use specific geographical examples (e.g. “the Amazon Basin as an open subsystem”).
- Include key terms from the specification, such as flows, stores, feedback, dynamic equilibrium, and cascading systems.
- Show understanding of scale — how global closed systems contain local open subsystems.