CAIE A-Level Geography

The drainage basin as an open system

A drainage basin (catchment) is the area of land drained by a river and its tributaries, bounded by a ridge of high ground called the watershed. It is an open system, because both matter (water, sediment) and energy cross its boundary. Water enters as inputs, is held in stores, moves between them by transfers, and leaves as outputs.

The balance is expressed as a water balance: precipitation = river discharge + evapotranspiration ± change in storage. When inputs exceed outputs the stores grow (soil moisture rises, the water table climbs); when outputs exceed inputs the stores shrink.

Inputs and outputs

The main input is precipitation: rain, snow and hail. The outputs are:

• evaporation, the change of liquid water to vapour from lakes, rivers, soil and wet surfaces;
• transpiration, water vapour lost through plant stomata, and evapotranspiration, the two combined;
• channel flow (discharge) to the sea or a lake;
• in a broader view, condensation of vapour in the atmosphere above the basin, which returns water to the air as cloud.

Stores

• Atmosphere: water vapour and cloud.
• Vegetation: water held on leaves, stems and bark (interception store) and water taken up by roots.
• Surface: puddles, lakes, ponds and the water in the river channel.
• Soil: moisture held between soil particles.
• Bedrock: groundwater stored in saturated rock below the water table, the upper surface of the saturated zone. A rock that holds and transmits groundwater is an aquifer.

Transfers

• Stemflow: water running down branches and trunks to the ground.
• Throughfall: water dripping from leaves or falling through gaps in the canopy.
• Infiltration: water soaking into the soil surface.
• Percolation: water moving downwards from the soil into the bedrock.
• Overland flow (surface runoff): water running across the surface. Hortonian overland flow occurs when rainfall intensity is greater than the infiltration capacity of the soil, so the surplus cannot soak in. Saturation-excess overland flow occurs when the soil is already full from below, so any further rain runs off even if it falls gently.
• Throughflow: lateral movement of water through the soil towards the channel, slower than overland flow.
• Groundwater flow (baseflow): slow movement through saturated rock, which sustains river flow in dry weather.

Overland flow is the fastest transfer and reaches the channel within minutes to hours, throughflow takes hours to days, and groundwater flow takes weeks to years. The speed of the transfers decides how quickly a river responds to rain.

Basin characteristics that control the movement of water

Size, shape and drainage density

A large basin collects more water, so peak discharge is higher, but it also takes longer for water from distant parts to arrive. A circular basin brings water from all its edges to the outlet at about the same time, producing a sharp, high peak. An elongated basin spreads the arrival of water over a longer time, producing a lower, flatter peak.

Drainage density is the total length of stream channels divided by the area of the basin (km per km²). A high density means many channels carry water quickly to the main river, so the response is faster and peaks are higher. Density is high where rocks are impermeable, slopes are steep and rainfall is intense.

Soils and rocks

Soil porosity is the proportion of the soil's volume that is pore space and so can hold water. Permeability is how easily water passes through. Clay is porous but has very small pores and low permeability, so water infiltrates slowly and runs off. Sand has larger pores and high permeability, so infiltration is fast and runoff is low. Silt lies between them.

Rock type matters in the same way. Granite is generally impermeable, so water runs over it and a basin on granite has high drainage density. Chalk and many limestones are permeable and store groundwater, so channels are fewer and flow is steadier; limestone is also permeable along joints and bedding planes. Sandstone is porous and often permeable; clay is impermeable and produces rapid runoff.

Vegetation and land use

Vegetation intercepts rainfall, delays its arrival at the ground, and returns part of it to the atmosphere by evaporation and transpiration. Roots open channels in the soil that raise infiltration. A forested basin therefore has lower and later peaks than a cleared one. Dense, evergreen vegetation intercepts all year; deciduous woodland intercepts less in winter. Urban land use does the opposite: roofs, roads and drains are impermeable and carry water to the river in pipes, so lag time is short and peaks are high. Ploughed fields, compacted soil and overgrazed land also increase runoff.

Climate

Precipitation type, duration and intensity all matter. Intense rain is more likely to exceed infiltration capacity and produce Hortonian overland flow. Long rain eventually saturates the soil. Snow stores water on the surface until it melts, so a rapid thaw can release a large volume at once. Temperature and evapotranspiration control how much water is lost before it reaches the river; high rates in summer dry the soil and leave room for rain to infiltrate. Antecedent moisture is the wetness of the soil before a storm: a basin that is already saturated has little capacity left, so a given storm produces more runoff.

Hydrographs

A hydrograph plots river discharge (in cubic metres per second, cumecs) against time. Discharge is the volume of water passing a point in a unit of time: the cross-sectional area of the channel multiplied by the mean velocity.

For example, a channel 12 m wide and 1.5 m deep has a cross-sectional area of $12 \times 1.5 = 18\ \mathrm{m^2}$. If the mean velocity is $0.8\ \mathrm{m\,s^{-1}}$, the discharge is $18 \times 0.8 = 14.4\ \mathrm{m^3\,s^{-1}}$.

Annual hydrographs (river regimes)

An annual hydrograph shows the average discharge for each month, and the shape is the river's regime. It reflects the seasonal pattern of precipitation, temperature and evapotranspiration:

• a river in a Mediterranean climate has high discharge in the wet winter and very low flow in the hot, dry summer;
• a river fed by snow and glacier melt has a peak in late spring or summer when melting is greatest, and low flow in winter when water is frozen in storage;
• a river in the wet tropics with rain all year has a high, fairly even flow, while a monsoon river has a very large peak in the rainy season;
• in a temperate maritime climate, discharge is highest in winter, when rain is frequent and evapotranspiration is low, and lowest in late summer.

Vegetation and land use modify the pattern: a forested basin gives a smoother regime, and a basin with much urban land responds more sharply to individual rainfall events.

Storm hydrographs

A storm hydrograph shows discharge during and after a single rainfall event. Its components are:

• Baseflow: the normal flow supplied by groundwater before the storm.
• Rising limb: the increase in discharge as overland flow and throughflow reach the channel. A steep rising limb means water arrives quickly.
• Peak discharge: the highest discharge reached.
• Lag time: the time between the peak of the rainfall and the peak discharge. A short lag time means a flashy river.
• Falling (recession) limb: the decline as the quickflow passes and discharge returns towards baseflow. A gentle recession means water is still arriving from slower transfers such as throughflow and groundwater.
• Bankfull discharge: the discharge at which the channel is full, above which the river floods.

The graph also records the amount and timing of precipitation, usually as bars at the top, so the lag is measured from the peak of rainfall.

A flashy hydrograph has a short lag time, steep rising limb and high peak. It is produced by intense rain, saturated soil, impermeable rock, steep slopes, a circular basin, high drainage density, little vegetation and urbanisation. A subdued hydrograph has a long lag time, gentle limbs and a low peak, and is produced by gentle rain, dry permeable soils, permeable rock, dense vegetation and a large, elongated basin.

The area under the curve is the total volume of water that passed during the storm, and the difference between the stormflow and baseflow is the volume added by the event. Snowmelt can lift a hydrograph in a way that does not match the rainfall record, because the water was stored earlier.