Rivers

Drainage Basins • Are local open systems within the closed global

hydrological cycle• A river’s drainage basin is the area surrounding the river

where the rain falling on the land flows into that river. This area is also called the river’s catchment

• The boundary of a drainage basin is the watershed- any precipitation falling beyond the watershed enters a different drainage basin

• Drainage basins are open systems with inputs and outputs

• Water comes into the system as precipitation and leaves via evaporation, transpiration and river discharge

A Typical Drainage Basin System

Inputs- water coming into the system

• Precipitation includes all the ways moisture comes out of the atmosphere

• Precipitation is mainly rain, but also includes snow, hail, dew and frost

Storage- water stored in the system

• Interception• Vegetation• Surface• Groundwater• Channel

Interception

• Interception is when some precipitation lands on vegetation or other structures, like buildings and concrete or tarmac surfaces, before it reaches the soil

• Interception creates a significant store of water in wooded areas

• Interception storage is only temporary because the collected water evaporates quickly

Vegetation

• Vegetation storage is water that’s been taken up by plants

• It’s all the water contained in plants at ny one time

Surface

• Surface storage includes water in puddles (depression storage), ponds and lakes

Groundwater

• Groundwater storage is water stored in the ground, either in the soil (soil moisture) or in rocks

• The water table is the top surface of the zone of saturation – the zone of soil or rock where all the pores in the soil or rock are full of water

• Porous rocks (rocks with holes in them) that hold water are called aquifers

Channel

• Channel storage is the water held in a river or stream channel

Flows and Processes- water moving from one place to another

• Surface runoff• Throughfall• Stemflow• Throughflow• Infiltration• Percolation• Groundwater flow• Baseflow• Interflow• Channel flow

Surface runoff

• Surface runoff/ overland flow is water flowing over the land

• It can flow over the whole surface or in little channels

• Surface runoff is common in arid areas where torrential rain falls on hard baked land

Throughfall

• Throughfall is water dripping from one (or other plant part) to another

Stemflow

• Stemflow is water running down a plant stem or tree trunk

Throughflow

• Throughflow is water moving slowly downhill through the soil

• Throughflow is faster through ‘pipes’- things like cracks in the soil or animal burrows

Infiltration

• Infiltration is water soaking into the soil• Infiltration rates are influenced by soil type,

soil structure and how much water’s already in the soil

• In a heavy storm, water can’t infiltrate fast enough, so it flows over the surface

Percolation

• Percolation is water seeping down through the soil into the water table

Groundwater flow

• Groundwater flow is water flowing slowly below the water table through permeable rock

• Water flows through most rocks, but rocks that are highly permeable with lots of joints (gaps that water can get through) can have faster groundwater flow, e.g. limestone

Baseflow

• Baseflow is groundwater flow that feeds into rivers through river banks and river beds

Interflow

• Interflow is water flowing downhill through permeable rock above the water table

Channel flow

• Channel flow is the water flowing in the river or stream itself

• This is also called the river’s discharge

Outputs- water leaving the system

• Evaporation• Transpiration• Evapotranspiration• River discharge

Evaporation

• Evaporation is water turning into water vapour- turning from a liquid to a gas

Transpiration

• Transpiration is evaporation from plant leaves- plants and trees take up water though their roots and transport it to their leaves where it evaporates it into the atmosphere

Evapotranspiration

• Evapotranspiration is the process of evaporation and transpiration together

Potential EvapotranspirationPET is the amount of water that could be lost by evapotranspirationActual evapotranspiration is what actually happense.g. in a desert, PET is high (because heat increases the amount of evaporation) but actual transpiration is low because there isn’t that much moisture in the first place

River discharge

• River discharge, or river flow is another output

The Water Balance

• The water balance shows the balance between inputs and outputs

• Water balance is worked out from inputs (precipitation) and outputs (channel discharge and evapotranspiration)

• The water balance affects how much is stored in the basin

The general water balance in the UK shows seasonal patterns:

1) In wet seasons, precipitation exceeds evapotranspiration. This creates a water surplus. The ground stores fill with water so there’s more surface runoff and higher discharge, so river levels rise

2) In drier seasons, precipitation is lower than evapotranspiration. Ground stores are depleted as some water is used (by plants and humans) and some flows into the river channel, but isn’t replaced by precipitation

3) So, at the end of a dry season, there’s a deficit of water in the ground. The ground stores are recharged in the next wet season i.e. autumn

Water balance for a temperate zone e.g. UK