The drainage basin is the catchment area fom which a river system obtains its water.
An imaginary line called the watershed delimits one drainage basin from another. The watershed generally follows a ridge of high land; any rain falling on the other side of the ridge will eventually flow into another river in the adjacent drainage basin.
The drainage basin hydrologicals system is an open ysem with inputs and outputs.
Inputs into the drainage basin include:
· Energy from the sun for evaporation
· Precipitation – rain & snow
Outputs move moisture out of the drainage basin and include:
· Evaporation and transpiration from plants (collectively called evapo-transpiration)
· Runoff into the sea
· Water percolating deep into underground stores where it can be effectvely lost from the system
Stores of water
· On the surface – glaciers, lakes, rivers, puddles
· Vegetation stores water by interception and plants
· The soil can hold water
· Groundwater is stored in permeable rocks
Transfers and flows- moves water through the system and enable inputs of water to be processed from one store to another.
· Transfers include throughfall, stemflow, infiltration, throughflow and groundwater flow.
Water balance or budget
Within the drainage basin, the balance between inputs (precipitation) and outputs (runoff, evapo-transpiration, soil and groundwater storage) is known as the water balance or budget. Rivers are presnet on the surfaxce only if the stores are capable of releasing water and if there is direct surface runoff. This is a dynamic relationship; river levels rise and fall over the short term follwoing heavy rainfall and often show an annual pattern (called the river’s regime) in terms of their discharge.
The water balance can be shown using the formula below:
Precipitation (P) = streamflow (Q) + evapo-transpiration (E) + or – changes in storage (S)
P = Q + E + or – S
When precipitation is greater than evapo-transpiration, at fries the pores of the soil are refilled with water. When the soil becomes saturated, excess water has difficulty infiltrating into the groun and may then flow over the surface.
The water balance of a particular location can be studied using a water budget graph (see below). This shows the relationship between temperature, precipitation and evaporation rates over the year. During the months of the year when precipitation exceeds evapo-transpiration, once the soil has been recharged there will be a water surplus available to supply rivers and streams.