Lesson 4-What are drainage basins and what is the water balance? Flashcards
Local water cycle
- In a local drainage basin system , water may be lost as an output through evapotranspiration and runoff, but more water may be gained as an input through precipitation.
- As the inputs and outputs are not balanced, it is an open system
Global water cycle
- The global water cycle is a closed system.
- No water can enter or leave the system
however energy can. - The process that occur define how water is transferred and stored within the system.
- Water is always on the move.
What is a drainage basin?
- A drainage basin (or catchment) is an area of land which is drained by a river and its tributaries.
- The boundaries are marked by the watershed.
- Drainage basins can cover many square miles/km over a number of countries
e.g. the Nile River basin drainage area is more than 3 million km2 over 12 countries with 73 % of the drainage basin in Sudan and Egypt
Characteristics of a drainage basin?
- Watershed> seperate to drainage basins
- Source>start of the river
- Confluence> where two rivers meet
- Tributary>when the river splits
- Mouth>meets the sea
What is the water balance?
- The water balance helps to understand the behaviour and water balance of individual drainage basins
When does the water balance vary?
- Dry seasons
- Wet seasons
Seasonal changes in water balance- In wet seasons
- In wet seasons, precipitation is greater than evapotranspiration which creates a water surplus.
- Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels.
- This means there is apositive water balance.
Seasonal changes in water balance- In dry seasons
- In drier seasons evapotranspiration exceeds precipitation, as plants absorb water, ground stores are depleted.
- This produces a water deficit at the end of a dry season generating anegative water balance.
What is the soil water budget?
- This shows the balance between precipitation (inputs) and potential evapotranspiration (outputs) of soil as a store over the year
- In summer there is more evapotranspiration than precipitation
- In winter there is more precipitation than evapotranspiration
Soil moisture graph?
• Pink- Water surplus
• Yellow- Water deficit
• Orange- Precipitation
• Green- Evapotranspiration
Soil moisture graph for an equatorial climate
- The temperatures vary very little throughout the year.
- This means potential evapotranspiration remails relatively constant throughout the year.
- The high rainfall fills the soils rapidly. In the short times between rainfall maxima soil water does not go into defecit and so rivers and plants have a source of water all year round
Soil moisture graph for a tropical wet and dry climate
- The tropical wet/dry area experiences a dry season (July to October) where soils are reduced until there is a surplus.
- This surplus does not last long when rain stops
-There is a short period of utilisation but from December to July the soil is dry. River levels fall with some drying up.
- Vegetation has adapted to this seasonal water supply by evolving to survive drought conditions
How does water move through the drainage basin?
- Above ground flows
- Below ground flows
Above Ground Flows
- Throughfall
- Drip Flow
- Stem Flow
- Overland flow
- Hortonian overland flow
Below Ground Flows
- Infiltration
- Throughflow
- Percolation
- Base flow/ ground water flow
Above ground flows- Throughfall
Precipitation reaches the ground without ever been intercepted
Above ground flows- Drip Flow
Water flows off leaves to the ground
Above ground flows- Stem Flow
Water flows down tree trunks and stems to the ground
Above ground flows-Overland Flow
Water flows over the ground either in a river channel (channel flow) or as sheet flow over the surface of the ground
Above ground flows- Hortonian overland flow
- Water flows horizontally across land when rainfall has exceeded infiltration capacity and storage capacity.
- The water cannot enter the ground, so it runs straight off the surface (surface run off)
Below ground flows-Infiltration
Water flows vertically into the ground from the surface
Below ground flows- Throughflow
Water flows horizontally under the ground in the soil through lines of seepage called percaline
Below ground flows- Percolation
Water flows vertically though the soil layers
Below ground flows- Base flows/ ground water flow
Water that has infiltrated and percolated flows in saturated rocks (aquifers)
Inputs and outputs of the drainage basin?
- Inputs-Rain, sleet, snow
- These vary throughout the year due to seasonal variation
- Outputs-Evapotranspiration, transpiration, evaporation
Improtant flows within the water cycle?
- Infiltration
- Ground water flow
Factors affecting infiltration
- Interception
- Soil and porosity storage
- Strom events and rainfall intensity
Factors affecting infiltration- Interception
- Some precipitation will land on a bare surface, but much of it will beinterceptedby the leaves and branches of the vegetation.
- This is important, as it reduces the amount available for overland/surface flow and therefore, there is a reduction in soil erosion and flooding
.
-Coniferous trees intercept 22% of rainfall
-Deciduous trees intercept 19% of rainfall
Factors affecting infiltration- Soil porosity and storage
- Soil porosity is the most important factor, and is controlled by the following factors:
-Texture of the soil (proportions of sand, silt and clays)
-Structure of the soil (arrangement of the soil)
-Organic content of the soil
-Coarse-textured soils have larger pore spaces than fine-textured soils, - Many plants utilise the water stored in the soil (vegetation storage) for growth.
-As a consequence water is lost from the system through the process of transpiration.
Factors affecting infiltration-Storm events and rainfall intensity
- Many plants utilise the water stored in the soil (vegetation storage) for growth.
-As a consequence water is lost from the system through the process of transpiration. - If rainfall intensity is greater than the infiltration rate then the soil has reached infiltration capacity (i.e. the soil is saturated).
-As a result water will build up on the surface as surface storage (e.g. puddles). Much surface storage evaporates and is lost from the system.
Flow in water cycle-Ground water flow
- Percolation occurs and is then be held in pore spaces in the rocks as groundwater.
- It then passes slowly into the zone of saturated rock where it can then move laterally by the process of groundwater flow.
- This is a slow movement and can feed rivers through long periods of drought.
- Porous and permeable rocks are able to store a lot of water, these are known as aquifers.
