Drainage basins

Question 1

What physical factors influence the drainage basin system ?

Answer 1

1. Climate - This hugely impacts the main input (precipitation) and output (evaporation) in the drainage basin system - thus influencing all flows and stores./ it also has specific impacts on certain stores and flows. E.G The climate can influence the vegetation that lives in the drainage basin - this therefore influences interception and infiltration.
2. Soils - Soils types vary from, place to place influencing the rate of infiltration and through flow
3. Geology - Permeable rocks (which can be porous or pervious0 allow water to percolate downward whilst impermeable rocks don’t. This influence the stores of groundwater as well as the height of the water table.
4. Relief - A steeper relief will result in faster run off. Altitude way also affect precipitation.
5. Vegetation - Areas with thick vegetation cover will have higher levels of infiltration and evapotranspiration. the type of vegetation may also influence the rate of infiltration as roots provide pathways for water to infiltrate the soil.

Question 2

What human factors influence the drainage basin system ?

Answer 2

1. Deforestation - This reduces interception and infiltration rates. thus increasing the rate of surface runoff - increasing risk of floods (as surface runoff is a faster flow of water into a river channel than throughflow or groundwater flow - meaning the river discharge reaches capacity far quicker)
2. River management - Constructing dams and reservoirs allows humans to control river discharge. Channelisation increases discharge by aiding water flow. This may increase the risk of flooding downstream as sections of the river which haven’t been managed may not have the capacity for higher discharge. Diverting channels for irrigation reduces total discharge.
3. Groundwater abstraction - Lowers water tables
4. Changing land use - agriculture - pastoral land (used for rearing livestock) has lower rates of infiltration as livestock compact the soil. this means higher rates of surface runoff and greater risk of flooding, Ploughing to form arable land increases interception.
5. Urbanisation - This involves deforestation (decreasing infiltration and interception) as well as the creation of impermeable concrete surfaces on soils. This means infiltration and percolation can no longer occur so higher rates of overland flow. Storm drains and sewage rapidly deliver precipitation to rivers and streams. the overall effect of this is an increased risk of flooding.

Question 3

Impacts of deforestation on Amazonia

Answer 3

Reduced evapotranspiration - in a typical rainforest rea, 75% of intercepted precipitation is returned to the atmosphere by evapotranspiration. this falls to 25% when the forest is cleared. This could lead to desiccation and degradation of the rainforest. it also impacts ENSO - increasing the risk of drought - causing forest fires.

Additionally, an increase in surface run off increases the risk of mudslides and flooding. It also means that, as less infiltration and percolation occurs, aquifers will be depleted as there is little water cannot reach them to recharge them. Floods cause soil erosion which degrades the soil - decreasing environmental productivity. This not only has an impact on the rainforest itself but also cattle ranching and commercial agriculture businesses - the largest causes of deforestation in Amazonia.