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2.1.2 Catchment hydrology – the drainage basin as a system - Coggle Diagram
2.1.2 Catchment hydrology – the drainage basin as a system
The Water Balance
The equation for water balance is P= Q +ET +/- S (Precipitation = Channel flow + EvapoTranspiration +/- Storage)
Water Surplus - When precipitation exceeds evapotranspiration and the excess is not being used by plants, there is a water surplus.
Water Defecit- When precipitation is below evapotranspiration, storage is reduced (water table will drop).
Precipitation in excess
Soil moisture utilisation- a time when water is withdrawn from soil moisture storage. This happens when potential evapotranspiration exceeds precipitation, e.g. in the summer.
Soil moisture recharge- when water is added to soil moisture storage. The recharge period occurs when precipitation exceeds potential evapotranspiration but the soil has yet to reach its field capacity, e.g. the autumn/ winter.
Field Capacity- The maximum amount of water that soil can hold before it becomes saturated, e.g. winter = more floods are likely as evapotranspiration is low, and the soil is full of water.
Case Studies
River Exe
Water Balance there- Precipitation (1295mm)= Evaporation +/- Soil water storage (451mm) + runoff (844mm)
Human activities affecting the River Exe and the water cycle
Peatland restoration on Exmoor
Since the late 1800s farmers have drained the moorland in the upper catchment area of the River Exe. This has increased the speed of water flow to the Exe, and reduced water quality as more silt is carried downstream. Peat has also been dug out as fuel. As the peat surface dried out, decomposition occurred, releasing carbon. This carbon store is now a source of carbon emissions in the form of carbon dioxide and methane.
The Exmoor Mires Project
this works to restore the peat bogs (mires) by blocking the drainage ditches with peat blocks or moorland bales. This increases water content and returns the ground to the saturated, boggy conditions that would naturally occur in this moorland environment. These saturated conditions help to retain carbon stored within the peat. The project aims to restore 2000ha of Exmoor.
Benefits of this are that there will be more water storage in the upper catchments, improved water quality, more carbon storage, improved opportunities for education, leisure and recreation and improved grazing and water supply
The Amazon
Rainfall across the Amazon is very high. Average rainfall across the whole Amazon basin is approximately 2300 mm annually. In some areas of the northwest portion of the Amazon basin, yearly rainfall can exceed 6000 mm
It is thought that Up to half of the rainfall in some areas may never reach the ground, being intercepted by the forest and re-evaporated into the atmosphere and that additional evaporation occurs from ground and river surfaces, or is released into the atmosphere by transpiration
This moisture contributes to the formation of rain clouds, which release the water back onto the rainforest. In the Amazon, 50-80 percent of moisture remains in the ecosystem’s water cycle.
The future of the amazon
More CO2 in the atmosphere has led to greater photosynthesis, and the forest growing at a faster rate, reaching maturity sooner than would usually occur. The trees are also taller, and can become more vulnerable to high winds and drought. Droughts are becoming more frequent, due to changes in rainfall patterns
In the eastern Amazon it is feared that clearance beyond 30% may trigger a decline in rainfall by 20% by 2100. This will create a positive feedback loop where the dynamic equilibrium will be irreparably altered. Experts are predicting the rainforest will be replaced by savanna-like vegetation as it becomes semi-arid.