The water cycle

  • Created by: barry
  • Created on: 22-05-18 20:55

Soil water budget

  • The soil water budget decribes the cange in the soil water storage during the course of the year
  • Soil water budget varies from place to place as it depends on the depth of soil but also the ease as to which water can percolate into the rock

Wet seasons

  • Percolation exceeds evaporation = water surplus.
  • Ground water stores fill with water
  • More run off:
    • Higher discharge
    • river levels rise

Drier seasons

  • Evaporation exceeds precipitation= ground store depletion
  • Some flows into the river channel (output) but is not replaced by precipitation

Back to wet seasons-

  • Deficit of water
  • Ground stores recharged
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Water balance

The water balance

This helps hydrologist to gain a better understanding of the hydrological characteristics of an individual drainage basin. This helps hydrologist plan for the future water supply and flood control by understanding the unique charastics of an indvidual basin.

P=O+E+/-S

  • P= Precipitation
  • O= tOtal run off
  • E= Evapotranspiration
  • S= Storage in soil and water

Variations of run off

  • Are caused by a difference in the amount of water in the sol, rock type (permeable rock) and if the land is vegetation covered (forestry).
  • Also, the time of year can affect evapotranspiration
  • The type and intensity of precipitation are also important, intense rainfall = pass quickly into rivers increasing run off.  
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Flood hydrograph

The storm hydrograph

Shows variations in a river's discharge over a short period of time, usually during a rainstorm.

  • The starting and finishing level show the base flow of the river.
  • As storm water enters the drainage basin the discharge rises, shown by the rising limb, to reach the peak discharge, which indicates the highest flow in the channel.
  • The receding limb shows the fall in the discharge back to the base level.
  • The time delay between maximum rainfall amount and peak discharge is the lag time.

 

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Flood hydrograph 2

Discharge variation:

  • Drainage basin size- small basin size leads to rapid water transfer
  • Drainage basin shape- steep slopes lead to rapid water transfer
  • Soil water- if the drainage basin is already saturated, then surface runoff increases 
  • Rock type- if the rock type within the river basin is impermeable surface runoff will be higher, throughflow and infiltration will also be reduced meaning a reduction in lag time and an increase in peak discharge.
  • Rainfall intensity- heavy rainfall may exceed the infiltration capacity of vegetation, which leads to rapid over land flow which reduces lag time

River regime

  • Shows the yearly dischargeg of a river 
  • In the summer season you will have low discharge due to high temperturesleading to quicker evapotranspiration as well as low rainfall however odd peak due to conventional rainfall due to summer storms
  • In the winter season there is a high river discharge as cooler tempertures and more tempertures leads to flucuation in river discharge
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The water cycle stores

Water stores:

  • 97.4%, is stores in oceans.
  • 3% water is fresh water stored as snow and ice.

Freshwater stores:

  • 69% in cyrosphere
  • 30% i lithosphere

Aquifers

Most commonly form in rocks such as chalk and sandstone which are porus rocks and permeable  leads to water to percolate into the rock. Many aquifers are being exploited unsustainably as more water is being extracted.

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Changes in the water cycle

Human causes- 

  • Land use change- Urbanisation results in impermeable surfaces which result in water can’t infiltrate the soil, which increases overland flow and makes flooding more likely. Deforestation reduces the amount water that is intercepted, increasing the amount that reaches the surface. In forested areas dead plant material on the forest floor helps to hold the water, allowing it to infiltrate they soil rather than run off.
  • Farming practises- Soils covered with plants have higher infiltration and soil water rates and therefore reduce run off. However, ploughing breaks up the surface so that more water can infiltrate, reducing the amount of water infiltrated. However, crops increase infiltation and interception compared to the bare ground reducing run off, evapotranspiration also increases, which can increase rainfall.
  • Water abstraction- The water is abstracted to meet demands in areas where population desinity is high. This reduces the amount of water stores like rivers, lakes and reservoirs. Consequently, it can result in low flow conditions in rivers.
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Changes in the water cycle 2

Natural causes

  • Natural variations in weather- Drought causes reduction in water stores in rivers and lakes. Vegetation dies which affects processes such as transpiration, interception and infiltration and soil water dries out which reduces the store and through flow stops. In winter there is greater quantities of precipitation which may make soils become more saturated leading to increased overland flow. Vegetation dies reducing interception and infiltration which all consequently results in a high river channel flow.
  • Storms-  intense rainfall increases the amount of rainfall reaching the ground and increasing the magnitude of stores, as infilltration capicty is reached and causes fast overland flow which can causes flooding in the river.

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The water cycle processes

Process:

Precipitation- transfer of water from the atmosphere to the ground, it can take the form of rain, snow, hail and sleet.

Evaporation- Transfer of water from liquid state to gas state (water vapour).

Condensation- Transfer of water from gaseous state to liquid state, eg formation of clouds.

Interception- water intercepted and stored on leaves of plants.

Infiltration- Transfer of water from the ground surface into soil where it may then percolate into underlying rocks.

Overland flow-  Transfer of water over the land surface.

Percolation- Water soaking into rocks.

Groundwater flow- Transfer of water very slowly through rocks.

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Types of precipitation

Orographic

Prevailing winds bring warm, moist air to the western British Isles. Air is forced to rise over high areas. Air cools and condenses. Clouds form and it rains. Air descends on the other side of the mountains. It warms up and hold more moisture and less rain

Frontal

When a cold air meets a warm air they do not mix – they form fronts. The colder air mass is heavier than the warmer air mass, therefore the lighter, warmer air rises over the top of the heavier, colder air. As the warm air is forced to rise it cools. Also, the warm air is in contact with the cold air along the fronts, and this also cools. Condensation occurs and clouds form. Rain occurs along the front.

Convectional

The moist air from over the oceans rises and cool. Condensation occurs forming clouds. If this process continues then rain will fall.

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The hill slope water cycle

Open system

The hill slope cycle (found within a drainage basin) and drainage basin are both open system and both have precipitation as their only input and some sort of run off as their output. Also, the way the water travels from the hillslope to the river as it travels by both overland and infiltration. 

Factors impacting water transfer:

  • Deforestation
  • Urbanisation
  • Farming
  • Storms
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Drainage basin

A drainage basin- is an area of land drained by a river and its tributaries. The edge is marked by a boundary called watershed. It is considerd as a open system with inputs and outputs.

Inputs:

Precipitation– all forms of moisture that reach the Earth’s surface e.g. rain, snow, sleet and hail.

Outputs

Run off- water leaving the river into the sea

Evapotranspiration – the loss of water from a drainage basin into the atmosphere from the leaves of plants + loss from evaporation.

 

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Drainage basin 2

Storage

  • Interception– lands on buildings, vegetation and concrete. Interception storage is only temporary as it is often quickly evaporated.
  • Vegetation storage– this is water taken up by vegetation. 
  • Surface storage– water held on the Earth’s surface in lakes, ponds and puddles.
  • Groundwater storage– the storage of water underground in permeable rock strata.
  • River storage- the water held in a river

Flows

  • Groundwater flow –  the deeper movement of water through underlying permeable rock 
  • Infiltration – the downward movement of water into the soil surface.
  • Percolation – the gravity flow of water within permeable rock.
  • Stemflow – water running down a plant stem or tree trunk.
  • Overland flow – the movement of water over the surface of the land
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Global atmospheric circulation

Within a Hadley cell air rises which creates clouds because the equator is warm which results in more evaporation. When the air descends no clouds are formed.

Cyrospheric water cycle glacier:

  • Cyrospheric processes such as accumlation and ablatioon change the amount of water stores as ice in the cyrosphere. The balance of accumlanation and ablation varies with tempertures
  • During period of global cold inputs are greater than outputs. Water is transferd in as snow and less water is transferrd in as snow, and less water is transferrd away due to melting.
  • During periods od warmer global tempertures as the magnitude of the cyrosphere store reduces are losses due to melting being larger than input of snow
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Feedback

Positive feedback

  • Precipitation increases due to more storm events (winter in UK)
  • Increased overland flow, throughflow groundwater flow
  • Amount of water in river store in a drainage basin increases
  • Potentially more evaporation, so more clouds, so more precipitation so accelerates initial change

Implications for life on Earth

  • More water in rivers and groundwater, so continuous supply of water for life from aquifers and rivers
  • Flooding

Implications for climate

  • •More clouds, so wetter
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Feedback 2

Negative feedback

  • Less water in peat bogs on Exmoor due to drainage ditches dug by farmers
  • Ditches blocked by Exmoor National Park
  • Amount of water in peat bogs increases as it is trapped
  • Peat is now waterlogged so lessens effect of original change

Implications for life on Earth

  • Peat bog restored – ecosystem supports flora and fauna
  • Less flooding

Implications for climate

  • More peat bog, so more carbon stored
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Case study

River Exe, Devon

The river Exe flow for 80km from its source in the hills of exmoor to the sea at exmouth. It has an extensive network of tributries and a high drainage density. 

Characterisitics- upper course:

  • Physical- the area of upper catchment is 600km2 and the land is much flatter
  • Geology- The area has an 84.4% of the catchment is impermeable rock and is Devonian sandstone.
  • Land use- most of the land is agricultural usage with some woodland and arable farmland, and peat pogs.
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