Understanding the Water Cycle: Processes and Mechanisms
The water cycle involves various processes such as air uplift, condensation, and precipitation. Cloud formation, rainfall mechanisms like orographic and frontal rain, and the Bergeron-Findeisen process are explained. Activities to understand condensation on windows and cloud precipitation are included. The interaction of different mechanisms in weather systems is discussed.
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WATER CYCLE 4: precipitation and runoff Cloud droplets Air uplift Condensation Precipitation Excess runoff We are grateful to Hodder for their permission to use the diagrams in this resource. WJEC Focus Box 3.1.4 Eduqas Focus Box 2.1.4
Water Cycle 4 Air uplift, condensation, clouds Air uplift leads to cooling and condensation because of the fall in pressure with altitude This results in of the expansion of the air As a result, there are fewer collisions between air molecules: this reduces the amount of heat energy per unit volume and air temperature falls Cloud formation occurs when temperature is low enough for vapour to condense into water droplets Class activity Why does condensation occur on windows? Why is your breath visible when you exhale on cold days?
Water Cycle 4 Air uplift (c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]
The feeder-seeder mechanism involves water droplets from high altitude seeder clouds falling through a lower- level orographic stratus cloud (the feeder cloud), collecting more cloud water as they do so, which results in heavier rainfall Water Cycle 4 Air uplift mechanisms Orographic rainfall is related to relief features, augmented by the feeder-seeder mechanism Convectional rain results from intense daytime heating of the land Frontal rain forms when two surface air streams meet e.g. when polar (cold) and tropical (warm) air masses meet over the North Atlantic ocean Class activity Think about how the three mechanisms could interact, e.g. when a weather front crosses a relief barrier
Water Cycle 4 Precipitation formation Air uplift and condensation does not always lead to precipitation; further physical processes must operate Cloud droplets Air uplift Condensation Precipitation Class activity Is it possible to tell which clouds will produce rain or snow? What evidence do you look for?
Water Cycle 4 Bergeron-Findeisen process Clouds at high altitude contain a mixture of water droplets and ice crystals; the ice crystals grow rapidly by attracting vapour from the water droplets Eventually, the ice crystals become too large to be held aloft; in falling to the ground, they pass through warmer air and melt to produce rain Class activity Watch this short film: https://www. youtube.com /watch?v=- UXpJ3KRO_sl
Water Cycle 4 Collision process A second theory explains rainfall in the warm tropics Super-sized condensation nuclei, e.g. large sea salt particles, provide seeds around which very large water droplets form The larger super droplets fall and collide with smaller droplets, absorbing them In support of this theory, unexpected downpours and flash flooding in arid areas demonstrate how high numbers of large raindrops can be generated quickly Class activity Watch this short film: https://www.youtube.com/watch?v=WByQ3yKxj1s
Water Cycle 4 Excess runoff generation (1) Runoff is all of the rainwater that contributes to river discharge following a rainfall event Three drainage basin water flows contribute to runoff: overland flow, throughflow and groundwater flow The balance between these three water pathways determines how flashy the river response is; and thus any likelihood of flooding Class activity Research how excess runoff results from (1) monsoon rainfall (2) UK storm events and (3) snowmelt (show your results in a table)
Water Cycle 4 Excess runoff generation (2) Prior discharge Litres per second Peak discharge Litres per second Storm Precipitation Storm amount (mm) Average intensity (mm / hr) Maximum intensity (mm / hr) A B C D 12 11 30 16 3 2 3 1 10 3 4 3 57 70 9 79 1034 694 1019 665 Class activity 1. Explain the variations in peak discharge for each of the four storm events 2. State and comment on three factors affecting run-off
Water Cycle 4 Land use changes and runoff (1) Urbanisation renders previously permeable ground surfaces impermeable Surfaces like concrete and tarmac increase overland flow generation and decrease the effectiveness of infiltration, throughflow and soil storage The more ground that is covered by impermeable hard surfaces such as concrete or paving slabs, the less rainfall will soak into the ground and the more will flow over the surface into drains and sewers Class activity Vegetation is removed to make room for urban areas. Why does this also contribute to increased runoff?
Water Cycle 4 Land use changes and runoff (2) In England, Wales and Scotland, much of the forest which once covered the land has been removed Flood risk in some parts of the UK is undoubtedly higher as a result of these changes One study in mid-Wales found that rainwater s infiltration rate into the soil was 67 times higher under trees than under sheep pasture Class activity What other examples are there of recent floods in the UK? Might greater forest cover have prevented them?
Water Cycle 4 River basin mismanagement (c) Hodder Education [Edexcel A- level Geography Book 2 (Dunn)] Class activity Describe and explain the hydrological changes resulting from river basin mismanagement asshown above
Water Cycle 4 Plenary There are three main causes of air uplift, condensation and cloud formation: they are the orographic, frontal and convection mechanisms There are two recognized theories of precipitation formation: the Bergeron-Findeisen process and the collision process Excess runoff generation occurs naturally in many parts of the world for climatic reasons. These include monsoon rainfall and annual snowmelt Increasingly, human factors are to blame for overland flow generation in many drainage basins. Two important causes are urbanisation and forest removal
