WATER CYCLE 4: precipitation and runoff - PowerPoint Presentation

1. WATER CYCLE 4: precipitation and runoffWJEC Focus Box 3.1.4Eduqas Focus Box 2.1.4 We are grateful to Hodder for their permission to use the diagrams in this resource.

2. Water Cycle 4Air uplift, condensation, cloudsAir uplift leads to cooling and condensation because of the fall in pressure with altitudeThis results in of the expansion of the airAs a result, there are fewer collisions between air molecules: this reduces the amount of heat energy per unit volume and air temperature fallsCloud formation occurs when temperature is low enough for vapour to condense into water dropletsClass activityWhy does condensation occur on windows? Why is your breath visible when you exhale on cold days?

3. Water Cycle 4Air uplift (c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]

4. Water Cycle 4Air uplift mechanismsClass activityThink about how the three mechanisms could interact, e.g. when a weather front crosses a relief barrierOrographic rainfall is related to relief features, augmented by the feeder-seeder mechanismConvectional rain results from intense daytime heating of the landFrontal rain forms when two surface air streams meet e.g. when polar (cold) and tropical (warm) air masses meet over the North Atlantic ocean 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

5. Water Cycle 4Precipitation formationAir uplift and condensation does not always lead to precipitation; further physical processes must operateClass activityIs it possible to tell which clouds will produce rain or snow? What evidence do you look for?

6. Water Cycle 4Bergeron-Findeisen processClouds at high altitude contain a mixture of water droplets and ice crystals; the ice crystals grow rapidly by attracting vapour from the water dropletsEventually, the ice crystals become too large to be held aloft; in falling to the ground, they pass through warmer air and melt to produce rainClass activityWatch this short film: https://www.youtube.com/watch?v=-UXpJ3KRO_sl

7. Water Cycle 4Collision processA 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 formThe 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 quicklyClass activityWatch this short film: https://www.youtube.com/watch?v=WByQ3yKxj1s

8. Water Cycle 4Excess runoff generation (1)Runoff is all of the rainwater that contributes to river discharge following a rainfall eventThree drainage basin water flows contribute to runoff: overland flow, throughflow and groundwater flowThe balance between these three water pathways determines how flashy the river response is; and thus any likelihood of floodingClass activityResearch how excess runoff results from (1) monsoon rainfall (2) UK storm events and (3) snowmelt (show your results in a table)

9. Water Cycle 4Excess runoff generation (2)Class activity1. Explain the variations in peak discharge for each of the four storm events  2. State and comment on three factors affecting run-offStormPrecipitationPrior dischargePeak discharge Storm amount (mm)Average intensity (mm / hr)Maximum intensity (mm / hr)Litres per secondLitres per secondA12310571034B112 370 694C303 4 91019D161 379 665

10. Water Cycle 4Land use changes and runoff (1)Urbanisation renders previously permeable ground surfaces impermeableSurfaces like concrete and tarmac increase overland flow generation and decrease the effectiveness of infiltration, throughflow and soil storageThe 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 sewersClass activityVegetation is removed to make room for urban areas. Why does this also contribute to increased runoff?

11. Water Cycle 4Land use changes and runoff (2)In England, Wales and Scotland, much of the forest which once covered the land has been removedFlood 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 activityWhat other examples are there of recent floods in the UK? Might greater forest cover have prevented them?

12. Class activityDescribe and explain the hydrological changes resulting from river basin mismanagement as shown aboveWater Cycle 4River basin mismanagement(c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]

13. Water Cycle 4PlenaryThere are three main causes of air uplift, condensation and cloud formation: they are the orographic, frontal and convection mechanismsThere are two recognized theories of precipitation formation: the Bergeron-Findeisen process and the collision processExcess runoff generation occurs naturally in many parts of the world for climatic reasons. These include monsoon rainfall and annual snowmeltIncreasingly, human factors are to blame for overland flow generation in many drainage basins. Two important causes are urbanisation and forest removal