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Teachers can: ● Use the Lesson Builder to plan and deliver outstanding lessons ● Share lessons and resources with students and colleagues ● Track student progress with Tests and Assessment Teachers can also combine their own trusted resources alongside those from A-level Geography which has a whole host of informative and interactive resources including: ● Key content PowerPoints with lesson starters, discussion activities and tasks that can be adapted to suit students’ varying needs ● Ready-made worksheets to support the key content PowerPoints ● Self-marking knowledge-check tests that allow students to quickly identify areas for revision Exam Question Practice helps prepare students for assessment through practice questions, sample answers and comments from teachers with examining experience. Teachers can: ● Work through questions as a class ● Assign questions to individual students ● Improve students’ confidence with exam and revision advice Edexcel A level Geography Book 2 Third Edition is available as a Whiteboard eTextbook which is an online interactive version of the printed textbook that enables teachers to: ● Download and view on any device or browser ● Add edit and synchronise notes across two devices ● Access their personal copy on the move Additionally the Student eTextbook of Edexcel A level Geography Book 2 Third Edition is a downloadable version of the printed textbook that teachers can assign to students so they can: ● Download and view on any device or browser ● Add edit and synchronise notes across two devices ● Access their personal copy on the move To find out more and sign up for free trials visit: www.hoddereducation.co.uk/dynamiclearning GEOGRAPHY EDEXCEL A LEVEL BOOK 2 2 Third Edition CAMERON DUNN, KIM ADAMS, DAVID HOLMES, SIMON OAKES, SUE WARN, MICHAEL WITHERICK Although every effort has been made to ensure that website addresses are correct at time of going to press, Hodder Education cannot be held responsible for the content of any website mentioned in this book. 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You can also order through our website: www.hoddereducation.co.uk ISBN: 978 1 4718 5653 2 © Cameron Dunn, Kim Adams, David Holmes, Simon Oakes, Sue Warn, Michael Witherick 2017 First published in 2017 by Hodder Education, An Hachette UK Company Carmelite House 50 Victoria Embankment London EC4Y 0DZ www.hoddereducation.co.uk Impression number 10 9 8 7 6 5 4 3 2 1 Year 2021 2020 2019 2018 2017 All rights reserved. Apart from any use permitted under UK copyright law, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or held within any information storage and retrieval system, without permission in writing from the publisher or under licence from the Copyright Licensing Agency Limited. Further details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited, Saffron House, 6–10 Kirby Street, London EC1N 8TS. Cover photo © Marcel Schauer – Fotolia Illustrations by Aptara Inc. Typeset in India by Aptara Inc. Printed in Italy A catalogue record for this title is available from the British Library. In order to ensure that this resource offers high-quality support for the associated Pearson qualification, it has been through a review process by the awarding body. This process confirms that this resource fully covers the teaching and learning content of the specification or part of a specification at which it is aimed. It also confirms that it demonstrates an appropriate balance between the development of subject skills, knowledge and understanding, in addition to preparation for assessment. Endorsement does not cover any guidance on assessment activities or processes (e.g. practice questions or advice on how to answer assessment questions) included in the resource nor does it prescribe any particular approach to the teaching or delivery of a related course. While the publishers have made every attempt to ensure that advice on the qualification and its assessment is accurate, the official specification and associated assessment guidance materials are the only authoritative source of information and should always be referred to for definitive guidance. Pearson examiners have not contributed to any sections in this resource relevant to examination papers for which they have responsibility. Examiners will not use endorsed resources as a source of material for any assessment set by Pearson. Endorsement of a resource does not mean that the resource is required to achieve this Pearson qualification, nor does it mean that it is the only suitable material available to support the qualification, and any resource lists produced by the awarding body shall include this and other appropriate resources. CONTENTS Introduction iv Unit 3: Physical Systems and Sustainability Topic 5: The Water Cycle and Water Insecurity Chapter 1: The operation and importance of the hydrological cycle 2 Chapter 2: Short- and long-term variations in the hydrological cycle 23 Chapter 3: Water security – is there a crisis? 45 Topic 6: The Carbon Cycle and Energy Security Chapter 4: The carbon cycle and planetary health 78 Chapter 5: Consequences of the increasing demand for energy 94 Chapter 6: Human threats to the global climate system 111 Unit 4: Human Systems and Geopolitics Topic 7: Superpowers Chapter 7: What are superpowers? 132 Chapter 8: Superpower impacts 145 Chapter 9: Superpower spheres of infl uence 159 Topic 8: Global Development and Connections Option 8A: Health, Human Rights and Intervention Chapter 10: Human development 174 Chapter 11: Human rights 191 Chapter 12: Interventions and human rights 204 Chapter 13: The outcomes of geopolitical interventions 219 Option 8B: Migration, Identity and Sovereignty Chapter 14: The impacts of globalisation on international migration 234 Chapter 15: Nation states in a globalised world 249 Chapter 16: Global organisations and their impacts 265 Chapter 17: Threats to state sovereignty 279 Chapter 18: Synoptic themes 292 Chapter 19: Independent investigation 304 Glossary 330 Index 338 INTRODUCTION This book has been written specifically for the new Edexcel specification introduced for first teaching in September 2016. The writers are all experienced authors, teachers and subject specialists with experience of examining. This book has been designed to cover the specification in a comprehensive, interesting and informative way. Edexcel A level Geography Book 2 covers the content which is only tested at A level. For most of you, this will mean what you cover in class in Year 13. For your A level exams you also need to cover the content in Book 1. All of the compulsory topics, as well as all of the options, are covered in this book. There is also a section on the Independent Investigation coursework (Chapter 19) to help you plan and complete that piece of work. Chapter 18 covers Synoptic Themes. These include the three synoptic themes in the specification: l Actions and attitudes l Players l Futures and uncertainties Chapter 18 also explains some specialist geographical concepts which cut across different topics. Introduction iv At the end of your two-year A level course you will sit three examinations and submit your Individual Investigation coursework. The tables below summarise these exams and show how they link to Books 1 and 2: A level Paper 1 l 2 hours, 15 minutes l 105 marks Section A Topic 1: Tectonic Processes and Hazards 16 marks Book 1 Section B Topic 2: Landscape Systems, Processes and Change (either Topic 2A Glaciated Landscapes and Change or Topic 2B Coastal Landscapes and Change) 40 marks Book 1 Section C Topic 5: The Water Cycle and Water Insecurity Topic 6: The Carbon Cycle and Energy Security 49 marks Book 2 Book 2 Summary of the speci cation and its coverage in Edexcel A level Geography Book 1 and Book 2 Book 1 Year 12 Compulsory content Topic 1: Tectonic Processes and Hazards Topic 3: Globalisation Year 12 Optional content Topic 2: Landscape Systems, Processes and Change Study one of these topics: l Topic 2A Glaciated Landscapes and Change l Topic 2B Coastal Landscapes and Change Topic 4: Shaping Places Study one of these topics: l Topic 4A Regenerating Places l Topic 4B Diverse Places Book 2 Year 13 Compulsory content Topic 5: The Water Cycle and Water Insecurity Topic 6: The Carbon Cycle and Energy Security Topic 7: Superpowers Individual Investigation Year 13 Optional content Topic 8: Global Development and Connections Study one of these topics: l Topic 8A Health, Human Rights and Intervention l Topic 8B Migration, Identity and Sovereignty v Introduction A level Paper 2 l 2 hours, 15 minutes l 105 marks Section A Topic 3: Globalisation Topic 7: Superpowers 32 marks Book 1 Book 2 Section B Topic 4: Shaping Places (either Topic 4A Regenerating Places or Topic 4B Diverse Places) 35 marks Book 1 Section C Topic 8: Global Development and Connections (either Topic 8A Health, Human Rights and Intervention or Topic 8B Migration, Identity and Sovereignty) 38 marks Book 2 A level Paper 3 l 2 hours, 15 minutes l 70 marks This exam paper is an Issues Analysis using an unseen resource booklet. It tests your understanding of different, linked parts of the whole two-year A level course 70 marks Book 2, Chapter 18 and the synoptic links in Books 1 and 2 A level Paper 4 l Individual Investigation coursework l 70 marks The coursework component is your own Individual Investigation. Your will choose your own topic, meaning it could link to either your Year 12 or Year 13 studies, or both 70 marks Book 2, Chapter 19 and relevant topics in Books 1 and/or 2 The three exams and your Individual Investigation contribute to your A level in this way: Paper 1 (Physical geography) 30% of the total marks Paper 2 (Human geography) 30% of the total marks Paper 3 (Synoptic issues analysis) 20% of the total marks Paper 4 (Coursework) 20% of the total marks Each chapter in this book covers one enquiry question in a topic. Within each chapter, each of the Key Ideas from the specification is covered in detail using a combination of text, figures and easy-to-access tables. There is also a range of features in each chapter designed to boost your skills, understanding and confidence, presented in an interesting and accessible way. These will also help you revise and prepare for the exams: l An introduction to each chapter gives you an overview of what is covered in that chapter. l Key terms are defined throughout to help increase your geographical vocabulary. l Key concepts explain important ideas and theories and how to apply them. l Skills focus features cover the compulsory skills in the specification, linked to a particular sub-topic. These skills can be tested in the exams so are important to understand and practice. l Synoptic themes in all topics indicate when content and examples need to be related to the synoptic themes of Actions and attitudes, Players, and Futures and uncertainties. This means thinking across topics so that you see links to other areas of study. l Place contexts are indicated with a globe symbol and show how an idea or theme can be applied to a particular place example. These link to the place contexts in the specification. l Fieldwork opportunities suggest ideas for carrying out fieldwork which could form part of your Individual Investigation. l A range of photographs, maps and graphs help you develop your data response skills. l Further research ideas are provided at the end of each chapter in the form of websites you could use to take some ideas further, perhaps as part of your Individual Investigation or to deepen your understanding. l Review questions at the end of each chapter are designed to enhance your understanding of key ideas and allow you to test that understanding. l Exam-style questions have been designed to provide practice with exam questions in the format in which they will be presented in your final exams. 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All rights reserved.; p.328 © David Holmes Acknowledgements p.31 Environmental Hazards , K. Smith, © 2013, Routledge. Reproduced with permission of Taylor & Francis Books UK; p.43, WWAP (United Nations World Water Assessment Programme) 2009. The United Nations World Water Development Report 3:Water in a Changing World . London/Paris, Earthscan/UNESCO. http://unesdoc.unesco.org/images/0018/001819/181993e.pdf; p.58 & p.72 The Atlas of Water: Mapping the World’s Most Critical Resource (The Earthscan Atlas) © 2013, Routledge. Reproduced with permission of Taylor & Francis Books UK; p.102 WCA, 2009: The Coal Resource: A Comprehensive Overview of Coal World Coal Association, London, UK; p.122 ‘Climate change: processes, characteristics and threats’. Used by permission of GRID-Arendal; p.127 Cornell, H. 2011. ‘Driver: Climate Change in the Salish Sea Ecosystem’ in Puget Sound Science Review . Puget Sound Partnership. Tacoma, Washington, U.S.A. Accessed from www.eopugetsound.org/science-review/section-2-driver-climate-change-salish-sea-ecosystem; p.228 United Nations Peacekeeping Operations, Map No. 4259 Rev.21 (E) UNITED NATIONS, October 2011 (http://www.un.org/Depts/Cartographic/map/dpko/PKOBN.pdf); p.237, Andrew Taylor 2008, ‘Workers of the world on the move,’ FT.com, 24 June. Used under license from the Financial Times . All Rights Reserved; p.241, 2015 ‘Migrant crisis explained in numbers,’ FT.com. Used under license from the Financial Times . All Rights Reserved; p.243, ‘Net migration flows between London and other settlements 2009–12’ © Guardian News & Media Ltd, 2016; p.246, Barney Jopson, 2014 Used under license from the Financial Times . All Rights Reserved; p.247 Used under license from the Financial Times . All Rights Reserved; p.252 Used under license from the Financial Times. All Rights Reserved; p.276 © SPUTNIK / sputniknews.com; p. 277 Infographic by Column Five for GOOD; p.274 The Guardian , 15 December 2015 © Guardian News & Media Ltd Every effort has been made to trace all copyright holders, but if any have been inadvertently overlooked, the Publishers will be pleased to make the necessary arrangements at the first opportunity. Chapter 1: The operation and importance of the hydrological cycle Chapter 2: Short- and long-term variations in the hydrological cycle Chapter 3: Water security – is there a crisis? The Water Cycle and Water Insecurity Topic 5 The Water Cycle and Water Insecurity 2 1.1 The operation of the hydrological cycle at a global scale In order to understand the operation of the hydrological cycle (also known as the natural water cycle) a systems approach is useful. Three concepts are key to understanding how water cycling operates: 1 Stores (stocks), which are reservoirs where water is held, such as the oceans. 2 Fluxes , which measure the rate of flow between the stores. 3 Processes , which are the physical mechanisms which drive the fluxes of water between the stores. The global hydrological cycle The global hydrological cycle is an example of a closed system driven by solar energy and gravitational potential energy. In a closed system there is a fi xed amount of water in the Earth–atmosphere system (estimated at 1385 million km 3 ). A closed system does not have any external inputs or outputs, so this total volume of water is constant and finite. However, the water can exist in different states within the closed system (liquid, vapour and solid) and the proportions held in each state can vary for both physical and human reasons. For example, in the last Ice Age more water was held within the cryosphere in a solid form as snow and ice; as less was held in the oceans, sea levels dropped considerably – over 140 m lower than they are today. Recent climate warming is beginning to reverse this with major losses of ice in Greenland and, more recently, Antarctica, and significant rises in sea level (see page 37 for the impacts of climate warming). At a small scale, humans have built numerous water storage reservoirs to complement natural lakes in order to increase the security of their water supplies. Figure 1.1 shows how the global hydrological cycle works. Essentially there are four major stores of water, of which the oceans are by far the largest: they contain an estimated 96.5 to 97 per cent of the world’s total water. The next largest stores occur in the cryosphere (1.9 per cent), and then terrestrial surface groundwater. The atmosphere is by far the smallest of the significant stores. Table 1.1 shows a recent estimate of the size of these stores. 1 The operation and importance of the hydrological cycle What are the processes operating within the hydrological cycle from global to local scale? By the end of this chapter you will: l understand the importance of the hydrological cycle in supporting life on Earth and how it operates at a range of spatial and temporal scales l know that the global hydrological cycle is a closed system and that the drainage basin, a subsystem within it, is an open system l understand how inputs, stores, flows and outputs operate within systems l understand how inputs, stores, flows and outputs contribute to contrasting water budgets, river regimes and storm hydrographs at a more local scale. Key terms Systems approach: Systems approaches study hydrological phenomena by looking at the balance of inputs and outputs, and how water is moved between stores by flows. Stores: Reservoirs where water is held, such as the oceans. Fluxes: The rate of flow between the stores. Processes: The physical mechanisms that drive the fluxes of water between the stores. Cryosphere: Areas of the Earth where water is frozen into snow or ice. 3 1 The operation and importance of the hydrological cycle Figure 1.1 The global water cycle Cryospheric processes Atmosphere 4 1 40 3 73 113 40 Vapour transport 413 373 Evaporation Precipitation Evaporation Transpiration Ocean 1 Stores Flows Fluxes in 10 3 km 3 /year Fluxes Precipitation Percolation Groundwater Surface run-off Soil 2 In the oceans the vast majority of water is stored in liquid form, with only a minute fraction as icebergs On land the water is stored in rivers, streams, lakes and groundwater in liquid form. It is often known as blue water, the visible part of the hydrological cycle. Water can also be stored in vegetation after interception or beneath the surface in the soil. Water stored in the soil and vegetation is often known as green water, the invisible part of the hydrological cycle. In the cryosphere water is largely found in a solid state, with some in liquid form as melt water and lakes. Water largely exists as vapour in the atmosphere, with the carrying capacity directly linked to temperature. Clouds can contain minute droplets of liquid water or, at a high altitude, ice crystals, both of which are a precursor to rain. Table 1.1 Details of the main global water stores; note that numbers are rounded so the totals may not add up to 100 Store Volume (10³ km³) Percentage of total water Percentage of freshwater Residence time Oceans 1,335,040 96.9 0 3,600 years Icecaps 26,350 1.9 68.7 15,000 years depending on size Groundwater 15,300 1.1 30.1 Up to 10,000 years for deep groundwater; 100–200 years for shallow groundwater Rivers and lakes 178 0.01 1.2 2 weeks to 10 years; 50 years for very large scale Soil moisture 122 0.01 0.05 2–50 weeks Atmospheric moisture 13 0.001 0.04 10 days Key terms Blue water: Water is stored in rivers, streams, lakes and groundwater in liquid form (the visible part of the hydrological cycle). Green water: Water stored in the soil and vegetation (the invisible part of the hydrological cycle). 1 2 3 4 The Water Cycle and Water Insecurity 4 In Figure 1.1 (page 3) the major fluxes are shown, driven by key processes such as precipitation , evaporation , cryospheric exchange, and a run-off generation (both surface and groundwater). These fluxes have been quantified, with the most important being evaporation from the oceans and precipitation on to land and the oceans. Table 1.1 (page 3) allows you to compare residence times These are the average times (it is an estimate, hence you will find considerable variation depending on source used) a water molecule will spend in that reservoir or store. Residence times impact on turnover within the water cycle system. Groundwater, if it is deep seated, can spend over 10,000 years beneath the Earth’s surface. Some ancient groundwater, such as that found deep below the Sahara Desert – the result of former pluvial (wetter) periods – is termed fossil water and is not renewable or reachable for human use. Major ice sheets too (such as Antarctica and Greenland) store water as ice for very long periods, so the figures in the table represent an average. Ice core dating has suggested that the residence time of some water in Antarctic ice is over 800,000 years. Conversely, some very accessible stores, such as soil moisture, and small lakes and rivers, have much shorter residence times. Water stored in the soil, for example, remains there very briefly as it is spread very thinly across the Earth. Because of its accessibility it is easily lost to other stores by evaporation, transpiration , groundwater flow or recharge. Atmospheric water has the shortest residence time of all, about ten days, as it soon evaporates, condenses and falls to the Earth as precipitation. There is a strong link between residence times and levels of water pollution: stores with a slower turnover tend to be more easily polluted as the water is in situ for a longer length of time. Accessible water for human life support Figure 1.2a–c summarises where the Earth’s total global water is stored, with an overwhelming 96 to 97 per cent stored in the oceans – only around 2.5 per cent occurs as fresh water. Figure 1.2d looks at the Earth’s fresh water supply. Around 69 per cent is locked up in snowflakes, ice sheets, ice caps and glaciers found in high latitudes and high- altitude locations. This water supply is largely inaccessible for human use, although some streams in mountain areas are ‘fed’ from ice and snow as melt water. Another 30 per cent occurs as groundwater, some of which is very deep seated as fossil water and, therefore, also inaccessible. This leaves only around one per cent of fresh water which is easily accessible for human use. Key terms Precipitation: The movement of water in any form from the atmosphere to the ground. Evaporation: The change in state of water from a liquid to a gas. Residence time: The average times a water molecule will spend in a reservoir or store. Fossil water : Ancient, deep groundwater from former pluvial (wetter) periods. Transpiration: The diffusion of water from vegetation into the atmosphere, involving a change from a gas to a liquid. Groundwater flow: The slow transfer of percolated water underground through pervious or porous rocks. Surface and groundwater freshwater 2.5% Surface/other freshwater 1.2% Other saline water 0.9% Ground- water 30.1% Glaciers and ice caps 68.7% Lakes 20.9% Ground ice and permafrost 69.0% (not accessible) Living things 0.26% Rivers 0.49% Swamps, marshes 2.6% Soil moisture 3.8% Atmosphere 3.0% Oceans 96.5% Rivers (the main source for people) 1% Living things 1% Atmosphere 8% Soil moisture 38% Lakes (natural and artificial) 52% Accessible surface freshwater Surface water and other freshwater Freshwater Total global water a b c d Figure 1.2 Where is the Earth’s water? (Source: Adapted from Igor Shiklomanov’s ‘World freshwater resources’ in Peter Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Freshwater Resources ) 5 1 The operation and importance of the hydrological cycle Figure 1.2c includes all sources of surface water, including ground ice and permafrost, which are very difficult to access. Figure 1.2d shows only fresh water that is accessible to humans with current levels of technology – note the importance of lakes and soil moisture. Rivers, which are currently the main source of surface water for humans, constitute only 0.007 per cent of total water. It is not surprising that there are so many concerns and disputes about the usage of this tiny, precious fraction. As with any global overview, the differences between places are masked and, in terms of availability of water, it is a very unequal world. It is also notable that technology is being used widely to extend the availability of fresh water supplies, for example by desalination of ocean water. 1.2 The operation of the drainage basin as an open system The drainage basin water cycle On a smaller scale (variable from regional to local, depending on the size of the drainage basin) the drainage basin is a subsystem within the global hydrological cycle. It is an open system as it has external inputs and outputs that cause the amount of water in the basin to vary over time. These variations can occur at different temporal scales, from short-term hourly through to daily, seasonal and annual (Figure 1.3). A drainage basin can be defined as the area of land drained by a river and its tributaries, and is frequently referred to as a river catchment . The boundary of a drainage basin is defined by the watershed , which is usually a ridge of high land which divides and separates waters flowing to different rivers. Drainage basins can be of any size, from that of a small stream possibly without tributaries up to a major international river flowing across borders of several countries. Figure 1.4 (page 6) shows how a drainage basin works. It has the advantage of showing the inter-linkages between components of the system. Key terms Catchment: The area of land drained by a river and its tributaries. Watershed: The highland which divides and separates waters flowing to different rivers. Precipitation Precipitation Precipitation Precipitation Evaporation and transpiration from vegetaion Evaporation and transpiration from vegetaion Evaporation from water surfaces Evaporation from water surfaces River discharge River discharge Interception Interception River flow River flow Channel fall Channel fall Throughfall Throughfall Groundwater Groundwater Surface Surface Soil Soil Soil Soil Rock Rock Base flow Base flow Lake Lake Overland flow Overland flow Throughflow Throughflow Infiltration Infiltration Energy from the Sun Energy from the Sun Watershed Watershed Stores Stores Outputs Outputs Inputs Inputs Flows Flows Figure 1.3 The drainage basin cycle The Water Cycle and Water Insecurity 6 Drainage basin system inputs Precipitation For precipitation (rain, snow, hail) to form, certain conditions are needed: l air cooled to saturation point with a relative humidity of 100 per cent l condensation nuclei, such as dust particles, to facilitate the growth of droplets in clouds l a temperature below dew point There are three main triggers for the development of rainfall, all of which involve uplift and cooling and condensation (Figure 1.5). As far as the impacts on the drainage basin hydrological system are concerned, there are six key influencing factors: 1 The amount of precipitation, which can have a direct impact on drainage discharge: as a general rule, the higher the amount the less variability in its pattern. 2 The type of precipitation (rain, snow or hail): the formation of snow, for example, can act as a temporary store and large fluxes (flows) of water can be released into the system after a period of rapid melting resulting from a thaw. 3 Seasonality. In some climates, such as monsoon, Mediterranean or continental climates, strong seasonal patterns of rainfall or snowfall will have a major impact on the physical processes operating in the drainage basin system. 4 Intensity of precipitation is also a key factor as it has a major impact on flows on or below the surface. It is difficult for rainfall to infiltrate if it is very intense, as the soil capacity is exceeded. 5 Variability can be seen in three ways: l Secular variability happens long term, for example as a result of climate change trends. l Periodic variability happens in an annual, seasonal, monthly or diurnal context. l Stochastic variability results from random factors, for example in the localisation of a thunderstorm within a basin. Key terms Condensation: The change from a gas to a liquid, such as as when water vapour changes into water droplets. Dew point: The temperature at which dew forms; it is a measure of atmospheric moisture. Variable Transpiration from vegetation Outputs Outputs Materials (water) Energy (solar, gravity) Precipitation Infiltration Groundwater store Deep groundwater flow Sub-surface throughflow Channel store Basin exit: lake, reservoir, ocean store Deep percolation Soil-water sub-surface store Surface store Interception by vegetation Inputs Surface run-off Evaporation Figure 1.4 A drainage basin as a hydrological system 7 1 The operation and importance of the hydrological cycle 6 The distribution of precipitation within a basin. The impact is particularly noticeable in very large basins such as the Rhone or the Nile, where tributaries start in different climatic zones. At a local scale and shorter time scale the location of a thunderstorm within a small river basin can have a major impact temporarily as inputs will vary, with contrasting storm hydrographs for different stream tributaries. Fluxes (flows) in the drainage basin Interception Interception is the process by which water is stored in the vegetation. It has three main components: interception loss , throughfall and stem flow Interception loss from the vegetation is usually greatest at the start of a storm, especially when it follows a dry period. The interception capacity of the vegetation cover varies considerably with the type of tree, with the dense needles of coniferous forests allowing greater accumulation of water. There are also contrasts between deciduous forests in summer and in winter – interception Key terms Convectional rainfall: Often associated with intense thunderstorms, which occur widely in areas with ground heating such as the Tropics and continental interiors. Cyclonic rainfall: A period of sustained, moderately intensive rain; it is associated with the passage of depressions. Orographic rainfall: Concentrated on the windward slopes and summits of mountains. Interception loss: This is water that is retained by plant surfaces and later evaporated or absorbed by the vegetation and transpired. When the rain is light, for example drizzle, or of short duration, much of the water will never reach the ground and will be recycled by this process (it’s the reason you can stand under trees when it is raining and not get wet). Throughfall: This is when the rainfall persists or is relatively intense, and the water drops from the leaves, twigs, needles, etc. Stem flow: This is when water trickles along twigs and branches and then down the trunk. Convectional rainfall This type of rainfall is common in tropical areas, and in the UK during the summer. When the land becomes hot, the air above it becomes warmer, expands and rises. As it rises, the air cools and its ability to hold water vapour decreases. Condensation occurs and clouds develop. If the air continues to rise, rain will fall. Cumulus cloud Further ascent causes more expansion and more cooling: rain takes place 3 The heated air rises, expands and cools; condensation takes place 2 Cool air descends and replaces the warm air 4 The Earth’s hot surface heats the air above it 1 Ground level Rain Rising warm air Cyclonic rainfall This happens when warm air, which is lighter and less dense, is forced to rise over cold, denser air. As it rises, the air cools and its ability to hold water vapour decreases. Condensation occurs and clouds and rain form. Warm air rises over cold air; it expands, cools and condensation takes place; clouds and rain form Warm air Cold air This line represents the plane separating warm air from cold air Warm air is forced to rise when it is undercut by colder air; clouds and rain occur Orographic rainfall When air is forced to rise over a barrier, such as a mountain, it cools and condensation takes place forming rain. The leeward (downwind) slope receives relatively little rain, which is known as the rain shadow effect. Condensation and rain Air cools Heavier rain on high land Rain shadow Warm, moist west winds Atlantic Rainfall: 1000 mm 3750 mm 1205 mm Less than 750 mm North sea Rain Cumulus cloud 3 2 Figure 1.5 The three types of rainfall Precipitation data It is important to recognise that data on precipitation may not always be reliable. In the UK, 200 automated weather stations spaced about 40 km apart continuously collect precipitation data. In the semi-arid Sahel countries of Mali, Chad and Burkina Faso roughly 35 weather stations collect data across an area of 2.8 million square kilometres (more than 10 times the area of the UK). Major storms can easily fall between these weather stations because rainfall is geographically patchy, especially when it is non-frontal. Understanding rainfall patterns and trends is critical in semi-arid areas (see page 26) but data reliability in these regions is often low. Rain shadow A rain shadow is a dry area on the leeward (downwind) side of the mountain. It receives little rainfall as the mountains shelter it from rain-producing weather systems. As the moist air is forced to rise on the windward side of the mountain, rainfall occurs as a result of adiabatic cooling (when the volume or air increases but there is no addition of heat), and condensation to dew point. The air, without much water left in it, is then drawn over the mountains where it descends and is adiabatically warmed by compression. This leads to a very dry 'shadow' area, for example, the Owens Valley is in the rain shadow of the Sierra Nevada range in California. The Water Cycle and Water Insecurity 8 losses are around 40 per cent in summer for certain Chiltern beech forests, but under 20 per cent in winter. Meteorological conditions also have a major impact. Interception varies by vegetation cover. Coniferous forest intercepts 25–35 per cent of annual rainfall, whereas deciduous forest only 15–25 per cent and arable crops 10–15 per cent. Wind speeds can decrease interception loss as intercep