Hydrometeorology
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Series Foreword xiv
Preface xv
Acknowledgements xvii
About the Companion Website xviii
1 The Hydrological Cycle 1
1.1 Overview 1
1.2 Processes comprising the hydrological cycle 3
1.3 Global influences on the hydrological cycle 4
1.4 Water balance 6
1.5 Impact of aerosols on the hydrological cycle 6
1.6 Coupled models for the hydrological cycle 7
1.7 Global Energy and Water Cycle Exchanges Project (GEWEX) 8
1.8 Flooding 8
Summary of key points in this chapter 9
Problems 10
References 10
2 Precipitation 11
2.1 Introduction 11
2.2 Equation of state for a perfect gas 11
2.3 Hydrostatic pressure law 12
2.4 First law of thermodynamics 12
2.5 Atmospheric processes: dry adiabatic lapse rate 13
2.6 Water vapour in the atmosphere 15
2.7 Atmospheric processes: saturated adiabatic lapse rate 16
2.8 Stability and convection in the atmosphere 16
2.9 The growth of precipitation particles 18
2.10 Precipitation systems 21
2.10.1 Localized convection 22
2.10.2 Mesoscale precipitation systems 23
2.10.3 Mid-latitude depressions 26
2.10.4 Tropical storms 30
2.10.5 Orographic effects on precipitation distribution 31
2.10.6 Topographical effects on precipitation distribution 33
2.11 Global atmospheric circulation 33
Appendix 2.1 Growth of a raindrop 33
Summary of key points in this chapter 35
Problems 36
References 37
3 Evaporation and Transpiration 41
3.1 Introduction 41
3.2 Modelling potential evaporation based upon observations 41
3.3 Aerodynamic approach 42
3.4 Energy balance 44
3.5 The Penman equation 44
3.6 Sensible and water vapour fluxes 45
3.7 Evaporation of water from wet vegetation surfaces: the interception process 47
3.8 Measuring evaporation and transpiration 47
3.9 Water circulation in the soil-plant-atmosphere continuum 48
3.10 Water circulation and transpiration 50
3.11 Water flux in plants 50
3.12 Modelling land surface temperatures and fluxes 51
3.13 Soil-vegetation-atmosphere transfer schemes 54
3.14 Estimation of large scale evapotranspiration and total water storage in a river basin 56
Appendix 3.1 Combination of aerodynamic and energy balance methods of computing lake evaporation 57
Appendix 3.2 Modelling soil moisture wetness 57
Summary of key points in this chapter 58
Problems 59
References 60
4 Snow and Ice 63
4.1 Introduction 63
4.2 Basic processes 63
4.2.1 Formation of snow 63
4.2.2 Formation of snow cover and its effects on the atmosphere 65
4.2.3 Formation of ice 67
4.3 Characteristics of snow cover 68
4.4 Glaciers 70
4.5 Sea ice 71
4.6 Permafrost 71
4.7 The physics of melting and water movement through snow 71
4.8 Water equivalent of snow 74
4.9 Modelling snowmelt and stream flow 76
4.10 Snow avalanches 80
4.11 Worldwide distribution and extremes of snow cover 81
Appendix 4.1 Estimates of catchment snowmelt inflow rates 83
Summary of key points in this chapter 84
Problems 86
References 87
5 Measurements and Instrumentation 90
5.1 Measurement, resolution, precision and accuracy 90
5.2 Point measurements of precipitation 90
5.2.1 Raingauge types 90
5.2.2 Measuring snow and hail 92
5.2.3 Errors in measurement 94
5.3 Areal measurements of precipitation using raingauge networks 96
5.4 Radar measurements of rainfall 96
5.4.1 Basics 96
5.4.2 Errors in radar measurements 97
5.4.3 Adjustment using raingauges 101
5.4.4 Summary of problem areas associated with radar measurements of precipitation 102
5.4.5 The use of multi-parameter radar 103
5.4.6 Drop size distributions 104
5.4.7 Rainfall estimation using parametric variables 104
5.4.8 Measurement of snow 106
5.4.9 Measurement of hail 107
5.4.10 Precipitation type 108
5.5 Soil moisture 109
5.5.1 Approaches 109
5.5.2 Gravimetric method 109
5.5.3 Electrical resistance method 110
5.5.4 Neutron method 110
5.5.5 Gamma ray attenuation method 110
5.5.6 COSMOS-UK 111
5.5.7 Dielectric methods 111
5.5.8 Tensiometric method 113
5.5.9 Satellite remote sensing 113
5.6 Evaporation and evapotranspiration 113
5.7 Flow measurement: basic hydrometry 113
5.8 Measuring stream discharge 115
5.8.1 The stage-discharge curve 115
5.8.2 Automated moving boat methods 117
5.9 Brief overview of modern telemetry 117
5.9.1 Ground-based telemetry links 117
5.9.2 VHF and UHF radio links 117
5.9.3 Satellite links 118
Appendix 5.1 Combining dissimilar estimates by the method of least squares 118
Summary of key points in this chapter 119
Problems 121
References 121
6 Satellite-Based Remote Sensing 125
6.1 Overview of satellite remote sensing 125
6.2 Surface scattering of electromagnetic radiation 129
6.3 Interaction of electromagnetic radiation with the atmosphere 131
6.4 Visible and infrared data 132
6.4.1 Precipitation 134
6.4.2 Snow depth 135
6.4.3 Soil moisture and evapotranspiration 136
6.5 Multispectral data 137
6.5.1 Precipitation 137
6.5.2 Cloud recognition 137
6.5.3 Snow 138
6.6 Passive microwave techniques 138
6.6.1 Precipitation 141
6.6.2 Global Precipitation Climatology Project (GPCP) 143
6.6.3 Global Precipitation Measurement mission (GPM) 143
6.6.4 Snow depth 143
6.6.5 Sea ice and sea surface temperature 145
6.6.6 Soil moisture and evapotranspiration 145
6.7 Active (radar) microwave techniques 147
6.7.1 Synthetic aperture radar 147
6.7.2 Radar systems 149
6.7.3 Tropical Rainfall Measuring Mission (TRMM) 150
6.8 The surface energy balance system (SEBS) 150
6.9 Summary of satellite measurement issues 151
Appendix 6.1 Radiation balance 154
Summary of key points in this chapter 155
Problems 157
References 157
7 Analysis of Precipitation Fields and Flood Frequency 163
7.1 Introduction 163
7.2 Areal mean precipitation 163
7.3 Spatial and temporal storm analysis 165
7.3.1 Spatial statistical analyses 165
7.3.2 Temporal analyses 167
7.3.3 Oscillations in precipitation 168
7.3.4 Conditional probabilities 169
7.3.5 Kriging 169
7.3.6 Accuracy of the precipitation products 171
7.4 Model storms for design 172
7.5 Approaches to estimating flood frequency 173
7.6 Probable maximum precipitation (PMP) 175
7.7 Probable maximum flood (PMF) 177
7.8 Flood Studies Report (FSR) 177
7.9 Flood Estimation Handbook (FEH) 180
Appendix 7.1 Three-dimensional description of a rainfall surface 182
Appendix 7.2 Gumbel distribution 183
Summary of key points in this chapter 183
Problems 185
References 185
8 Precipitation Forecasting 188
8.1 Introduction 188
8.2 Nowcasting 188
8.2.1 Definition 188
8.2.2 Impact of errors in precipitation measurements 189
8.2.3 Extrapolation of radar data 189
8.3 Probabilistic radar nowcasting 192
8.4 Numerical models: structure, data requirements, data assimilation 194
8.4.1 Probabilistic quantitative precipitation forecasting 194
8.4.2 Mesoscale models 197
8.4.3 Data assimilation 197
8.4.4 Performance of high resolution mesoscale model-based nowcasting systems 198
8.5 Medium range forecasting 198
8.6 Seasonal forecasting 201
Appendix 8.1 Brier skill score 203
Summary of key points in this chapter 203
Problems 205
References 205
9 Flow Forecasting 209
9.1 Basic flood forecasting techniques 209
9.2 Model calibration and equifinality 210
9.3 Flood forecasting model development 210
9.4 Conversion of detailed hydrodynamic models to simplified models suitable for real-time flood forecasting 213
9.5 Probabilistic flood forecasting and decision support methods 215
9.6 Derivation of station rating (stage-discharge) curves 216
9.7 Performance testing of forecasting models and updating procedures 216
9.8 Configuration of models on to national and international forecasting platforms 218
9.9 Flood warnings and levels of service 222
9.9.1 United Kingdom 222
9.9.2 United States and Canada 222
9.10 Case studies worldwide: river and urban 224
Appendix 9.1 St Venant equations 224
Appendix 9.2 Flow in unsaturated and saturated zones 226
Summary of key points in this chapter 227
Problems 228
References 229
10 Coastal Flood Forecasting 233
10.1 Types of coastal flooding 233
10.2 Models used to predict storm surge flooding 233
10.2.1 Empirical models 234
10.2.2 First-generation models 235
10.2.3 Second-generation models 235
10.2.4 Third-generation models 235
10.2.5 Wave, tide and surge models 235
10.3 Probabilistic surge forecasting 238
10.4 Tsunamis 239
10.5 Examples of coastal flooding in the United Kingdom 241
10.5.1 The surge of 1953 241
10.5.2 Wirral floods 2013 241
10.5.3 Surges along the east coast of England, December 2013 241
10.5.4 Aberystwyth floods January 2014 242
10.6 Some examples of coastal flooding worldwide 243
Appendix 10.1 Wave overtopping at the coast 244
Summary of key points in this chapter 245
Problems 247
References 247
11 Drought 249
11.1 Definitions 249
11.2 Drought indices 250
11.3 The physics of drought 253
11.4 Frequency analysis: predictability 254
11.5 Modelling the occurrence of drought 256
11.6 Major drought worldwide 258
11.7 Examples of the consequences of drought 258
11.8 Strategies for drought protection, mitigation or relief 260
Appendix 11.1 Defining aridity 261
Summary of key points in this chapter 261
Problems 263
References 263
12 Wind and the Global Circulation 266
12.1 Equations of motion 266
12.2 Atmospheric Ekman layer 268
12.3 Fronts 269
12.4 Jet streams 270
12.5 Hurricanes 271
12.6 Lee waves 272
12.7 Land and sea breezes 272
12.8 The wind structure of the atmospheric circulation 273
12.9 Hadley cell 273
12.10 Polar cell 274
12.11 Ferrel cell 275
12.12 Walker circulation 275
12.13 El Niño/Southern Oscillation 276
12.14 Monsoons 276
Appendix 12.1 Large scale air motion 278
Appendix 12.2 Ageostrophic motion 278
Summary of key points in this chapter 279
Problems 281
References 282
13 Climatic Variations and the Hydrological Cycle 284
13.1 An introduction to climate 284
13.2 Evidence of climate change 286
13.2.1 Climatology of the last ice age 292
13.2.2 Intergovernmental Panel on Climate Change (IPCC) 295
13.3 Causes of climatic change 297
13.3.1 The natural energy system 298
13.3.2 The hydrological cycle 299
13.3.3 The carbon cycle 301
13.3.4 Other biochemical cycles 301
13.4 Modelling climatic change 303
13.5 Possible effects of climate change upon the hydrological cycle and water resources 307
Appendix 13.1 Estimating return times for events in a long term climate record 310
Summary of key points in this chapter 310
Problems 313
References 314
14 Hydrometeorology in the Urban Environment 318
14.1 Introduction 318
14.2 Urban boundary layer and the water cycle 318
14.3 Urban development and rainfall 320
14.4 Sewer flooding 322
14.5 Surface runoff from urban areas 324
14.6 Floodplain development 326
14.7 Acid rain 327
14.7.1 Basics 327
14.7.2 Modelling wet deposition 328
14.8 Urban air and water pollution 329
Appendix 14.1 Number of runoff events from an urban drainage system 330
Summary of key points in this chapter 331
Problems 332
References 333
Glossary 336
Index 347
1
The Hydrological Cycle
1.1 Overview
The hydrological cycle describes the continuous movement of water above, on and below the surface of the Earth. It is a conceptual model that describes the storage and movement of water between the biosphere (the global sum of all ecosystems, sometimes called the zone of life on Earth), the atmosphere (the air surrounding the Earth, which is a mixture of gases, mainly nitrogen (about 80%) and oxygen (about 20%) with other minor gases), the cryosphere (the areas of snow and ice), the lithosphere (the rigid outermost shell of the Earth, comprising the crust and a portion of the upper mantle), the anthroposphere (the effect of human beings on the Earth system) and the hydrosphere (see Table 1.1).
Table 1.1 Water in the hydrosphere and the distribution of fresh water on the Earth (from Martinec, 1985)
(a) Distribution of water in the hydrosphere Forms of water present Water volume(106 km3) As % Oceans, seas 1348 97.4 Polar ice, sea ice, glaciers 28 2.0 Surface water, ground water, atmospheric water 8 0.6 Total 1384 100.0 Total fresh water 36 2.6 (b) Distribution of fresh water on Earth Forms of water present Water volume (106km3) As % * ┼ * ┼ Polar ice, glaciers 24.8 27.9 76.93 77.24 Soil moisture 0.09 0.06 0.28 0.17 Ground water within reach 3.6 3.56 11.17 9.85 Deep ground water 3.6 4.46 11.17 12.35 Lakes and rivers 0.132 0.127 0.41 0.35 Atmosphere 0.014 0.014 0.04 0.04 Total 32.236 36.121 100.0 100.0
*Based on Volker (1970).
┼Based on Dracos (1980), referred to in Baumgartner and Reichel (1975).
Models of the biosphere are often referred to as land surface parameterization schemes (LSPs) or soil-vegetation-atmosphere transfer schemes (SVATs). An example of an SVAT is described by Sellers et al. (1986). The water on the Earth's surface occurs as streams, lakes and wetlands in addition to the sea. Surface water also includes the solid forms of precipitation, namely snow and ice. The water below the surface of the Earth is ground water.
Most of the energy leaves the ocean surface in the form of latent heat in water vapour, but this is not necessarily the case for land surfaces. Hence maritime air masses are different to continental air masses. The atmosphere and oceans are strongly coupled by the exchange of energy, water vapour, momentum at their interface, and precipitation. The oceans represent an enormous reservoir for stored energy and are denser than the atmosphere, having a larger mechanical inertia. Therefore ocean currents are much slower than atmospheric flows. The atmosphere is heated from below by the Sun's energy intercepted by the underlying surface, whereas the oceans are heated from above. Lakes, rivers and underground water can have significant hydrometeorological and hydroclimatological significance in continental regions. The hydrological cycle is represented by the simplified diagram in Figure 1.1.
Figure 1.1 Simplified representation of the hydrological cycle
(NWS Jetstream NOAA, USA, www.srh.noaa.gov/jetstream/atmos/hydro.htm)
1.2 Processes comprising the hydrological cycle
There are many processes involved in the hydrological cycle, the most important of which are as follows:
- Evaporation is the change of state from liquid water to vapour. The energy to achieve this may come from the Sun, the atmosphere itself, the Earth or human activity.
- Transpiration is the evaporation of water from plants through the small openings found on the underside of leaves (known as stomata). In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Only 1% of the transpired water passing through a plant is used by the plant to grow, with the rest of the water being passed into the atmosphere. Evaporation and transpiration return water to the atmosphere at rates which vary according to the climatic conditions.
- Condensation is the process whereby water vapour in the atmosphere is changed into liquid water as clouds and dew. This depends upon the air temperature and the dew point temperature. The dew point temperature is the temperature at which the air, as it is cooled, becomes saturated and dew can form. Any additional cooling causes water vapour to condense. When the air temperature and the dew point temperature are equal, mist and fog occur. Since water vapour has a higher energy level than liquid water, when condensation occurs the excess energy is released in the form of heat. When tiny condensation particles, through collision or coalescence with each other, grow too large for the ascending air to support them, they fall to the surface of the Earth as precipitation (Chapter 2). Precipitation is the primary way fresh water reaches the Earth's surface, and on average the Earth receives about 980 mm each year over both the oceans and the land.
- Infiltration of water into the land surface occurs if the ground is not saturated, or contains cracks or fissures. The flow of water into the ground may lead to the recharge of aquifers, or may move through unsaturated zones to discharge into rivers, lakes or the seas. Between storm or snowmelt periods, stream flow is sustained by discharge from the ground water systems. If storms are intense, most water reaches streams rapidly. Indeed, if the water table - the boundary between the saturated and unsaturated zones - rises to the land surface, overland flow may occur.
- The residence time of water in parts of the hydrological cycle is the average time a water molecule will spend in a particular area. These times are given in Table 1.2. Note that ground water can spend over 10,000 years beneath the surface of the Earth before leaving, whereas water stored in the soil remains there very briefly. After water evaporates, its residence time in the atmosphere is about nine days before it condenses and falls to the surface of the Earth as precipitation. Residence times can be estimated in two ways. The first and more common method is to use the principle of conservation of mass, assuming the amount of water in a given store is roughly constant. The residence time is derived by dividing the volume of water in the store by the rate by which water either enters or leaves the store. The second method, for ground water, is via isotropic techniques. These techniques use either in-stream tracer injection combined with modelling, or measurements of naturally occurring tracers such as radon-222 (see for example Lamontagne and Cook, 2007).
- Human activities release tiny particles (aerosols) into the atmosphere, which may enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to the availability and quality of fresh water (see Ramanathan et al., 2001).
Table 1.2 Average residence times for specific stores (see for example www.physicalgeography.net/fundamentals/8b.html)
Reservoir Average residence time Antarctica 20,000 years Oceans 3,200 years Glaciers 20 to 100 years Seasonal snow cover 2 to 6 months Soil moisture 1 to 2 months Ground water shallow 100 to 200 years Ground water deep 10,000 years Lakes 50 to 100 years Rivers 2 to 6 months Atmosphere 9 days1.3 Global influences on the hydrological cycle
Differential heating by the Sun is the primary cause of the general circulation of the atmosphere. There are a number of regional differences which influence the hydrological cycle in addition to the relative...
