Water Circulation

Water circulation plays a dominant role in the dispersion of isotopes which are biogeochemically 'passive' (eg,3H, Rn), whereas biological uptake and release, solute–particle interactions and chemical scavenging exert major control in the distribution of biogeochemically 'active' elements (eg, C, Si, Th, Pb, Po).

From: Encyclopedia of Ocean Sciences (Third Edition) , 2016

Estuarine water circulation

Eric Wolanski PhD, DSc, FTSE, FIE Aust , in Estuarine Ecohydrology, 2007

Publisher Summary

Water circulation in estuaries is forced by the riverine inflow, the tides, rainfall and evaporation, the wind, and oceanic events in coastal waters such as an upwelling, the passage of an oceanic eddy, and storms. Some water particles that leave an estuary at falling tide re-enter the estuary at a later time, possibly at the next rising tide. This leads to the concept of the exposure time, i.e. the time spent in the domain of interest until the particles never return back in the estuary. The exposure time is much larger than the residence time if most of the particles that exit the estuary at ebb tides return at the following rising tide. The ratio between the number of particles returning and the number of particles leaving is called the return coefficient r, and it is smaller than 1. To quantify the exposure time, one needs to know the water circulation outside the estuary. Furthermore, the water circulation in estuaries can vary markedly across the width. In wide estuaries, the Coriolis force causes a horizontal shear of the flow; as a result, seaward flow occurs on the right (left) hand side in the northern (southern) hemisphere, and landward flow on the left. In wide estuaries, this creates a tidally averaged, net inflow on one side, and a net outflow on the other side.

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The PEM Water Electrolysis Plant

Dmitri Bessarabov , Pierre Millet , in PEM Water Electrolysis, 2018

1.5 The Liquid–Gas Separation Unit

1.5.1 General Description

Some general characteristics of the liquid–gas separation unit are compiled in Table 1.6.

Table 1.6. Main Characteristics of the Liquid–Gas Separation Unit

Item Unit Reference Value
Water level monitoring Yes/no Yes
Online temperature regulation Yes/no No
Online cooler Yes/no Yes
Cyclonic separation Yes/no Yes

1.5.2 Example

The WCU (Figs. 1.11 and 1.12 ) contains the water circulation loops placed at the anode and cathode of the electrolysis unit. Some technology suppliers use only one pump at the anode. Others use one on each side. Some systems operate at equi-pressure. Others operate under a difference of pressure. A purification unit is placed on the anodic loop as well as a heat exchanger to cool down circulating water and set nominal operating temperature. The electroosmosis flow of water collected on the cathode side is transferred back to the anode at regular time intervals or sent back to water treatment unit.

Figure 1.11. Water circulation circuitry of a laboratory 25   kW PEM water electrolyzer.

Figure 1.12. Liquid-gas separators of a 2   MW PEM water electrolyzer.

 Source: Courtesy Siemens Co.

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Surf-Zone Zooplankton and Nekton

Anton McLachlan , Omar Defeo , in The Ecology of Sandy Shores (Third Edition), 2018

Food and Feeding Relationships

Water circulation in the surf zone concentrates particulate food, such as detritus and phytoplankton. Clutter (1967) suggested that the surf zone was characterized by much detrital food in suspension and that this decreased offshore, thus explaining the greater zooplankton biomass inshore. Certainly, many zooplanktonic species are attracted to phytoplankton blooms in the surf and feed on the diatoms (Webb et al., 1987). Although they may be attracted primarily to the phytoplankton, and also feed on detritus, the larger forms of zooplankton are usually omnivorous and switch readily to an animal diet, whereas smaller forms (such as many copepods) are purely herbivorous. The zooplankton is, in turn, a major component of the food of surf-zone fishes and thus occupies a key position at the center of the surf-zone food chain.

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Surf-zone Fauna

A. McLachlan , A.C. Brown , in The Ecology of Sandy Shores (Second Edition), 2006

Food and Feeding Relationships

Water circulation in the surf zone concentrates particulate food, such as detritus and phytoplankton. Clutter (1967) suggested that the surf zone was characterized by much detrital food in suspension and that this decreased offshore, thus explaining the greater zooplankton biomass inshore. Certainly, many zooplanktonic species are attracted to phytoplankton blooms in the surf, to feed on the diatoms (Webb et al. 1987). Although they may be attracted primarily to the phytoplankton (and also feed on detritus), the larger forms of zooplankton are usually omnivorous and switch readily to an animal diet, whereas smaller forms (such as many copepods) are purely herbivorous. The zooplankton is, in turn, a major component of the food of surf-zone fishes and thus occupies a key position in the center of the surf-zone food chain.

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Greece

Nomiki Simboura , ... Panayotis Panayotidis , in World Seas: an Environmental Evaluation (Second Edition), 2019

9.2    Physical Oceanography

Water circulation in the basins surrounding the Hellenic Peninsula (Aegean, Ionian, and Levantine Seas) is determined by the general circulation of the Eastern Mediterranean Sea and the local interaction with the atmosphere and bottom topography. The surface circulation of the Eastern Mediterranean tends to form a cyclonic pattern, and surface water flow over the Hellenic Seas follows this broader cyclonic pattern. This pattern is fairly sustained by the water inflow, which counterbalances the excess of evaporation over the water input from precipitation and river runoff. In terms of water masses, the Hellenic Seas are of high importance not only on regional basis but also for the whole Mediterranean Sea. Within the Hellenic Seas, during the cold period of the year, formation of both intermediate and deep water occurs at several sites ventilating the seawater column and contributing to the basin-wide thermohaline conveyor belt of the Mediterranean Sea.

Fig. 9.3 provides a schematic map depicting the general surface circulation of the Hellenic Seas. Waters entering the Hellenic Seas are characterized by three different origins (Malanotte-Rizzoli et al., 1997; Robinson et al., 1991). From the west, the Modified Atlantic Water reaches the Eastern Mediterranean and has a characteristic signal of relatively low salinity 38.5–38.8. It can be traced over large parts of the Ionian Sea and south of Crete with a meandering flow toward the easternmost part of the Mediterranean through the Cretan Passage (between Crete and Africa). Occasionally, waters of Atlantic origin are detected inside the Cretan Arc mainly in patches of low salinity over the western parts of the Cretan Sea.

Fig. 9.3

Fig. 9.3. Schematic surface circulation of the Hellenic Seas. 1, Modified Atlantic Water input; 2, Pelops Gyre; 3, Cretan Gyre; 4, Ierapetra Gyre; 5, Rhodes Gyre; 6, Asia Minor Current; 7, Black Sea Water input; 8, Central Aegean Gyre; 9, Cretan Sea Gyres.

Surface Levantine Water, which is a relatively warm and saline water mass (salinity >   39), inflows into the Aegean through the eastern Cretan Straits mainly via the Asia Minor Current. This current system appears in the northern part of the Levantine Basin and the Eastern Cretan Arc Straits. It then inflows into the Cretan Sea and continues further north to the central and northern Aegean Sea through numerous straits between the Hellenic Islands and the Western Turkish coastline (Theocharis, Balopoulos, Kioroglou, Kontoyiannis, & Iona, 1999). The least saline water found in the Hellenic Seas comes from the Straits of Dardanelles in the northeastern Aegean Sea.

Low-salinity water of Black Sea origin with typical salinities around 30 enters the Aegean Sea and flows westwards following the Hellenic Peninsula coastline (Georgopoulos, 2002). Thus, in the Aegean Sea, the combination of the northward Asia Minor Current to its eastern coastline with the southward flow of less saline waters along its western coastline forms an overall cyclonic surface circulation (Olson, Kourafalou, Johns, Samuels, & Veneziani, 2007). This general regime of the surface circulation incorporates a number of permanent or quasi-permanent cyclonic or anticyclonic gyres like the Rhodes Gyre over the Levantine Sea, the Ierapetra, Cretan, and Pelops Gyres at the periphery of the Cretan Arc, and the Central Aegean cyclonic gyre (Fig. 9.3).

Current speeds are generally weak with typical values of 10–20   cm   s−   1, but over some straits in the Aegean Sea (e.g., between Rhodes Island and Turkey and around Limnos Island) current speeds higher than 50   cm/s can be observed (Kontoyiannis et al., 1999). Although tidal ranges are low throughout the Hellenic Seas, at several straits that constrict sea bodies from the open sea (e.g., semi-enclosed or elongated gulfs) the tidal currents are very strong with speeds exceeding 1   m   s−   1.

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Estuarine Ecohydrology Modeling: What Works and Within What Limits?

Eric Wolanski , in Coasts and Estuaries, 2019

2.1 Water Circulation Models

The water circulation in estuaries is controlled by physical forcing at the open boundaries, namely the river inflow, the groundwater inflow, the wind, and the oceanic forcings mainly in terms of tides, fluctuations of the mean sea level (e.g., storm surges and shelf waves), the waves, and the net oceanic currents. This circulation has been extensively studied and this knowledge has been used to develop numerical models of the water circulation; these models are generally reliable ( Valle-Levinson, 2010; Uncles and Monismith, 2011; Ganju et al., 2016; Bruner de Miranda et al., 2017). These models are routinely used to study the distribution of passive waterborne particles that behave conservatively (i.e., that do not settle or erode from the bottom or change while in the water column). Salinity is such a conservative tracer; thus, these models can readily be used to study changes in salinity of an estuary due to dredging, changing river flows, or passing storms. However, these models still do not reproduce well estuarine fronts, that is, discontinuities in the salinity; yet, these fronts are important for fish larvae in their strategy to recruit (Kingsford and Suthers, 1994; Uncles, 2011; Teodosio et al., 2016). Thus there are strong limitations to the use of these models for modeling fish dynamics in a stratified estuary.

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Coastal Wetlands

Charles S. Hopkinson , ... Mark M. Brinson , in Coastal Wetlands (Second Edition), 2019

2.8 Observations Across Ecosystem Types

Surface and underground water circulation are vital to the sedimentation processes, chemistry, and biology of coastal wetlands. There is a strong feedback between water circulation and sediment dynamics and the constantly changing geomorphology of coastal wetlands. The circulation of surface and sediment porewaters in coastal wetlands also plays a key role in the adjoining estuarine and coastal ecosystems, as well as their biology and biodiversity. The processes vary spatially from small to large scales and temporally at time scales from that of individual mixing events to tidal to geomorphological scales.

Almost every day of field work in coastal wetlands reveals new aspects of the complex interplay between hydrodynamics, sedimentology, and flora and fauna. To cite a few examples, we compare attributes of the five major ecosystem types (Table 1.1) in terms of their geographic distribution, physical limits, food web dynamics, and responses to human activities. In making such comparisons, there is a tendency to overgeneralize and to gloss over the diversity of patterns found within each of the types. We make these comparisons in the spirit that they may lead to insight not obvious by working within or examining a single ecosystem type (the remainder of this section refers to the contents of Table 1.1).

Table 1.1. Comparison of Coastal Wetland Ecosystem Characteristics for Five Major Coastal Ecosystem Types

Characteristics Seagrass Meadows Intertidal Flats Salt Marshes Mangrove Forests Freshwater Tidal Wetlands
Geographic Distribution
Geographic range a Absent in polar regions Unlimited Mid to high latitudes; replaced by mangroves in subtropics Mean monthly temps >20   C Low to high latitudes
Global abundance b 180,000   km2 Widespread; undocumented 60,000   km2 150,000   km2 Grossly underestimated
Physical Limits
Maximum water depth limitation/minimum elevation c Approximately 20% incident light Light limitation of net primary production where P/R   <   1 Varies with amplitude and temperature Unresolved Unresolved
Maximum elevation/minimum hydroperiod d Intertidal conditions (exposure to some drying) Supratidal during storms Groundwater discharge zone; salt accumulation in arid climates Groundwater discharge zone; salt accumulation in arid climates No information
Salinity range e Euryhaline to polyhaline Variable Hyperhaline to oligohaline Hyperhaline to oligohaline Fresh to oligohaline
Source of material for vertical accretion f Also organic accretion Allochthonous inorganic only (biofilm stabilizing effect) Also organic accretion Also organic accretion Also organic accretion
Interannual variation in sea level; ENSO cycles g Not applicable Not applicable Demonstrated effect on primary production Fish nursery variation Not known
Hurricanes and cyclones h Temporary unless burial by sediments or severe erosion Likely short-term effects Local shore erosion and sediment redeposition Decades-long effects after blowdown of trees Surge may affect soil salinity
Food Web Dynamics
Typical dominant primary producers i Submersed vascular plants Epipelic microalgae (esp. diatoms) and macroalgae Emergent grasses and forbs Trees, shrubs Ranges from herbaceous to forest dominance
Grazing food webs j Large mammals and turtles Infauna, epifauna Snails and insects Tree crabs Rodents
Detrital food webs k Invertebrate dominated Indistinguishable from grazing Invertebrate dominated Burrowing crabs Invertebrate dominated
Openness of organic matter fluxes l Exports and imports well-documented No information; presumed open Evidence of net exports Evidence of net exports Few studies
Response to Human Activities—Global Change
Response to climate warming: ambient temperatures m Thresholds not established No information Possible distribution to higher latitudes Possible distribution to higher latitudes lacking frost No information
Response to climate warming: acceleration of rising sea level n Thresholds not established No information Strong sediment sources needed for survival (glacial rebound areas excluded) Strong sediment sources needed for survival Strong sediment sources needed for survival (little information)
Altered salinities in response to climate drying or wetting o Hypersalinity in seasonally isolated lagoons; freshening in others No information Expansion or reduction of salt flats Expansion or contraction of salt flats Transformation to greater or lesser salinity tolerant species
Response to Human Activities—Local and Upstream
Increased salinity from reduced freshwater flows (upstream withdrawals, irrigation, etc.) p Hypersalinity in seasonally isolated lagoons Increasingly stressful Expansion of salt flats Expansion of salt flats Transformation to salinity tolerant species
Reduced sediment supply due to reduced freshwater flows (dams, etc.) q No information (reduced suspended sediments beneficial to water clarity) Reduced development of tidal flats Erosion Erosion; hypersaline conditions Erosion
Enhanced or excessive sediment supply r Reduced water clarity limits primary production Accretion with shift to marsh and mangrove colonization Accretion self-limiting Sensitive to burial of pneumatophores Massive development of marshes historically (Chesapeake Bay, USA)
Tidal barriers—dyking and bulkheads s Not applicable Not applicable Eliminates physical and biotic exchanges Eliminates physical and biotic exchanges Eliminates physical and biotic exchanges
Eutrophication t High sensitivity to nutrients; macroalgal smothering Proliferation of macroalgae Increased herbivory No information No information
Harvesting of plants and animals u Trawling for benthic organisms is extremely destructive Shellfish harvesting (e.g., hand tonging for oysters and clams), disruption of substrate Harvesting of marsh hay (can be done sustainably) Logging for firewood (can be done sustainably); aquaculture impoundments (see#22, very destructive) Cypress logging
Bottom disturbance: channel dredging, fish/shrimp ponds, etc. v Extremely destructive, direct loss, and declining water clarity Deepening beyond euphotic zone Extremely destructive Extremely destructive Extremely destructive
Fragmentation within habitat w No information No information No information No information No information
Loss of connectivity with other habitats x No information No information No information No information No information
Response to Human Activities—Biotic
Invasive species y Caulerpa taxifolia aggressive clones No information Phragmites in spring tidal and fresh zones No information Phragmites throughout
Diseases z Wasting disease historically No information No information No information No information

Superscript alphabets corresponds to footnotes that refer to chapters in this volume and additional sources listed at the bottom of the table.

a
Chapters 2–4 and 28 Chapter 2 Chapter 3 Chapter 4 Chapter 28 ; Conner et al., 2007.
b
Duarte et al. (2008) for all but intertidal flats and tidal freshwater wetlands, the latter grossly underestimated (Chapter 18). Mcowen et al. (2017) for saltmarshes and Giri et al. (2011) and Hamilton and Casey (2016) for mangroves. Green and Short (2003), Nellemann et al. (2009a) and Hopkinson et al. (2012) for seagrasses.
c
Chapter 13; McKee and Patrick (1988) for saltmarshes, Gallegos and Kenworthy (1996) for seagrass.
d
Chapters 3 and 16 Chapter 3 Chapter 16 .
e
Chapters 2, 3 and 4 Chapter 2 Chapter 3 Chapter 4 .
f
Chapters 2, 3, and 4 Chapter 2 Chapter 3 Chapter 4 .
g
For saltmarshes, Morris et al., 1990; for mangroves, Rehage and Loftus, 2007.
h
For mangroves (Smith et al., 1994); for marshes and mangroves (Cahoon, 2006); for marshes (van de Plassche et al., 2004), for seagrasses Holmer Chapter 13.
i
Chapters 2, 3, and 4 Chapter 2 Chapter 3 Chapter 4 .
j
Chapters 2, 3, and 4 Chapter 2 Chapter 3 Chapter 4 ; Chapter 11 for tidal flats; Chapter 15 for saltmarshes.
k
Chapters 2, 3, and 4 Chapter 2 Chapter 3 Chapter 4 ; Chapter 11 for tidal flats; Chapter 15 for saltmarshes.
l
Chapters 11, 13, 15, 19 Chapter 11 Chapter 13 Chapter 15 Chapter 19 .
m
Chapters 2, 20, 22, 28 Chapter 2 Chapter 20 Chapter 22 Chapter 28 .
n
Chapters 2, 3, 4, and 28 Chapter 2 Chapter 3 Chapter 4 Chapter 28 .
o
Predictions depend upon local effects of precipitation/evapotranspiration change as they may affect continental sediment supplies.
p
No documented examples in chapters.
q
Pasternack et al., 2001.
r
Pasternack et al., 2001; van Katwijk et al., 2016.
s
Chapter 19.
t
Seagrasses particularly vulnerable: Chapter 13, van Katwijk et al., 2016 and Waycott et al., 2016.
u
Chapter 24 (Holmer) and van Katwijk et al., 2016 for seagrasses; Buchsbaum et al., 2009 and Holden et al., 2013 for saltmarsh haying; Perkins, 2017 and Mancil, 1972 for cypress tree logging; Goessens et al., 2014 for mangrove logging.
v
Chapter 24.
w
No information available.
x
No information available.
y
Chapter 13.
z
Chapter 13 and Waycott et al., 2016.

Some patterns are obvious, such as climatic differences between the latitudinal ranges of saltmarshes and mangroves. Although the geographic distribution is reasonably well-established, inventories of the area covered by each ecosystem type are less reliable. For example, tidal freshwater wetlands have not been much studied outside of the North American and European continents, so their distribution has not been mapped to our knowledge (Conner et al., 2007; Whigham et al., Chapter 18). In fact, most of these ecosystems are associated with the large deltas of the world where dominance by freshwater discharge results in salinities lower than is often associated with the term "coastal."

At a particular site, physical limits and factors (water depth, wave energy, salinity range, rate of rising sea level, etc.) determine the type of coastal wetland. Because of the high diversity in life forms across ecosystem types, ranging from diatoms to trees for just the primary producers, there is great variability in the capacity of biotic structure to influence physical processes. For tidal flats and seagrasses, both of which are dominated by obligate aquatic taxa, maximum depth distribution (or lowest elevation) is limited by light availability when flooded. The upper elevation of disturbance is limited by desiccation when the ecosystem is exposed. For emergent life forms (marsh grasses, shrubs, and trees), the vertical range is highly influenced by tidal amplitude, i.e., greater tidal amplitudes allow a greater elevational range of distribution (McKee and Patrick, 1988).

Variations in intertidal soil salinity are largely a consequence of climate (precipitation–evapotranspiration intensity and periodicity) and freshwater discharge, when significant. In some humid climates, groundwater discharge establishes the upper boundary of mangroves and marshes (Plater and Kirby, 2006), whereas in some arid climates, high evapotranspiration leads to high soil salinities that restrict the landward extent of coastal wetlands (Pratolongo et al., Chapter 3). Flooding frequency and bioturbation contribute to porewater exchange and soil salinity. With regular tidal flooding, porewater exchange maintains salinity (in psu) so that it is rarely above 50 in a mangrove forest or a saltmarsh, compared to 32–37 in an adjacent tidal creek and 100 in salt pan porewater (Sam and Ridd, 1998; Gardner, 2005; Boorman, Chapter 17). The intensity of the groundwater salinity intrusion in a wetland depends on the tidal range, occurrence of an impermeable clay/silt layer underneath the wetland soils penetrated by roots, and the fresh groundwater discharge from the upland (Barlow, 2003; Wilson and Morris, 2012). With changes in the relative position of sea level, the boundary at which coastal effects, such as salinity, are no longer apparent—and this boundary controls the vegetation—changes over time as wetlands migrate landward in response to rising sea level (most regions; see Fig. 1.3) and regress as sea level drops (mainly at high latitudes) (Figs. 1.3 and 1.6).

Virtually all coastal wetlands respond to and rely on sediment sources and exchanges. The effects of suspended sediment on light transmission in the water column are critical for survival of seagrasses and likely have short-term effects on the primary productivity of tidal flats. For marsh and forest ecosystems, however, sediment accumulation through deposition is a critical process because it maintains the relationship between a wetland surface and sea level change. This dynamic interaction is to some degree self-maintaining because too much accretion places the sediment surface too high for flooding and sediment deposition, whereas too little accretion has the opposite effect (Rybczyk and Callaway, 2009; Morris, 2016).

Interannual variation in sea level stand adds another dimension of complexity, but it has been demonstrated only for primary production rates in saltmarshes (Morris et al., 1990). Of course, hurricanes, cyclones, and tsunamis, where they occur, can produce short-term disruptions of tidal flats but highly variable effects locally for other coastal wetland types (Cahoon, 2006). For saltmarshes, sediment erosion is a shore phenomenon that in extreme cases can remove large areas in a single storm. In an example studied by van de Plassche et al. (2004) in Connecticut (USA), massive removal of saltmarsh sediments could be returned to intertidal status only with substantial infilling and regrowth of vegetation. In contrast, mangroves can experience massive blowdown with return to full growth forests only on decadal time scales (Cahoon et al., 2003; but see also Danielson et al. 2017) where mangrove recovery is seen in relation to the degree of disturbance. The effects of soil salinity on tidal freshwater swamps in temperate zones have been shown to cause tree mortality and replacement with marsh vegetation (Conner et al., 1997). Fire is likely a disturbance only in the upper portions of tidal wetlands where mortality of trees can accelerate the landward movement of marsh vegetation (Poulter, 2005).

Food web dynamics vary greatly among coastal ecosystem types. Tidal flats represent the largest departure from other ecosystem types in that the grazing food web is dominated by deposit-feeders rather than the more typical herbivory of higher plants. This is somewhat deceiving because marshes and swamps also have epiphytic communities that support substantial food webs. Algal production in both seagrasses and saltmarshes can be substantial, especially with regard to macroelements, such as N. This is not to diminish the role of living plant tissue in supporting grazing food webs because there are examples of substantial herbivory, ranging from sea turtles and dugongs for seagrasses and invertebrates for mangroves and saltmarshes. Regardless, plant tissue is often unpalatable to many potential consumers resulting in the majority of primary production being entrained in the detrital food webs (Visser et al., Chapter 15). In spite of the openness of most tidal ecosystems to organic matter exchanges, both particulate and dissolved, there is general agreement that greater tidal amplitudes not only facilitate imports and exports but also that associated currents serve to amplify nutrient cycling and associated primary productivity. Even seagrasses that are completely open to exchange have the capacity to trap particulate organic matter through their baffling effect in comparison with bare sediments (Holmer, Chapter 13). Nevertheless, it should be pointed out that shallow bare sediments used for comparison are similar to the intertidal flats that fully contribute to the habitat complexity of coastal wetlands. Migrating birds also provide a connection, through biomass transfer, between coastal wetlands that can be adjoining or widely separated, and even in different hemispheres.

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Water

Fiona Allon , in International Encyclopedia of Human Geography, 2009

Big Water: Water and Modernity

The historical geography of water circulation highlights the political and economic discourses and practices, as well as social and cultural power relationships that actually become 'built, etched, or baked into' the steel and concrete of the technological and ecological structures of the water system itself. To illustrate this, we could take the example of modern hydrological engineering systems organized by large, centralized decision-making bureaucracies. Such systems are still the basis of current infrastructures for water delivery throughout most Western industrial societies. These large-scale projects that developed during the twentieth century have been termed 'Big Water', and are typified by the creation of new landscapes: giant reservoirs and catchment areas, mountains pierced with pipelines and aqueducts, and of course, massive dams, pipe networks, pumps, and central treatment plants. As one of the quintessential products of modernity's 'engineering era', Big Water embodied long-standing fantasies of mastering nature, including the dream of 'making the desert green', and of creating abundant, always available, flows of water.

The building of dams – the Hoover Dam on the Arizona–Nevada Border, the Marathon Dam in Athens, and the Snowy Mountains Scheme in Australia, to give a few iconic examples – became a key expression of Big Water's vision of technological modernity. At the same time, large-scale water resources development and complex networks such as the urban water supply became the basis of urban expansion and, together with the 'harnessing' of the power of water for energy sources such as hydroelectricity, became a guarantee of a country's status as a modern, modernizing industrial nation. The construction of big dams and massive water infrastructures all over the world in the twentieth century represents the apogee of modernity's Promethean project of conquest and control. It was also the pinnacle of the so-called 'hydraulic mission', an approach to water resources controlled by the engineering profession and technical sciences, guided by the domination of nature, undertaken by powerful colonial and national water agencies, and resulting in around 50 000 large dams and 280 million hectares of irrigated land.

At the beginning of the twenty-first century, both faith in technological progress and the optimistic belief in a steady flow of always-available water have faded. Many dams once seen as engineering triumphs are now regarded as ecological disasters. A survey of the world's 45 000 biggest dams found their construction had displaced around 40–80 million people, majority of whom were poor and vulnerable. Many contemporary dam projects are highly controversial, and international social movements have been formed to highlight the environmental and social costs to local communities. These struggles have included indigenous people and ethnic minorities protesting against the loss of lands, livelihoods, and cultural heritage, and claiming the right to retain traditional social rituals and forms of collective water management. Recent examples include: The Itapu Dam bordering Paraguay, Brazil, and Argentina, the Tucuri Dam in Brazil, the Chixoy Dam in Guatemala, the Malaysian Pergua Dam, the Namarda and Sardar Sarovar Dams in India, the Biobia in Chile, the Arun Dam in Nepal, and the Ilisu Dam in Turkey. In Cochabamba, Bolivia, however, local support for a big-dam project became part of a wider struggle to resist water privatization and World Bank-led neo-liberal water reforms, producing new and sometimes conflicting constructions of regional and national identity and modernity. This also resulted in Cochabamba's highly controversial 'water wars' (Guerra del Agua), which are celebrated as a victory for local communities and social movements against privatization and have become emblematic of the anti-water privatization campaigns and protests that have occurred around the world since the 1990s.

Despite widespread criticism, development 'mega projects' such as new dams, pipelines, desalination plants, and even membrane-covered canals are still promoted as the answer to current water problems. In China, for example, the Three Gorges Dam promised 'water and electricity for all', and went ahead even though 700 000 people were displaced by the project. In Spain, the country's national hydrological plan is one of the most ambitious dam projects ever undertaken, and in the UK new reservoirs (such as the Upper Thames Reservoir) are being built to support new urban housing developments. Meanwhile in Australia some states (e.g., New South Wales and Western Australia) are building large, expensive, and energy-intensive desalination plants to support their urban water networks.

Big Water, however, refers not only to the giganticism of the engineering projects themselves, but also to a whole network of relationships between human beings and nature, nature and the city, the state and citizens, and domestic users and technical authorities/experts. Within these massive infrastructures, water is controlled, circulated, standardized as 'potable', supplied to users, and also disposed of and eliminated, often after an all-purpose, one-time use. The actual system of water management – the dams, treatment plants, pipes, drains, water meters, pricing mechanisms, and taps – not only controls the production, circulation, and delivery of water, but also guides the social and human interactions associated with its use, along with the meanings and values attached to its status as a tamed and plentiful resource.

These large-scale systems and government-owned utilities delegated with responsibility for supplying the continuous flows of water across the modern city went hand in hand with a dispersed system of domestic users who naturally came to expect the ready availability of water in their households. Simply by turning a tap the consumer accessed a seemingly constant flow of water whose source (both physically and symbolically) was far removed from the urban environments where it was actually used, but whose presence symbolized the engineer's promise of a rational, comfortable, modern life.

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Seafloor Processes

M. Maslin , ... V. Ettwein , in Encyclopedia of Ocean Sciences (Third Edition), 2019

Causes of Millennial Climate Fluctuation During the Holocene

As we have already suggested, deep water circulation plays a key role in the regulation of global climate. In the North Atlantic, the northeast-trending Gulf Stream carries warm and relatively salty surface water from the Gulf of Mexico up to the Nordic seas. Upon reaching this region, the surface water has cooled sufficiently that it becomes dense enough to sink, forming the North Atlantic Deep Water (NADW). The "pull" exerted by this dense sinking maintains the strength of the warm Gulf Stream, ensuring a current of warm tropical water into the North Atlantic that sends mild air masses across to the European continent. This circulation is called the Atlantic Meridional Overturning Circulation (AMOC). Formation of the NADW can be weakened by several processes, including: (1) The presence of huge ice sheets over North America and Europe, changing the position of the atmospheric polar front, preventing the Gulf Stream from traveling so far north. This reduces the amount of cooling and the capacity of the surface water to sink. Such a reduction of formation occurred during the last glacial period. (2) The input of fresh water, forming a lens of less-dense water, and preventing sinking. If the AMOC is reduced, the weakening of the warm Gulf Stream causes colder conditions within the entire North Atlantic region and has a major impact on global climate. Bianchi and McCave (1999), used deep-sea sediments from the North Atlantic to show that during the Holocene there have been regular reductions in the intensity of NADW (Fig. 2E), which they link to the 1500-year D/O cycles (O'Brien et al., 1996; Bond et al., 1997). There are two possible causes for the millennial-scale changes observed in the intensity of the NADW: (1) instability in the North Atlantic region caused by varying freshwater input into the surface waters; and (2) the "bipolar seesaw".

There are a number of possible reasons (Wanner et al., 2011, 2015) for the instability in the North Atlantic region caused by varying fresh water input into the surface waters (option 1):

• Internal instability of the Greenland ice sheet, causing increased meltwater in the Nordic Seas that reduces deep water formation.

• Cyclic changes in sea ice formation forced by solar variations.

• Increased precipitation in the Nordic Seas due to more northerly penetration of North Atlantic storm tracks.

• Changes in surface currents, allowing a larger import of fresher water from the Pacific, possibly due to reduction in sea ice in the Arctic Ocean.

The other possible cause for the millennial-scale changes is an extension of the suggested glacial intrinsic millennial-scale "bipolar seesaw" to the Holocene (option 2). One of the most important finds in the study of glacial millennial-scale events is the apparent out-of-phase climate response of the two hemispheres seen in the ice core climate records from Greenland and Antarctica. It has been suggested that this bipolar seesaw can be explained by variations in the relative amount of deep water formation in the two hemispheres and heat piracy (Fig. 4). This mechanism of changing the AMOC by altering dominance of the NADW and the Antarctic Bottom Water (AABW) can also be applied to the Holocene. The important difference with this hypothesis is that the trigger for a sudden 'switching off' or a strong decrease in rate of deep water formation could occur either in the North Atlantic or in the Southern Ocean. AABW forms in a different way than NADW, in two general areas around the Antarctic continent: (1) near-shore at the shelf–ice, sea-ice interface and (2) in open ocean areas. In near-shore areas, coastal polynyas are formed where katabatic winds push sea ice away from the shelf edge, creating further opportunity for sea ice formation. As ice forms, the surface water becomes saltier (owing to salt rejection by the ice) and colder (owing to loss of heat via latent heat of freezing). This density instability causes sinking of surface waters to form AABW, the coldest and saltiest water in the world. AABW can also form in open-ocean Antarctic waters; particularly in the Weddell and Ross Seas; AABW flows around Antarctica and penetrates the North Atlantic, flowing under the less dense NADW. It also flows into the Indian and Pacific Oceans, but the most significant gateway to deep ocean flow is in the south-west Pacific, where 40% of the world's deep water enters the Pacific. Interestingly, Seidov and Maslin (1999) and Seidov et al. (2001) have shown that the Southern Ocean is twice as sensitive to meltwater input as is the North Atlantic, and that the Southern Ocean can not be seen as a passive player in global climate change. The bipolar seesaw model may also be self-sustaining, with meltwater events in either hemisphere triggering a train of climate changes that cause a meltwater event in the opposite hemisphere, thus switching the direction of heat piracy (Fig. 5).

Fig. 4

Fig. 4. Atlantic Ocean poleward heat transport (positive indicates a northward movement) as given by the ocean circulation model (Seidov and Maslin, 1999) for the following scenarios: (1) present-day (warm interglacial) climate; (2) last glacial maximum (LGM) with generic CLIMAP data; (3) "Cold tropics" LGM scenario; (4) a Heinrich-type event driven by the meltwater delivered by icebergs from decaying Laurentide ice sheet; (5) a Heinrich-type event driven by meltwater delivered by icebergs from decaying Barents Shelf ice sheet or Scandinavian ice sheet; (6) a general Holocene or glacial Dansgaard–Oeschger (D/O) meltwater confined to the Nordic Seas. Note that the total meridional heat transport can only be correctly mathematically computed in the cases of cyclic boundary conditions (as in Drake Passage for the global ocean) or between meridional boundaries, as in the Atlantic Ocean to the north of the tip of Africa. Therefore, the northward heat transport in the Atlantic Ocean is shown to the north of 30°S only.

Fig. 5

Fig. 5. Possible deep water oscillatory system explaining the glacial and interglacial Dansgaard–Oeschger cycles. Additional loop demonstrates the possible link between interglacial Dansgaard–Oeschger cycles and Heinrich events.

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WATER CYCLE

D.K. Cassel , B.B. Thapa , in Encyclopedia of Soils in the Environment, 2005

Introduction

The water cycle is an endless process of water circulation on the planet Earth. Water is an essential constituent of all plants and animals, most of which contain more than 60% water, and many contain more than 95%. The planet Earth is dominated by the hydrosphere, which contains all of the Earth's water. Approximately 1.39  billion   km3 of liquid water is stored in depressions, primarily oceans, which occupy over 70% of the Earth's surface. Water is unique in many ways: it has a high heat capacity, high heats of vaporization and fusion, and can exist in solid, liquid, and gaseous phases at temperatures commonly encountered on Earth. The ease with which water can move from one phase to another in response to additions or losses of heat or energy allows water to move or cycle from one storage field to another.

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