The people involved in formulating projections of water needs for food production relied on the use of fairly robust data sets and sophisticated models to reach their compelling conclusion. They estimated that a 17 percent increase in water consumed by irrigated agriculture would be necessary to provide for the nutritional needs of a world population likely to rise to somewhere between 7 and 11 billion by the year The chief spokesman for this subsector made an impressive presentation, replete with charts and graphs.
Thirsty Planet: Strategies for Sustainable Water Management by Constance Elizabeth Hunt - olagynulehyb.gq
The nature conservation people looked rather alarmed at the philosophical gulf that yawned between the two subsectors. Nature is the source of water; therefore our ability to support additional human lives on planet Earth depends upon the protection of nature and the continued operation of the water cycle. The water cycle is the combination of natural physical, chemical and biological processes that constantly recycle water, ensuring a steady supply to support life on Earth.
Our survival depends on this machinery to provide us with essential water. Neglect of the integrity of the water cycle could have severe consequences for people and other living things. Virtually all known forms of life that people consume as food or in commerce must have water to survive. The aspirations of human society expand the applications of water from the simple survival and reproduction needs shared by most other forms of life to comfort, convenience, enterprise and recreation.
Every newborn baby and most human struggles to achieve a higher standard of living place additional stress on the natural processes that provide our planet with water. If we take too much water, or severely damage the ecosystems that constantly renew our water supplies, the water cycle will break down. Water will become scarce or too polluted to use. According to one estimate, by the year , nearly two billion people will live in regions or countries with absolute water scarcity — defined as the lack of sufficient water to maintain current per capita levels of food production and meet expanding urban demands for water even at a high level of irrigation efficiency.
We refrained from using biological weapons during the Second World War and nuclear weapons during the Cold War. The restraint shown in both of these cases was exemplary of the human survival instinct operating in the absence of binding agreements or functional global governance. Similar restraint in the context of water use will be needed to avoid a global water crisis. We must call upon our creativity and generosity to craft solutions that will perpetuate the water cycle. These solutions will consist of a wide array of ideas and technologies, ranging from the communications and networking potential of the Internet to the many generations of ecological knowledge passed down orally in remote corners of the developing world.
We must also recognize the complexity of the problems. The global water cycle comprises many interlocking and nested regional and local water cycles. Similarly, a global water crisis would not be truly global in nature but the combined result of ubiquitous local shortages of water. The solutions must be tailored to the causes of the problems. There is no global solution. In the interest of the long-term sustainability of the water cycle, world leaders must commit their countries to protecting the natural ecosystems that sustain it. In many instances, this may require substantial changes to current patterns of water use.
Uses that change the quantity, quality and timing of water flows to various parts of the natural ecosystem may introduce disruptions to the water cycle that eventually result in a reduction of good-quality water. The objectives that those uses have supported — including food production, flood damage reduction, provision of energy, transport of goods to markets and others — need to be met in ways that sustain the water cycle if we are to ensure an adequate future supply of water for our planet. This argument responds sceptically to the idea that human ingenuity can overcome whatever insults we inflict upon the natural world.
While we have had some limited success in turning salt water to sweet and wastewater to drinking water, the expense of these practices prohibits any reasonable hope of a panacea. This vision rises above the debate over whether or not to rely on controversial infrastructure such as dams to avoid a global water crisis. The first chapter describes how the water cycle works and why its continued functioning relies on the presence of intact ecosystems. Chapter 2 also challenges the reader to consider the viability of broad-brush approaches to resolving the global gap between demand for and supply of water.
Chapters 3—6 present a discussion of how current approaches to water management disrupt the water cycle and what alternatives are available to achieve human objectives in a more sustainable fashion. Topics covered include food production, water supply and sanitation, flood damage reduction and navigation. Chapter 7 addresses the issue of global warming, including its effects on aquatic ecosystems. This chapter also discusses the pros and cons of alternative energy sources in the context of water cycle maintenance. Chapter 8 discusses the protection and restoration of aquatic ecosystems — actions that are essential to the continued operation of the water cycle.
Chapter 9 delves into current policy debates. What institutions are needed to ensure global preservation of the water cycle? How do we ensure adequate representation of and participation by local people in water management decisions that affect their welfare? Will we be able to strike a balance between global governance and decentralization or between the efficiency of markets and the protection of human rights and the environment?
The implications of these policy issues for the continued function of the water cycle are examined. Collectively, these chapters offer an analysis of how current water management practices may need to change if we are to sustain the water cycle. While some of the prescriptions may seem radical or potentially expensive, it is important to note their synergy. Larger investments in watershed management and soil and water conservation practices could improve water yield and consistency and crop production while decreasing rates of erosion and sedimentation that increase the costs of dredging.
The increased use of non-hydropower, renewable energy sources — such as solar, geothermal and wind — could help address air quality and global climate change concerns, both of which affect the availability of potable water. The author hopes that the following compilation of ideas, information and case studies will help place the international dialogue over the potential for devastating water shortages in a new light. Barker, As the planet cooled, its constituents separated like curdled milk. Within about fifty million years, the iron of which much of the Earth was comprised had sunk to the core, and the lighter elements silicon, aluminum, calcium, magnesium, sodium, potassium, and oxygen, along with some remaining iron formed a rocky crust at the surface — just as slag floats on top of molten iron in a smelter.
While the Earth was molten, these volatile compounds were dissolved in the magma, but as the molten rock cooled and solidified, the vapours were released in a process called degassing. And then sometime between 4. Clouds massed in the sky, and the oceans rained down. Water provides lubrication for living cells, the very building blocks of life. Water performs similar functions for entire ecosystems as it does for individual organisms.
It circulates around the world, transporting nutrients and building materials to ecosystems, facilitating the chemical communication between ecosystems, and cleansing ecosystems so that they can maintain optimal performance. What is more, the reliable provision of high-quality water for human uses depends on the healthy functioning of ecosystems, particularly freshwater ecosystems. Water and rock come into being and metamorphose into various forms as a function of the nihilistic physics of the Universe. The evolution of ecosystems with living components that we recognize as the biosphere buffers the raw physical processes that create and destroy matter.
This sophisticated and delicate living skin on the surface of the Earth regulates the movement and quality of water in ways that perpetuate its own well-being and that of humankind. Because more water evaporates from the oceans than falls on them as precipitation, there is a continuous transfer of freshwater from the oceans to the continents. Water is captured and temporarily detained by wetlands, ponds and lakes which eventually release it back into circulation through evaporation, seepage into groundwater systems, or discharge into rivers.
On an annual basis, roughly , cubic kilometres per year evaporates from the oceans, equivalent to the top metre and a half of the sea. Water has a number of exceptional properties that provide it with the capacity to perform these functions. First, it has a high and universal dissolving power that is essential for distributing geochemical material and nutrients and for removing waste substances from living organisms. Second, it has a high surface tension, causing high capillary forces together with osmotic forces, which enables water and solute transport within living things.
Third, a maximum density above the freezing point at 4 degrees centigrade allows freezing to proceed from the surface downward, slowing down both the heat release and the advancement of the freezing process, and thus protecting living organisms. Fourth, water has high freezing and boiling points relative to its molecular weight in comparison to similarly structured compounds such as H2S and H2Se.
All of these properties stem from the high cohesion and pseudo-crystalline structure of water. The total amount of water present in the solar system is estimated at approximately , times the 1,,, km3 mass of water in our oceans. Some scientists theorize that the processes that generated the hydrological cycle continue to operate today. This process is driven by thermal convection currents, which move over the rigid lithosphere that forms a coherent layer over the more plastic asthenosphere. Magma moves upward into the spreading centres, or fractures, in the ocean floor, where it forms new oceanic crust and causes the lithosphere to move away from the fractures.
Subduction occurs at the other end of the convection cell, where the lithosphere sinks downward. The comets would have delivered enough moisture to Earth to cover the entire surface of the planet with 2. The Earth apparently exchanges water with the rest of the solar system as well: solar winds may be taking about four cubic metres of water out of the atmosphere each day in the form of ionized hydrogen and oxygen gases. The atmosphere recycles its entire water content 33 times each year total precipitation divided by total atmospheric precipitable water vapour , giving water vapour a mean global residence time of about eleven days.
Water vapour constitutes twothirds of the total amount of greenhouse gases that warm the planet by trapping heat in the atmosphere. Without these gases, the mean surface temperature of the planet would be well below freezing and liquid water would be absent from much of the planet. This movement eventually propels them into the air as vapour. Water vapour also rises from ice in a process called sublimation.
As mentioned above, warm air can hold more water vapour than cold air. Evaporation also requires energy to convert water into vapour. For example, when a cold, dry air mass passes over a warm ocean current, the high vapour pressure gradient between the warm water and dry air stimulates upward vapour transport. Cooling usually results from the adiabatic expansion of uplifted air caused by the decrease in atmospheric pressure with height.
Heat release by condensation can subsequently provide additional energy to cause a further rise of the air mass, which can result in convective thunderstorms. Ponds evaporate, streams flow to the sea, icicles melt and dribble away, rain falls: water is forever on the move, repeatedly changing its state — liquid, ice or vapour — in the process. In a word, it is dynamic. In much the same way that every living organism has a life cycle, water has a water cycle; it circulates.
Indeed all the water on earth is constantly circulating. The droplets form clouds. Within the clouds, droplets move and may bump into each other, forming larger droplets as a result. When the droplets become too heavy to remain aloft, they fall to the ground as rain, sleet, snow or hail, losing volume to evaporation as they fall.
At low latitudes the air column rises and moves towards the poles on either side of the equator. Cooling air causes the air column to descend at higher latitudes. These tendencies result in the development of convective wind cells with low pressure at the equator and high pressure in the region of 30 degrees latitude. The rotation of the Earth causes the divergent spiralling of low-pressure cells.
The compensating back-flowing air produces the north-easterly and south-easterly trade winds that converge near the equator in the Inter-Tropical Convergence Zone, or ITCZ. Rising air in the equatorial convergence zone produces the high rainfall of the west-tropical rainforest areas, while the heating of the air by compression that occurs in the high-pressure belt creates the low rainfall zone of the dry, subtropical steppes and desert areas. The distance from the moist oceanic air extends these arid zones northwards into remote interiors of the northern hemisphere. Part of the warm air from the subtropical high-pressure belt moves to higher latitudes and forms the prevailing westerlies of the northern hemisphere.
Where this occurs, the warm, subtropical air rides over the polar air and forms polar fronts, which are the cold front that separates the main body of colder, drier air emanating from the poles from the warmer, moister air closer to the equator. The forced uplift of moist, warm air triggers the development of anticyclonic, or low-pressure, cells.
Low-pressure pulses also drive formulation of low-pressure cells in the meandering jet stream in the upper part of the westerlies. These low-pressure cells cycle inward in a convergent, spiralling movement that produces the frontal rains of mid-latitude convective cells in overheated air. Over 60 to 70 percent of North America and Eurasia typically receives a snow cover that reaches its maximum volume in March or April, with maximum depletion at the end of summer.
This lag provides the oceans with their maximum storage around October. In addition, the distribution of precipitation, evaporation and inflow from the continents produces a surplus in the Indian and Atlantic oceans and a deficit in the Pacific and Arctic oceans. Volume I. As soon as water falls on land, it interacts with both the biotic living and abiotic non-living components of the biosphere, making life possible as it flows back to the sea. While some of this water moves over the surface as runoff or in lakes, streams and wetlands, 35 times as much freshwater, or about 30 percent of all freshwater on Earth, is underground at any point in time.
Water enters the system at a recharge point from precipitation or seepage from surface water bodies and leaves the system through evapotranspiration discussed in the next section or by seeping into surface water bodies at a discharge point. If the water table is not at the soil surface, as it often is in wetlands, the soil profile will have an unsaturated zone, known as the vadose zone, above the saturated zone, where available spaces contain some air as well as water. The positions of these features in the landscape relative to the position of surface water systems is a major factor determining the pattern and rate of exchange between groundwater and surface water ecosystems.
Areas that freely allow movement of water into the groundwater are called recharge areas. Areas where groundwater emerges above receiving aquatic ecosystems are called springs or seeps. The configuration of the water table changes seasonally and year to year according to groundwater recharge and discharge. Groundwater recharge, which is the accretion of water to the upper surface of the saturated zone, is related to the wide variations in the quantity, distribution and timing of precipitation.
The same stream channel segment can also function as a discharge area during the dry part of the year and as a recharge area during the rainy season. The most common type of groundwater is meteoric water, which circulates as part of the water cycle. The interactions between surface and underground water are complex; any single water molecule may pass from above the ground to below it many times before it returns to the sea.
Most movement is driven by hydraulic head, which is the sum of elevation and water pressure divided by the weight density of water. The greater the hydraulic head, the more quickly water will move through the saturated zone. The permeability of the substrate also influences the rate at which groundwater flows.
For example, sand is more permeable than clay because the pore spaces between sand grains are larger than those between clay particles. In impermeable substrates, groundwater may be contained in fractures and between layers of rock. Groundwater moves along flow paths of varying lengths from the point of recharge to the point of discharge. In the uppermost, unconfined portion of the saturated zone, flow paths near a stream can be tens to hundreds of metres long and have travel times of days to a few years.
The longest and deepest flow paths may be thousands of metres to tens of kilometres in length, with travel times ranging from decades to millennia. In local flow systems, water that recharges at a water table discharges to adjacent lowland. Local flow systems are the most dynamic and shallowest, and therefore have the most interchange with surface water systems. When the deep flow systems eventually discharge into surface waters, they can substantially influence the chemical characteristics of the receiving water body.
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The groundwater component of stream flow is called baseflow. While baseflow is usually more consistent than flow contributed by surface runoff, it may also vary during the course of a day or over periods of weeks, months, years and decades. Groundwater inputs may also vary over the length of a stream course, with some reaches of a stream receiving water from the ground and other reaches discharging into the ground. During a flood, the rapid rise in stream stage level causes water to move from the channel into the streambanks in a process known as bank storage.
This temporary storage can reduce flood peaks and later supplement streamflows. If the rise in stream stage does cause the water to overtop the bank, widespread recharge to the water table can occur in the floodplain. When this happens, the time required for the floodwater to return to the stream channel by groundwater flow may be weeks, months or years because the lengths of the groundwater flow paths are much longer than those resulting from local bank storage.
The interactions between groundwater and streams and groundwater and lakes differ in several ways. First, the water levels of natural lakes do not change as rapidly as the water levels of streams, and therefore bank storage is less important. On the other hand, because the surface areas of lakes are greater than those of rivers, because there is less shading by vegetation, and because replenishment of lakes requires more time than replenishment of rivers, evaporation exerts more influence on lakes than on streams.
Finally, lake sediments typically have a higher organic matter content than stream sediments. The lower permeability of organic deposits on the lakebed can affect the distribution of seepage and biogeochemical exchanges of water and solutes more in lakes than in streams. Natural renewal rates vary substantially. Renewal rates for rivers average about 18 days. If water is removed and consumed from these components of the hydrological cycle faster than it is replenished, even accounting for the possibility that comets may contribute relatively small amounts to the water stock each year, eventually the planet will run out of water.
However, two-thirds of the 70 centimetres of precipitation that falls over land each year is made available through evapotranspiration. This movement of water through the biosphere is closely coupled with the cycling of biologically important materials such as carbon and nitrogen. In the process of moving water from the soil to the atmosphere, plants combine water with carbon dioxide using solar energy to produce carbohydrates, which form the basis of nutrition for most animals. Together, precipitation from water evaporated by the ocean and that evapotranspired from vegetation account for the , km3 of renewable freshwater available each year.
Freshwater ecosystems are particularly important in this regard because they form the reservoirs that make water available for use by humans, plants and animals. The living components of a freshwater ecosystem can affect the characteristics of the water cycle. For instance, low rates of decomposition in some types of wetlands can cause basins to fill with undecomposed plant material, thus altering water conditions.
Another example is that the water tables of some forested wetlands are regulated by evapotranspiration. When trees are removed from these ecosystems, standing open water and marsh vegetation can develop. The activities of animals such as alligators, muskrats and beaver also contribute to the naturally dynamic nature of these systems. Female alligators, for example, fill in marshes by building nest mounds in which to lay their eggs. These mounds serve as nesting sites for turtles and colonization areas for plants.
Trees and shrubs root on old mounds and may initiate the transformation of an open-water marsh into an upland forest. They may consume enormous amounts of vegetation, allowing water to inundate areas that previously were fairly dry. On the other hand, they also build dens made of piles of reeds, sticks and other vegetative matter plastered with mud, which are often the first places in an aquatic ecosystem to be colonized by plants. Beaver dams, which are often constructed in small, headwater streams, can increase surface water area, forcing water into riparian groundwater systems where it moistens the soil.
Water stored in these areas can provide sources of recharge for the streams during dry periods, adding stability to flows that would otherwise fluctuate or disappear. These are evapotranspiration, infiltration and runoff. Because it is difficult to measure vegetative evaporation and transpiration separately, the two processes are generally considered as a package. A portion of precipitation never reaches the ground because it is intercepted by vegetation and other natural and constructed surfaces. The amount of water intercepted in this manner is determined by the amount of interception storage available on the above-ground surfaces.
This will vary depending on characteristics of the ecosystem such as vegetation type and density. The intensity, duration and frequency of precipitation also determine the amount of water intercepted above ground. Much of this water is subject to evaporation, as is water in the vadose zone. Transpiration is the diffusion of water vapour from plant leaves to the atmosphere. Unlike intercepted water, which originates from precipitation, transpired water originates from water taken in by the roots from the vadose zone.
Vegetation transfers enormous amounts of water from the soil to the atmosphere through the process of transpiration. One Douglas fir tree, for example, can transpire litres in a summer day. The term infiltration refers to the portion of precipitation that soaks into the ground.
Water that reaches the ground may be stored there for relatively long periods of time. Suctioned downward by gravity and capillary action, water infiltrates into the soil through channels formed by soil pores, as discussed above, animals ranging from bacteria and worms to rabbits and groundhogs, and root systems. Areas with natural vegetative cover and leaf litter usually have high porosity and infiltration rates. This is because these features protect the surface soil pore spaces from becoming clogged by fine soil particles created by raindrop splash. Infiltration rates change throughout the duration of a storm.
After a storm passes, gravity drains water out of upper soils. The water that remains in the soil maintains soil moisture, which provides terrestrial plants with water. Water that is not held in the upper reaches of the soil will continue to move downward until it reaches the groundwater table. When the rate of rainfall or snowmelt exceeds infiltration capacity, excess water collects on the soil surfaces in small depressions.
After these depression storage spaces are filled, excess water begins moving down-slope as overland flow, either as a shallow sheet of water or as a series of small rivulets or rills. The sheet of water increases in depth and velocity as it moves downhill. Factors that affect runoff processes include climate, geology, topography, soil characteristics and vegetation.
Runoff typically occurs as overland flow, subsurface flow, and saturated overland flow. Overland flow takes place in areas with low infiltration rates, such as unvegetated slopes and paved, urban areas. Subsurface flow refers to the portion of water that runs off the landscape below the soil surface. Subsurface flow combines with groundwater flows and increases the total amount of groundwater that discharges to a stream channel during a storm.
Saturated overland flow is groundwater that rises above the soil surface or the stream channel elevation and mixes with overland runoff. Water Circulation through Freshwater Ecosystems Water is constantly circulating on and below the surface of the land, and in the atmosphere.
The constant interactions between water and the rocks and soils it encounters on its journey are major factors in the formation of landscape features geomorphology. These features — floodplains, channels and terraces for example — provide the physical habitat for freshwater organisms. Freshwater ecosystems are generally characterized by five broad landscapes. These are riverine landscapes, which include rivers and streams; depressional landscapes, which include lakes and ponds; and three landscape types that define wetlands: fringe, or wetlands at the margins of lakes or estuaries; slopes, or springs and seeps that emerge on hillsides; and flats, or extensive peatlands and mineral soil flats.
Three general sources of water may feed these systems: precipitation, overland flow, and groundwater discharge. It is, therefore, a major determinant of the biological community occupying that ecosystem. Rivers and streams, collectively referred to as lotic or running water ecosystems, are appropriately thought of as the venous system of the biosphere. The quantity and temporal distribution of stream flow, known as the hydrological regime, can be viewed as a complex, interactive function of at least three important extrinsic controls: climate, vegetative characteristics of the river basin, and physical characteristics of the basin.
Climate controls the quantity and temporal distribution of precipitation and the form in which it falls, rain or snow, and thus sets an upper limit to runoff over an arbitrary time limit. As the water drains away from its source, it follows the force of gravity, seeking the lowest path in the landscape subject to the malleability of the underlying substrate. At the top of a watershed, where the descent of a nascent stream is likely to be most steep, a stream will typically carry a scouring load of debris that continues to cut the channel until it eventually settles out in a region of lower energy.
Streams that have no tributaries are labelled first-order streams. Streams with one tributary are second-order streams, and so on from the tiniest trickle to the muscular rivers that empty into the sea. First-order streams tend to originate in steep and often mountainous landscapes; in the process of creating their channels they flow furiously in a relatively straight channel, creating sections of rapids where they meet resistance from the bedrock. At higher orders, which are lower in the landscape, the land is more level and the velocity of the river decreases substantially.
The river may then create a meandering channel, eroding material from the outside of a curve where the water must move faster to catch up with the water moving through the shorter inside of the curve , and depositing material as point bars on the more sluggish inside of the curve. Three primary processes are involved with flowing water: erosion, sediment transport and sediment deposition.
Erosion forms the beginning of most geomorphic processes. The occurrence, magnitude and distribution of erosion processes in watersheds affect the yield of sediment and associated contaminants to receiving water bodies. Soil erosion can occur gradually over a long period, or it can be cyclic or episodic, accelerating during certain seasons or during highintensity storms. Soil conditions influence erosion rates from the land and change with changing temperature, soil moisture content, amount and growth stage of vegetation, and land use.
Substrate particle size, streambed gradient, and flow rates and volumes influence erosion rates within stream channels. Sediment transport is the movement of eroded material within the banks of a river. Sediment transport redistributes material of many different sizes and origins, including eroded material from the watershed and from the bed and banks of the channel itself.
The energy that sets sediment particles in motion is derived from differences in flow velocity in different parts of the channel. The gradient of the stream, the size of sediment and bed materials, and the volume of water flowing through it at any given time largely determine the amount of sediment that a stream can transport.
Sediment deposition refers to the resettling of sediments out of the water column onto a receiving substrate. Sediments tend to drop out of suspension when the flow of water slows, as it does where a sediment-bearing stream enters a pool of quiet water such as a lake or a reservoir. Especially during floods, sediments also deposit on floodplains, where they serve as areas for plant colonization when the floodwaters recede. Sediment deposition within lakes can promote eutrophication by reducing water depth, which leads to relatively warmer water temperatures and larger zones where primary production photosynthesis is possible.
The patterns of sedimentation and erosion vary from stream to stream and sometimes from upper portions of one stream to another. In higher-order streams and large river channels, a more gentle slope gives rise to patterns of channel braiding multiple channels separated by numerous islands and sandbars in a braided pattern and meandering the pattern of winding back and forth across the floodplain like ribbon candy, a result of sediment erosion and deposition as described above. The processes of sediment erosion and deposition are dynamic in nature, and therefore create a constantly changing mosaic of in-stream habitat types.
The physical variables within a stream and river system generate a continuous gradient of conditions, including width, depth, velocity, flow volume and temperature from the headwaters to the final river mouth. Biological communities organize themselves in response to these changes. In the headwaters, which typically include stream orders one through three, streamside or riparian vegetation provides the bulk of food, and thus of energy. In these reaches, invertebrate communities composed of shredders and collectors dominate.
Rivers of medium size rely on algae or macrophytes vascular aquatic plants for food and energy inputs. Scrapers, invertebrates that are adapted primarily for shearing attached algae from surfaces, dominate in these rivers. Large rivers receive quantities of fine-particulate organic matter from upstream processing of dead leaves and woody debris.
In these reaches, depth and turbidity often limit primary production by plant photosynthesis. Here, fish that eat plankton often dominate — a situation that closely mimics the lentic or standing-water systems described below. The interactions between rivers and their floodplains support both terrestrial and aquatic life. Many fish receive cues from seasonal increases in river flows to move upstream to reproduce. During a flood, fish move into ponded areas formed on the floodplain to reproduce.
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When the flood water recedes, the ponds are isolated from the main stem of the river, providing a nursery area where young fish can grow without facing threats from aquatic predators. In rivers with two flood seasons per year, a fall flood will reconnect the river with the floodplain, and the young fish will swim into the main stem. These ponds are also important breeding habitats for amphibians and for the insects that nourish the young fish and amphibians.
In rivers with two flood seasons, the spring flood will also dampen the floodplain soil, setting the stage for the germination and growth of moist-soil plants during the summer, after the floodwater recedes. The second flood in the fall comes just in time to create ponded areas on the floodplain where migrating waterfowl can rest on their long journey in between their summer and winter ranges.
The moistsoil vegetation provides seed heads and tubers for the waterfowl to feed on while the birds remain safely in an open-water habitat, a serendipity that would not occur in the absence of floodplain dynamics. The riparian forests of the American South-west are critical to the survival of many neotropical bird species, because they provide a shaded refuge from the hot climate in the protective limbs of tall trees. Rivers can also be categorized into reaches based on similar characteristics, such as the ratio of shallow rapids riffles and pools, channel width, substrate sand, gravel, cobble or bedrock, for example and depth.
Many rivers exhibit alternation of reach type, with one type becoming more dominant between the upstream and the downstream reaches. In the headwaters, for example, bedrock and cobble-bottomed reaches may be more common, while sand and silt dominate in the lower reaches. In contrast to rivers and streams, lakes, which are lentic systems, cannot create their own beds; the existence of a basin must precede the birth of a lake.
Lake basins form as a result of the interruption of a drainage pattern. The formation of either a basin or a barrier restricts the flow of water. The oldest lake basins on the planet formed this way, including the great rift valleys of East Africa that formed about ten to twelve million years ago. Solution lakes form when materials such as salt, limestone and gypsum dissolve, and tend to be more saline than other lakes. Barrier lakes occur mainly behind sand or gravel bars in coastal areas and contain brackish water because of tidal flow or salt spray. Seepage lakes generally have small watersheds and obtain their water through groundwater inflow, while tributary channels feed drainage lakes.
Physical variability is often expressed in changes in light levels, temperatures, water currents, and sedimentation. Under natural conditions, chemical changes manifest mostly in the composition and concentration of nutrients and major ions. Biological variability includes changes in the structure, function, biomass, species composition, population and growth rates of living organisms in the lake. Light absorption and attenuation by the water column are major factors determining water temperature and potential photosynthesis. The deeper light penetrates, the more photosynthesis can take place.
Photosynthetic organisms include phytoplankton, periphyton algae attached to surfaces , and macrophytes. Temperature has a substantial effect on water density and chemistry, and therefore on the diversity and distribution of organisms in a lake. In contrast to most other compounds, water is denser as a liquid than as a solid.
The maximum density of water is at four degrees centigrade; water any warmer or colder is less dense. In climates that experience two or more seasons, this quality leads to the seasonal mixing and stratification of lake waters, with important biological consequences. During the winter in four-season, temperate climates, the temperature of the water near the bottom of a lake will typically be around four degrees centigrade, while the water at the top of the lake may be substantially cooler — approaching zero.
At this point, the density difference results in the layering, or stratification, of the lake. Assuming that the surface of the lake is covered with ice, and if there is no snow cover on top of the lake to block light infiltration during the winter season, phytoplankton and some macrophytes may continue to photosynthesize. As a result a slight increase in dissolved oxygen may occur just below the ice cover. As microorganisms continue to decompose material in the lower water column and sediments, however, they may deplete any dissolved oxygen that is available to them.
The ice cover would prevent the addition of oxygen from the air. If snow cover blocks light penetration below the ice, no photosynthesis will occur. When the temperature of the surface water equals the density of the bottom water, the lake mixes and becomes essentially homogeneous with respect to temperature and oxygen concentrations.
In the summer, the surface layer will warm and become less dense than the colder bottom water, leading again to stratification. During the summer, photosynthesis in the upper layer of the water is high, leading to high concentrations of dissolved oxygen. In the bottom layers there is no source of oxygen but organisms continue to respire and decompose organic matter, depleting oxygen in the process. The decrease in surface water temperature during the fall season causes the lake to mix once more, and the cycle begins again in the winter with lake stratification.
Some lakes, particularly in warm climates, stratify only once a year during the summer. Other lakes never mix completely, with the result that organic matter accumulates in the bottom water layers and can create a permanent, anoxic, or oxygen-free, environment. Other lakes, particularly in tropical climates, may mix only once a year. Lakes can also be classified according to their biological productivity. A eutrophic lake is highly productive. Accumulation of organic material in eutrophic lakes can lead to oxygen depletion during decomposition, and to eventual filling in of the lake.
An oligotrophic lake has a relatively low rate of productivity. Populations of algae and the animals that feed on them are also relatively low. Some species of fish require cold, well-oxygenated water. A lake can change from oligotrophic to mesotrophic to eutrophic over time, and may eventually fill in altogether. This process is known as eutrophication. Different zones of a lake house different biological communities and these interact with and influence the physical and chemical characteristics of the lake.
The near-shore zone where light penetrates to the bottom and allows macrophytes to grow is called the littoral zone. Macrophytes in this area provide a habitat for species of fish, amphibians and invertebrates that may differ substantially from deeper parts of the lake. In eutrophic lakes, a dense cover of leaves from floating plants such as water lillies Nymphaeaceae family or pond weed Potamogeton spp.
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At the same time, however, the leaf cover prevents photosynthesis in all but a very shallow layer beneath it by blocking out the sunlight. The detritus produced by these plants may rain through the shaded water and is consumed by bacteria and other microorganisms as it sinks. These microorganisms respire in the process, consuming oxygen. Lakes also have open water areas, known as pelagic or limnectic zones, further out from shore where light does not generally penetrate all the way to the bottom.
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These areas are too deep to support vascular plants, but photosynthesis by phytoplankton still occurs. Oxygen depletion may again occur in lower layers of the water column because, even though the phytoplankton are too small to block out sunlight significantly, as they die they also provide a substantial source of nutrition to microorganisms. The surface layer of the benthic zone is rich in life, primarily invertebrates that serve as an important food source for fish.
Wetland is a bit of a catch-all category that includes a very broad range of ecosystem types, generally considered to be intermediate between terrestrial ecosystems, or uplands, and open water or aquatic ecosystems. Under this definition, wetlands generally include swamps, marshes, bogs and similar areas.
The processes that create wetlands are also very diverse and dynamic. Some wetlands, such as the waterfowl-rich prairie potholes of the midwestern United States, were formed by glacial scour. Other wetlands develop at the fringe of aquatic ecosystems, including lakes, rivers and oceans, largely through soil saturation. Springs and seeps, the ecosystems that form when the water table intersects the surface of the land, are also wetlands. Peat wetlands and bogs form through the accumulation of plant material where cold water and anoxic conditions inhibit the breakdown of organic material by bacteria.
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Fens develop in depressions between gravel-filled hills and develop delicate and unique floristic communities dependent on the solution of minerals in water moving through the gravel fields into the depressional wetland areas. Hydrology controls abiotic characteristics such as soil colour, soil texture and water quality, as well as biotic features such as the abundance, diversity, and productivity of plants, vertebrates, invertebrates and microbes.
This control function is not unidirectional, however. The biotic component of a wetland can affect hydrology by increasing or decreasing water level or flow through such processes as interception or transpiration. Low rates of decomposition of organic material can cause basins to fill with undecomposed plant material, thus altering system hydrology.
Water flows and levels in most wetlands are dynamic, fluctuating daily in coastal marshes and seasonally in almost all wetlands. Water supplies to wetlands also vary significantly from year to year. In addition, moisture gradients vary temporally as well as spatially at the margin of a wetland, with plants, animals and microoganisms responding to the gradient. Difference in water sources can often explain temporal differences in water supply to wetlands. Overland flow, groundwater and precipitation maintain depressional wetlands. Changes in channel flow, including overbank flows carrying nutrients and organic matter, create seasonal or periodic pulses of water level in riparian wetlands.
Daily tides pulse estuarine fringe wetlands. Precipitation alone can be sufficient to maintain peatlands. Groundwater-dominated wetlands generally maintain higher productivity than precipitation-dominated ones because the nearly continuous flow of the former supplies nutrients and displaces sulphide and other potentially toxic compounds. Along the edges of rivers, for example, opportunistic species are the first to invade newly deposited sediments, but are eventually displaced by different plants and animals as the system matures.
At the edges of continents, salt marsh grasses and succulents colonize new mud flats formed by alluvial outwash. These plants trap sediments and build up the topography, attracting additional plants and animals. Sphagnum moss and herbaceous plants develop a mat that eventually supports bog shrubs and bog trees.
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The nutrient content of the soil and the biomass of plants and animals increase over time in all of these ecosystems, as does species diversity and ecosystem complexity. This process of succession is bi-directional, as disturbances such as floods and fires can set back maturing ecosystems to earlier stages of succession. Estuaries occur at the mouths of rivers where seawater becomes diluted by freshwater draining from the land. Estuaries are highly complex and variable ecosystems, driven by both riverine and oceanic hydrology operating on different time scales, with ocean tides cycling on a daily basis and river flow varying according to season.
The degree to which estuarine ecology is driven by the contributing river or the sea also depends largely on whether the estuary mouth is open to the sea or closed by a sandbar or other obstruction. Incoming flood tides force seawater into estuaries, thus raising the water level. Water levels near the mouth decrease more quickly than those higher up near the outflowing ebb tides. The difference in water level can create strong, outflowing currents that keep the mouth open. Conversely, rivers carry sediments into estuaries, where the relatively slow, shallow water causes the sediment load to drop out of suspension.
Wind and coastal currents also transport marine sediments into estuaries. When river flow is low, the river and tides may carry in more sediment than outflowing tides can remove, causing the mouth of the estuary to shallow or close. In addition, the salt concentration in the water increases between the river and the mouth of the estuary, and this gradient is an important factor determining the distribution of plants and animals within the ecosystem.
The gradient moves, of course, in response to changes in river flow and the direction of the tide. The runoff of a river into an estuary produces a distinct circulation pattern where the flow of freshwater over the top of the denser salt water causes nutrientrich bottom water from the river to upwell to the surface. This leads to the particularly high productivity of estuaries. Floristic components of estuaries range from algae and seaweed to submerged macrophytes and emergent reeds to mangrove forests.
The distribution and composition of these plant communities change with shifts in salinity, nutrient content and sediment deposition that favour one set of species over another in the process of succession. Floods, flushing and scouring can reinstate earlier conditions favouring formerly successful species. These cycles occur over time periods that range from days in the case of microscopic plants and animals to many years in the case of larger plants, such as reeds and mangroves.
Many estuarine organisms, including prawns, crabs and some fish, have to spend some time at sea to complete their life cycles. Seasons of the year and lunar cycles often determine the timing of these movements. Estuaries are also important nursery areas for many species of marine fish. Warm surface water currents flow from the equator towards the North Atlantic, where they cool as they exchange heat with the colder atmosphere.
As the seawater cools, its density increases and it sinks to the bottom of the ocean and returns to the tropics. This system transports huge quantities of heat to high latitudes. It also produces a global-scale flux between the major ocean basins of oxygen, organic nutrients such as nitrate, phosphate, and silicate and various trace metals. The quantity of water that returns to the sea, and the points at which it returns, exert significant influence on thermohaline circulation because freshwater alters the salinity and thus the density of the receiving seawater.
The loss of the density differential would effectively shut down the motor that powers the conveyor belt and tropical waters would no longer move northward in the Atlantic Ocean. Internal Ecosystem Dynamics Although aquatic ecosystems regulate the water cycle, the life forms within these ecosystems are also extremely dependent on the complex processes that drive the cycle. The abundance, diversity and productivity of plants, vertebrates, invertebrates and microbes all depend on the distribution and movement of water. The distribution of organisms in lakes, for example, and thus the entire organization of the food chain, is determined by the depth, temperature and water quality of the lake at various times throughout the year.
For example, high flows can trigger spawning runs in migratory fish populations.
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- Thirsty planet: strategies for sustainable water management.!
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Conversely, low flows in river ecosystems can provide opportunities for moist soil vegetation to grow in parts of the floodplain that are inundated under high-flow conditions. Patches of herbaceous vegetation provide food for semi-aquatic animals such as waterfowl as they migrate along river corridors. Flooding also nourishes floodplains with sediments and nutrients and provides habitat for invertebrate communities. Many birds that breed in desert ecosystems depend on the relative cool of riparian forests for reproductive success; were it not for the shade of the tree canopies next to rivers, their eggs would literally cook in the heat.
Similarly, as discussed above, the water cycle influences the structure and function of freshwater ecosystems.
The role played by freshwater ecosystems in the water cycle frequently provides services that are valued by people. These services arise from natural functions and therefore occur at no financial cost to society. However, if an ecosystem is damaged or destroyed it will not continue to function in the same manner and natural services may be lost. This is likely to have adverse impacts on the health, welfare and safety of the communities that benefited from the natural services previously provided, as well as entailing a financial cost to society.
In addition, annual costs will accumulate for whatever mechanisms the communities implement to replace the services that had been provided by the natural environment. Many millions of people, particularly in rural areas of developing countries, rely on rivers directly for many of their needs, including food, drinking water, sanitation and transport.
While people in the developed world often have alternatives to these very basic uses of rivers, they still gain many important social and economic benefits. Floodplain grasses contain a relatively high protein content, making them especially good livestock fodder. Pastoral communities synchronize their livestock cycle to the annual river flood, moving their herds onto arid, rain-fed rangelands as the water recedes.
The post-flood plain pastures of the delta support the highest density of herds in Africa, including cattle, sheep and goats, as well as over , hectares of rice. During drought and before rice harvests, the people depend on the natural production of wild grains in the delta. During times of flood, 80, fishermen depend on the fish catch, which amounts to over 61, tons. In addition, most of the African lakes and rivers, and a number of African wetlands support tribes whose livelihood has historically depended on fishing.
Large lakes can help to moderate the climate of adjacent landscapes by retaining and radiating heat in winter and creating a cooling effect in the summer. In addition, lakes can be significant sources of scientific information. This book looks at the complexity of the problem. It provides a wide array of ideas, information, case studies and ecological knowledge - often from remote corners of the developing world -- that could provide an alternative vision for water use and management at this critical time.
Essential and compelling reading for students on courses related to water resource management and development; water managers and decision makers, and non-specialists with an interest in global water issues. Water Cycle Dynamics and Freshwater Ecosystems. Water Use and Growing Shortages. Food Supply and the Water Cycle. Water Supply and Sanitation in a Watershort World.
Water Management for Flood Damage Reduction. Inland Waterways and the Water Cycle. The Potential for Restoration.