1.1
Water functions
Resources of drinking water and process water: a minute proportion of global water stocks – Water as food, habitat and regulator – Utilization and pollution of water resources by people
The entire stock of water on the “blue planet”, Earth, is estimated at approx. 1.4 billion km3 (Fig.
D 1.1-1). The world’s oceans, which cover 71% of the Earth’s surface, account for 96.5% of that total. The rest is found as polar ice masses and glaciers (1.77%), as groundwater (1.7%) and as the waters of lakes, swamps, rivers, permafrost and the atmosphere (0.03% all told). Freshwater stocks account for only 2.5% of the total volume of water in the hydrosphere (35.1 million km3), of which 69% is stored in glaciers and permanent snow cover and around 30% as groundwater. A mere 0.3% of all surface water is freshwater. Although the water volume on the plan-et has been constant for a long time, distribution among the three different water phases (ice, water and water vapor) has been subject to major fluctua-tions during Earth’s history.
The water resources of crucial importance for hu-mankind and which are continuously renewed
through rapid turnover are the small amounts of wa-ter in lakes, streams and “active” groundwawa-ter.Annu- groundwater.Annu-al runoff to the oceans is about 41,000 km3, of which 28,000 km3per annum flows directly as surface run-off, and 13,000 km3through groundwater to rivers.
Around 8% of this renewable resource is used at present, 69% being agricultural use, 23% industrial use and 8% domestic use. Regional variations are very large – for example, 96% of all industrial water use occurs in North America and Europe.
Before we deal in detail with freshwater stocks, freshwater turnover and freshwater use, an overview of the manifold functions of wateron Earth, the “ter planet”, should first be given.A description of ter’s functions shows not only the significance of wa-ter for life and the balance of nature, but also the role of water as a renewable resource and as a cultural as-set for people. A list of these functions is found in Box D 1.1-1.
Oceans 96.5%
3.5%
Ice caps and glaciers
1.77%
Groundwater 1.7%
0.03%
Permafrost, lakes, swamps and
rivers 0.03%
Atmosphere 0.001%
Figure D 1.1-1 Global distribution of water resources.
Source: Gleick, 1993
46 D 1.1 Water functions
1.1.1
Natural functions
1.1.1.1
Life-sustaining function
Not only do all organisms consist primarily of wa-ter (plants and animals 50–95%, humans 60%), but their physiological processes require an aqueous mi-lieu as well. Terrestrial organisms therefore need a regular input of water due to the unavoidable water losses to their surroundings. Lack or scarcity of water can mean the loss of vital functions within a short space of time, which means death if the organism is unable to survive protracted periods in a suspended, water-conserving state. Active life on land is insepa-rable from the availability or consumption of water, and the assimilation of CO2for biomass production.
Depending on prevailing climate conditions, crop stands consume between 10 and 100 cubic meters of water per hectare and day.
1.1.1.2
Habitat function
For many organisms, water represents not only an essential foodstuff, but also their very habitat. This holds true not only in the oceans, where life first orig-inated, but also in surface waters such as rivers, lakes
and swamps, groundwater, and the interstitial water of the pervious zone (soils). The planet could never have acquired its present state without the many dif-ferent life forms that evolved in the various aqueous habitats. In addition, aquatic habitats are in close interaction with terrestrial ecosystems and the atmo-sphere. Habitats exert mutual influence on each oth-er, especially through substance loads and exports.
1.1.1.3
Regulatory functions
Water’s different phases and its specific phase transition properties make it a critical regulator of the Earth’s energy and mass balance. Ice surfaces in-fluence the radiative balance on account of their back radiation (albedo). The energy consumed by evaporation and thawing and the energy released when water condenses and freezes are a fundamental driving force of the Earth’s weather and climate sys-tems. Water vapor in the atmosphere is the most im-portant greenhouse gas, without which the annual mean temperature would be significantly below the present level.
Due to its properties as a solvent and transporta-tion medium, water also influences the mass balance on the planet. During heavy rains and floods, water causes erosion, moving soil material to sink areas or into the ocean. Rainwater infiltrating the ground chemically alters rock and soil (weathering). In doing BOX D 1.1-1
Functions of water
Natural functions
• Life-sustaining function
• Habitat function
• Regulatory functions – Energy balance – Hydrological cycle – Matter balance
– Morphological function (floods, erosion, sedimentation)
– Solvent and transportation medium – Self-purification of nature
Cultural functions
• Consumption, withdrawals – Foodstuff function
– Drinking water
– Preparation of meals and beverages – Cleansing function
– Production functions
– Raw material (chemistry, food produc-tion)
– Process water (transport medium, deter-gent, solvent, extinguishing adeter-gent, etc.) – Crop and animal needs (irrigation,
aqua-culture, watering places, etc.)
• Utilization
– Therapeutic function – Energy carrier function – Transportation function – Recreation function – Design function – Aesthetic functions – Religious functions – Supply function (fisheries)
• Pressures
– Disposal function – Self-purification function
47 Cultural functions D 1.1.2
so, dissolved ions and other components are flushed into groundwater, waterbodies or the sea. Biological communities in waterbodies, soils and in groundwa-ter may absorb and decompose nutrients and energy carriers (organic substances) that are dissolved in water. This is of major importance for the biogeo-chemical cycles of carbon, nitrogen and other nutri-ents.
1.1.2
Cultural functions
Clean and fresh water is quintessential for life on Earth and human activities. In many regions of the world, however, people have virtually no access to clean drinking water or water for industrial use. This results not only from very uneven spatial distribution of water resources, but also and especially from the irresponsible way in which they are managed.
Box D 1.1-1 lists the manifold functions that water performs for humans. In order to use water, it must first be withdrawn from surface waterbodies or groundwater. It is then either consumed, i.e. convert-ed from the liquid to the gaseous or chemically bound state, or it is used in some way and thus changed in its quality. The wastewater that ultimate-ly results is channelled in most cases to surface wa-ters or to groundwater after passing through the soil.
Water resources perform a function here as a medi-um for disposal, transport and distribution.
In order to survive, people require between 3 and 8 liters of water per day, depending on prevailing cli-mate, in order to cover their needs and to prepare their food. People consume additional quantities of water to varying extents. Consumption occurs when liquid water is evaporated, for example in crop irriga-tion or when used as a coolant. Water is also con-sumed, however, when it is used as a raw material in the chemical and food industries. Water use refers to all those activities in which water undergoes a marked change in quality and loses some of its origi-nal characteristics. Such changes occur in cleaning, through use as process water, in crop and animal pro-duction and in aquaculture.
Waters thus contaminated are returned to the en-vironment, where they pollute soils, surface water-bodies and groundwater. The disposal and purifica-tion funcpurifica-tions of waterbodies are exploited in order to dispose of waste. However, critical loads are often exceeded, with the result that the habitat functions of lakes and streams are severely impaired. The filter and cleansing function of water in the unsaturated (vadose) zone is used in a similar way when unclean water is purified during its passage through soil, after which it is clean groundwater and ready for re-use.
Water is exploited by humans in various ways that involve no change in the actual volume of water. For example, water is utilized in power generation, while waterways serve on a large scale for the transporta-tion of goods and passengers. The therapeutic and recreational functions of water are commercially ex-ploited. In addition, water is used as a design element in the human environment, e.g. in fountains and land-scaping. Even when these types of use do not result in volume reductions, they may involve pollution that in turn impairs the habitat functions of waterbodies.
Because of water’s mobility and properties as a solvent, contaminants may be transported along with water into neighboring ecosystems, where they can cause severe damage. Examples include acid rain, sa-linization of soils, as well as nitrate and pesticide loads to groundwater.Waterbodies are also used on a large scale for waste disposal, with negative impacts on the environment.
All forms of water resource management, be it withdrawal and consumption, use or contamination, are influenced to a greater or lesser degree by the values operating in the respective sociocultural con-text. This specific context, which may be termed wa-ter culture, can be classified in wa-terms of several fea-tures. These, in turn, are interdependent and can be observed in different societies, separated in space and time, in specific “mixes” (see Section D 5.2). The most important dimensions of water culture include:
– the scientific and technological dimension, – the economic dimension,
– the legal-administrative dimension, – the religious dimension and
– the aesthetic and symbolic dimension.
These dimensions of water culture overlie the way in which a society manages its water resources, as manifested on different levels of social action. This generates a kind of matrix, with the cultural functions of water and the sociocultural dimensions as the two axes, and the cells of the matrix having a different content depending on the society in question.
This description shows clearly that water resource management can be sustainable and environmental-ly sound onenvironmental-ly if due consideration is given to the manifold and interrelated functions of water, and ex-isting knowledge is used to develop wise and appro-priate strategies for water use.
48 D 1.2 Water as habitat
1.2
Water as habitat and its importance for neigh-boring environments
Uneven global distribution of freshwater lakes – 10–400 liters of water in 1 cubic meter of soil – Func-tions of wetlands – 66% of all extinct species lived in freshwater ecosystems
Freshwater ecosystems (lakes and streams) cover 2.5 million km2in total, or less than 2% of the Earth’s surface (Wetzel, 1983). Together with soils, they con-tain only 0.014% of the Earth’s total water stock.
Freshwater ecosystems are highly diverse in their physical structure and species composition (Hutchin-son, 1957, 1967 and 1975; Hynes, 1970; Wetzel, 1983).
Through the exchange of water and the material dis-solved or suspended in it, freshwater habitats are linked to the surrounding terrestrial ecosystems, and via atmospheric pathways are exposed to impacts from distant regions.
1.2.1
Standing waters
The properties of inland lakes are closely linked to the way in which they were formed, the size and char-acteristics of their catchments, geological factors and climate (Hutchinson, 1957). Most lakes are young in geological terms (10,000–20,000 years; Wetzel, 1983).
The exceptions are lakes of tectonic or volcanic ori-gin (Lake Baikal, lakes of the East African rift, lakes of volcanic origin). Geologically old lakes often ex-hibit high species diversity and high endemism, i.e.
they are host to species that are found nowhere else in the world (Snimschikova and Akinshina, 1994) (see also Box D 1.2-1).
The water stored in freshwater lakes is unevenly distributed over the Earth’s surface. Approximately 18% of all liquid freshwater is stored in Lake Baikal alone (23,000 km3). The Great Lakes in North Amer-ica hold the same volume of water in total.
The unique physical properties of water – its high density (780 times that of air), heat capacity and op-tical density – are overriding determinants of the bi-otic conditions in aquatic habitats. Standing waters have vertical gradients of temperature and light, as well as varying concentrations of gases. The amounts of dissolved and particulate substances as well as or-ganisms vary with water depth, and in many cases are subject to daily or seasonal changes.
The cycles of many chemical elements and nutri-ents in lakes are closely linked to the biological com-munities present. Producers (especially green plants)
form living organic material that is used by fauna (consumers), and finally decomposed to its inorganic constituents by bacteria and other microorganisms (decomposers). Organic material that is less easily decomposed is deposited as sediment on the lake floor. The manifold biological functions operating within an aquatic ecosystem are maintained by a large number of species. Bacteria play a particularly important role in nutrient cycling and biological self-purification.
Biological communities in the benthic zone (the floor of lakes and oceans) have certain features in common with terrestrial communities. However, the high specific density of the water as compared to air allows a diverse community in the pelagial zone to which there is no equivalent in non-aquatic habitats (Wetzel, 1983). Open-water biological communities are composed primarily of microscopically small or-ganisms called plankton, which are able to remain passively suspended in water. Plankton consists of predominantly unicellular cell algae (phytoplank-ton), bacteria (bacterioplank(phytoplank-ton), as well as small an-imals (zooplankton). Phytoplankton forms its organ-ic substance through photosynthesis. With the help of pigments (especially chlorophyll), light is absorbed and used as an energy source to form biomass from carbon dioxide and nutrients (primary production).
Because the intensity of light declines exponentially as depth increases, primary production in inland lakes is confined to water layers near the surface (less than 20 meters in most lakes). Zooplankton, in turn, feeds on phytoplankton, suspended bacteria and pro-tozoa, which it takes up and assimilates (secondary production). In inland lakes, the main zooplankton are cladocerans (“water fleas”), copepods and roti-fers. Cladocerans, which are at most only a few milli-meters in size, are the preferred food source for plankton-feeding fish. Most bacteria live on organic substances dissolved in water. Bacteria and zoo-plankton secrete nutrients, which in turn can be tak-en up by phytoplankton. The regtak-eneration of inor-ganic nutrients (especially phosphorus and nitrogen) allows for efficient use of these resources, which are often in short supply. The maximum possible biomass depends on the amount of available nutrients, which determines the trophic state of a lake and influences the species composition of the biological commu-nities (Lampert and Sommer, 1993). The state of in-land waterbodies is closely linked to the characteris-tics of the drainage basin, and is subject to anthropo-genic influences on the supply of nutrients (Section D 4.4).
At greater depths where there is insufficient light for primary production, organisms depend on the or-ganic material that is present. Oxygen is then con-sumed when such material is decomposed. Where
49 Running waters D 1.2.2
there is a high concentration of biomass, as is often the case in nutrient-rich (eutrophic) lakes, the dis-solved oxygen may be depleted fully at deeper layers that receive no light, thus depriving many organisms of life support.
The characteristics of the biological communities of the benthic zone differ markedly between shallow areas in the vicinity of lakeshores, and the lake floor.
Sufficient light for plants to grow (emergent, float-ing-leaved and submersed macrophytes, sessile al-gae) is available only near lakeshores, where species-rich biological communities can develop. On the lake floor, organisms depend for nutrition on organic ma-terial that sinks to the bottom. This habitat is colo-nized by various species such as horsehair worms, ol-igochaets (Tubifex), insect larvae, mussels and bacte-ria. In cases where there is no dissolved oxygen in bottom waters, colonization of the lake floor is usual-ly limited to bacteria. Freshwater fish feed on plank-ton, or draw nutrition from the benthos. Many of them, especially salmon-like species (e.g. trout), are highly sensitive to low oxygen levels and elevated water temperatures.
1.2.2
Running waters
Only about 0.004% of the total liquid freshwater on Earth is found in streams and rivers (Brehm, 1982;
Hynes, 1970). Communities in running water are dominated by fish, sessile organisms or species (such as freshwater snails and blackfly larvae) that are suit-ed to life in water currents (e.g. through adaptations such as flattened body shape and attachment mecha-nisms). Plankton are found only in smoving low-land rivers (Schwoerbel, 1987). Sessile algae and higher aquatic plants (e.g. amphibious buttercup) predominate in the riverbed flora. Among the ani-mals, water insects play a dominant role. In running waters, the most important source of nutrition is the organic material deposited from neighboring terres-trial areas. In addition to predatory invertebrates and fish, the dominant organisms are those which take up decomposing organic matter (detritus eaters).
Along the course of running waters, the conditions for life change gradually (Niemeyer-Lüllwitz and Zucchi, 1985).Water temperature generally increases from the spring to the estuary, with daily fluctuations decreasing in amplitude. The flow of water enhances exchange processes with the atmosphere. The upper reaches of rivers with intense mixing are therefore saturated with oxygen as a rule. Here, cold-adapted animals with high oxygen requirements (larvae of mayflies and stoneflies) predominate. In lower reach-es, the river depth increases and gas exchange with
the atmosphere declines. Turbidity increases, the sub-stratum is more and more finely grained, and the con-centration of dissolved and particulate organic sub-stance increases. Oxygen is consumed when organic material is decomposed by microbes. If oxygen is not replenished in organically or thermally stressed wa-terbodies, fish and other water organisms may perish (see Section D 4.4).
Along the course of European rivers, a distinction is made between four regions with declining oxygen demand. These are designated according to the typi-cal fish species present there (trout zone, grayling zone, barbel zone and bream zone) (Fig. D 1.2-1).
This zoning is paralleled by invertebrate commu-nities, and is comparable to the zoning patterns of other continents (Illies, 1961). Water quality usually declines along the course of running waters due to civilizatory factors such as waste discharges, despite dilution and self-purification (Schmitz, 1961).
1.2.3
Soil and groundwater
Depending on its type, 1 cubic meter of soil may contain between 10 and 400 liters of water. In the un-saturated zone above the groundwater, water and air fill the pore spaces. Soil water and its biological com-munities are immensely important for the stability of terrestrial ecosystems and the maintenance of nutri-ent cycles. Water is the solvnutri-ent and transportation medium for many substances, and provides plants with the water and nutrients they need to grow. Soils manifest steep vertical physico-chemical gradients and differences in colonization. Photosynthesis is not possible due to the absence of light. With the excep-tion of bacteria that can use special chemical reac-tions to produce their own energy, living organisms in soil and interstitial water are therefore heterotroph-ic, i.e. they are dependent on organic substance as their source of energy and carbon. These include fun-gi and invertebrates such as horsehair worms, earth-worms, freshwater isopods, wingless insects and lar-vae of winged insects. Loads of organic carbon to top-soil may amount in productive ecosystems to 5–10 tons per hectare and year, falling to a mere 10–100 kg under the rooting zone. By far the greatest
Depending on its type, 1 cubic meter of soil may contain between 10 and 400 liters of water. In the un-saturated zone above the groundwater, water and air fill the pore spaces. Soil water and its biological com-munities are immensely important for the stability of terrestrial ecosystems and the maintenance of nutri-ent cycles. Water is the solvnutri-ent and transportation medium for many substances, and provides plants with the water and nutrients they need to grow. Soils manifest steep vertical physico-chemical gradients and differences in colonization. Photosynthesis is not possible due to the absence of light. With the excep-tion of bacteria that can use special chemical reac-tions to produce their own energy, living organisms in soil and interstitial water are therefore heterotroph-ic, i.e. they are dependent on organic substance as their source of energy and carbon. These include fun-gi and invertebrates such as horsehair worms, earth-worms, freshwater isopods, wingless insects and lar-vae of winged insects. Loads of organic carbon to top-soil may amount in productive ecosystems to 5–10 tons per hectare and year, falling to a mere 10–100 kg under the rooting zone. By far the greatest