Freshwater, People and Ecosystems
by: Chris Scott
The objective of
this presentation is to provide participants with a basic understanding
of freshwater hydrology, by linking physical and ecological processes
with external influences and impacts, both human and biotic. The basic
unit of analysis is the river basin, incorporating upper watersheds,
river corridors and floodplains, land use, and ultimately the downstream
estuary that is the focus of the next presentation. The river basin
water balance will be briefly introduced, although this topic will be
covered more extensively in the tools session on hydrologic assessments.
The hydrologic cycle
is driven by solar radiation that results in precipitation (often as
rainfall), which represents a net transfer of water from the earth's
oceans to land. The majority of precipitation on land, approximately
two-thirds on global basis, returns to the atmosphere as evaporation
from open water surfaces or moist soil, and transpiration by vegetation.
These two processes are difficult to separate; collectively, they are
referred to as evapotranspiration, and have been dubbed 'green water'
because they are the basis for the planet's terrestrial vegetative growth.
The remaining one-third of precipitation on land is partitioned among
several important 'blue water' flows, i.e., in liquid not vapor form,
specifically runoff, infiltration, and groundwater recharge. Runoff
occurs as surface water flow in rivers and streams, while infiltration
recharges groundwater aquifers. The bulk of the world's developed water
resources occurs as surface and groundwater. The relative magnitude
of these flows constitutes the water balance, which is generally defined
on an annual basis for a specific geographical area. Because surface
water flows are relatively easy to delineate, and groundwater resources
often (although by no means always) roughly coincide with surface water
flow delineations, the most appropriate geographical area to determine
water balances is the river basin, or that area of land drained by a
river or stream. The basic components of the river basin are upper watersheds,
stream corridors, basin land uses, floodplains, wetlands, underlying
aquifers, and the estuary.
A complex set of
natural and human factors affects the water balance. Because precipitation
and evapotranspiration flows are so large in magnitude, climatic conditions
(and climate change) are primary drivers of water abundance or scarcity.
At the same time, land use influences the water balance by altering
the partitioning of flows and, insofar as land use is determined by
economic development, it is an important human determinant of the water
balance. Forest cover in upper watersheds tends to distribute runoff
more evenly through wet and dry seasons of the year; however, there
is broad consensus that forests 'consume' more water than most other
land uses with the result that reforestation is not considered to be
a solution to water shortage. Agriculture is a major consumer of blue
(and green) water-up to 85% or greater of blue water flows in tropical
countries-through a variety of irrigation systems. It is also the primary
user of recycled water (drainage flows, treated or untreated wastewater,
etc.) and contributes to water quality degradation through runoff to
surface water bodies and leaching of pollutants to groundwater. Urban
land use often influences the water balance and water quality in ways
that are disproportionate to the area of urban land, through the creation
of impervious surfaces that accentuate surface runoff, and as the source
of chemical and biological pollution of surface and groundwater. Ecosystem
requirements for water that have intrinsic value or that provide essential
services are increasingly being recognized for their importance.
Human water use
takes place through the development of infrastructure that permits surface
water diversions-e.g., dams, reservoirs, canals, drinking water aqueducts-or
groundwater extraction through pumps. Conjunctive use takes place when
a particular demand is met through a combination of surface and groundwater
sources. Water demand has tended to be classified by economic sectors,
e.g., urban, industrial, agricultural, and environment. However, there
is growing recognition that this approach tended to fracture water resources
into artificially separate categories that did not permit meaningful
consideration of mutually positive or negative interactions among sectors.
A more holistic approach that considers various facets of water resources
use, conservation, planning and management is referred to as integrated
water resources management (IWRM) and has received worldwide endorsement.
The USAID Water Team aims to work with Missions to improve IWRM approaches
in the agency's activities around the world.
Estuarine and Coastal Processes:
The Vital Role of Freshwater
by: Richard Volk
This presentation
explores the continuation of the hydrological cycle in the lower basin,
and the central role of freshwater in the physical, chemical, and biological
integrity of estuarine and marine waters. Estuarine environments are
among the most productive on earth, creating more organic matter each
year than comparably sized areas of forest, grassland, or agricultural
land. They are not "delicate" and "fragile" systems,
but rather tough and resilient systems considering the wide swings in
physical and chemical conditions that are inherent in their natural
state of function. Salinities and temperatures can both dramatically
change over the course of a few hours creating extreme stress on resident
fauna and flora. Estuarine ecosystems are typically dominated by a relatively
few, hardy and prolific species adapted to the vagaries of estuarine
conditions. Tides, patterns of sediment movement, freshwater inputs,
geological history, geographical location, and shoreline structure -
combined with human land use and activities - all influence the health
and productivity of estuarine biota.
Although all estuaries
are similar, in that they are semi-enclosed bodies of brackish water,
scientists have used a variety of criteria to classify them. Estuaries
were once defined based on their rate of freshwater evaporation, but
today scientists categorize estuaries with regard to geological characteristics,
stratification, and circulation patterns. A simplified system classifies
estuaries in four categories: drowned river valleys or coastal plain
estuaries, bar-built estuaries or lagoons, fjord-type estuaries, and
tectonically caused estuaries. Eustatic sea level rise over the past
few centuries, and projected future sea level rise, have had and will
likely have significant effects on the extent, structure, and composition
of tidal wetland communities.
An estuary can be
thought of as a factory whose products (and benefits) are seafood, recreation,
flood control, water purification, maritime commerce, and habitat for
aquatic and wildlife species (among others). Energy that drives the
system comes primarily from the sun, but freshwater inflow is the mechanism
that transports the raw materials to the factory. This inflow initiates
the fundamental chemical and physical reactions that begin to convert
these materials into finished products. It also creates the variety
of physical environments such as marshes, mangroves, and seagrass beds
that come together to produce the basic component for all of the factory's
products and benefits - a prolific abundance of fish, shellfish, and
crustaceans. Freshwater inflows provide three important ingredients
fundamental to the success of the factory: (1) fresh water to dilute
sea water and to create a series of salinity gradients which allow estuarine
species to escape predators and parasites at critical stages of their
life cycles; (2) nutrients to fuel the production of shrimp, fish, and
other organisms; and (3) sediments spread over river deltas and marshes
to keep them from subsiding, drowning, and ultimately disappearing.
The Marine Environment The End
of the Watershed…. Or The Beginning?
by: Barbara A. Best
Beyond
the estuarine environments are the marine environments—coastal
and oceanic—where the salinity of the water is normally quite constant.
Although the freshwater running off the land no longer causes a noticeable
reduction in salinity, this marine-based watershed is biologically,
ecologically and hydrologically linked with the land-based watershed.
In many ways, the coastal and oceanic components of the "conceptual
watershed" are the beginning of the watershed—through evaporation
of surface waters, the world's oceans and seas release vast amounts
of water into the atmosphere. Freshwater re-enters the watershed in
the form of rain, fog and dew. The oceans and seas are also critical
drivers of the earth's weather and climate, both globally and locally,
and constitute the largest carbon sink.
In essence, rivers
and streams running into the sea constitute the lifeblood of the oceans,
renewing the waters lost to evaporation. However, in addition to freshwater,
runoff from the land into the sea can also include sediments, nutrients
and pollutants. Major impacts from humans activities in the land-based
watershed are now reducing the amount of freshwater entering coastal
waters in some areas, and accelerating the rate of transport of sediments,
excess nutrients and pollutants into other areas.
The marine watershed
is critically important to the natural and human food supply. Although
coastal waters over continental shelves cover only 10 % of the ocean
surface, they account for 20 % of the marine plant production, which
feeds into the oceanic food chain. Unlike on land, animal life makes
up the majority of biomass in the oceans. The world's primary fishing
grounds are located in these fertile coastal waters, and constitute
90% of the marine catch. Marine fish now provide humankind's largest
single source of animal protein, larger than beef or chicken.
Two major types
of marine environments in the tropics and sub-tropics will be contrasted.
Upwelling areas are created when prevailing trade winds carry surface
waters away, and bring cold, deep, nutrient-rich ocean water up to the
surface. Major upwelling areas occur off the west coasts of North and
South America and Africa. Nutrients in the water fuel algal growth in
the water column, leading to biologically rich food chains, usually
composed of only a few species but with very large populations. (Such
as the sardine fisheries off Peru). In contrast, coral reef ecosystems
are found in nutrient-poor, warm waters where little or no upwelling
occurs. Clear, nutrient-low waters allow light penetration down to the
corals, where primary production and photosynthesis occur within the
tissues of the corals. Coral reefs are characterized by tight-nutrient
recycling, high biodiversity of species, but low population numbers.
These systems are not conducive to high levels of mass transport out
of the system, and can be easily over-exploited.
For the marine watershed,
there are vital linkages between the water column and the bottom habitats,
which can be disrupted due to human activities. In addition to the impacts
of siltation and pollution from poor-land use practices or loss of wetlands
or mangroves, bottom habitats can be altered by destructive fishing
practices, such as trawling, poison-fishing and blast-fishing. The establishment
of marine ecological reserves, or "no-take" areas, are now
recognized as important and effective management tools for both fisheries
and biodiversity conservation.
Other major impacts
can occur from wastewater discharge, ocean dumping and vessel discharge.
Human viruses can remain viable for years in marine waters. Persistent
organic pollutants and heavy metals can enter the food chain and be
dispersed by ocean currents around the world, leading to bioaccumulation
in the food chain and human food supply. Pollutants are also leading
to outbreaks of animal diseases and the disruption of their physiology.
Recently, the concept
of the "airshed" has been introduced to complement the "watershed"
and characterize the far-reaching impacts that humans can have on natural
geo-chemical processes in coastal waters. An "airshed" is
defined as that geographic area from which aerosols originate and are
normally deposited into specific coastal waters. It is estimated that
one-third of the pollutants entering the marine environment come from
air emissions, a large portion of which settle into coastal waters.
For many heavy metals and volatile organic chemicals, air can be a major
or primary route to the sea. For example, in the Chesapeake Bay, farms
contribute about 30% and air pollution about 25% of the nitrogen pollution
leading to eutrophication. In the Baltic Sea, aerosols may contribute
up to 60% of pollutants.
The practice of
intensive open water aquaculture, such as for salmon, is increasing
and leading to the increased incidence of disease in both cultured and
native fish populations; decreases in the populations of native fishes;
releases of non-native fishes; harmful algal blooms; destruction of
bottom habitats under the farm sites; and instability of international
fish markets.
While the land-based
watershed is usually acknowledge as a discrete unit for integrated watershed
management, the full scope of the marine "watershed" is often
ignored. For many island and coastal countries, their Exclusive Economic
Zone (EEZ) is several times larger in geographic area than their land
area. Future development strategies should incorporate the potential
contribution of the marine watershed to the economic growth and food
security of countries, and the need for integrated management of this
extended.
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