Modern assessments of water
focus largely on the availability of water for unrestricted human
use, but have yet to explicitly consider the environmental needs of
the aquatic ecosystems. In order to provide for the sustainable utilization
of water resources, such assessments must determine the extent to
which a river's flow can be altered from its natural condition, while
still maintaining the integrity or an acceptable level of degradation
of the ecosystem. Worldwide recognition of this need has produced
the concept of "environmental flows" or "environmental
water requirements."
Environmental water requirements are defined as the quality and quantity
of water required by an aquatic ecosystem for the protection and maintenance
of its structure, functioning, and dependent species. If ecosystems
are to sustain the crucial water-related goods and services humans
depend upon to survive, a certain amount of water must be reserved
for them.
The Needs of Aquatic Ecosystems
Freshwater ecosystems and their associated coastal areas need maintained
water regimes to support their plant and animal communities and
ecological processes. A water regime is the prevailing pattern of
water flow for that system, over a given time. Some rivers have
a naturally constant flow of water with higher seasonal floods.
Intermittent rivers in arid areas, on the other hand, do not have
a constant flow, but rather periods of high peak flows. Native species
are accustomed to and rely on these flow patterns for their survival.
Determining environmental water requirements for an ecosystem involves
identifying those aspects of the natural water regime that are most
important for sustaining its key ecosystem features and processes,
then ascertaining the minimum quantity of water needed to sustain
them.
Unfortunately, population growth, industrialization, and the expansion
of irrigated agriculture have increased demand for many water-related
goods and services, straining the capacity of freshwater ecosystems.
These phenomena make water scarcity a top priority for many countries.
Ideally, only the excess water in the system-the amount of water
above and beyond what the ecosystem requires to maintain its ecological
processes and dependent species-should be taken for agricultural,
domestic, or industrial uses. However, in reality, this is rarely
the case. Observing environmental demands will guide wise resource
use by providing specific target benchmarks for water withdrawals.
The Global Picture
The study of environmental water requirements of aquatic ecosystems
is a rapidly developing field. Current methods for estimating environmental
water requirements differ in input information requirements, types
of designated ecosystems, time required for application, and the
level of confidence in the final estimates. They range from hydrological
methods, largely data-driven, to multidisciplinary models, involving
expert panel discussions and ecological information.
As a rule, ecosystem water requirements are case-specific, and must
be carried out at the scale of individual river basins. In an effort
to gauge the general state of the world's water resources situation,
however, a global model of water availability that takes ecosystem
requirements into account was developed by the International Water
Management Institute, the World Resources Institute, the Center
for Environmental Systems Research, and the World Conservation Union
in 2003.
Preliminary results of the study portray a bleak picture of the
condition of the world's river basins. In many parts of the world,
humans are tapping into water that is needed to sustain healthy
ecosystems, damaging the livelihoods of fishers and local communities.
Incorporating environmental requirements into water assessments
highlights long-ignored, yet vital, aspects of water scarcity. This
new information shows the world's water resources to be in a much
more troubling state than originally assumed.
The map above (Fig. 2) represents the traditional method in which
water scarcity is assessed. It compares water withdrawal to water
availability without taking into account the environmental needs of
aquatic systems. This is the approach used in most current water resources
assessment models and scenarios. The map below (Fig. 3) measures the
proportion of water withdrawal with respect to water available to
human use, where the portion of the basin's water that is estimated
to constitute an environmental water requirement is not open for human
use. A comparison of the two maps shows that when the ecosystem's
water requirements are taking into account, more basins have a high
degree of water stress. In addition, the circles in Fig. 3 identify
perfect examples of basins where excessive extraction of water is
causing problems to the ecosystem and to the people that depend on
the environmental services that the ecosystem provides.
The Future
of Environmental Water Requirements
A few countries, including Australia,
the United States, and South Africa, possess a solid track record of applying
different water assessment methodologies. Sadly, they make up the exception
rather than the rule. Most nations, even those privileged with considerable
financial and technical resources, lack the information necessary to determine
or even estimate environmental water requirements. The aforementioned global
model relies exclusively on hydrological data and simple conceptual rules, but
ideally, basin-level environmental requirements should also factor in water
quality, biodiversity, runoff, drought conditions, the desired future state
of a basin, and differences in availability of water during different seasons
of the year. The scarcity of such data makes the obstacle of measuring environmental
water requirements an even greater challenge.
Fortunately, a few ongoing freshwater assessments and initiatives
currently focus on improving access to basin-level data, as well as
information on freshwater ecosystem goods, services and resources.
Firmly establishing accurate environmental water requirements would
necessitate the collection and analysis of local and/or regional information
in the context of variable flows. This would allow for a better understanding
and quantification of hydrology; realistic management targets for
different ecosystems, basins, and regions; and the proper evaluation
of trade-offs. It is envisaged that information generated through
such initiatives will reinforce the case for environmental water requirements
and provide insights into their development.
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