Agriculture Reference
In-Depth Information
water from irrigated activities to meet current or future environmental demands will be
greater than in wet years. This reflects the fact that the marginal cost of the last irrigated
activity to which water is allocated will tend to be highest when water availability is
lowest. At the same time, an extended period with low river flows is a frequent and
natural phenomenon. In wetter years the cost of reallocating water from irrigation to the
environment will be lower. Further, in wetter years, tributary inflows will tend to be
highest, and the level of supplementary releases from storages needed to generate high
flow events will tend to be lower.
Exploiting this complementarity or counterseasonal pattern in irrigation and
environmental demands requires appropriately structured institutional and management
arrangements. As recognised in the Australian National Water Initiative, released in 2004,
trade between agricultural and environmental water uses has the potential to limit the
costs of any conflicts between irrigation and environmental demands (COAG 2004). At
the same time, both irrigators and environmental managers need to be able to deal with
the uncertainty associated with environmental objectives that depend on high flow
conditions. Uncertainty arises because neither the timing, nor the volume of water
required to augment naturally high flow events to meet environmental objectives, is
known with any degree of long-term predictive accuracy.
The objective in this paper is to establish a framework for defining environmental
water demands to meet specific flow based objectives that characterises this inherent
uncertainty in a useful way. The approach is applied to billabong and wetland
management in the New South Wales Murrumbidgee River through the supplementation
of natural flows using dam releases. The approach is intended to be adaptable to other
river systems, and environmental objectives that can be met through altered flow
management.
Background
River hydrology and the river ecosystem are interlocked in a variety of ways. The
most obvious are the requirement of all life for water to support metabolic processes and
the fact that aquatic organisms require water as a medium for existence. The natural link
between river systems and wetland environments in a river system is through high flow
events that now occur much less frequently in highly regulated river systems, where the
major share of water may be stored upstream and diverted for other uses, rather than
allowed to flow. As a consequence there has been a loss of connectivity, both laterally
across the flood plains and longitudinally between upstream and downstream
environments. Lateral connectivity provides a conduit for the movement of resources and
biota to and from the flood plain. Longitudinal connectivity permits the translocation of
larger aquatic organisms through, for example, fish migration. High flows also help
harvest food and snags from the banks, modify internal architecture such as benches and
gravel bars, and create new channels and cutoffs.
The case study river, the Murrumbidgee, rises south of Canberra in south eastern
Australia and runs westerly for 1700 kilometres before entering the Murray River. The
Murrumbidgee River catchment is 84 000 square kilometres, and is part of the much
larger Murray Darling Basin. The Murrumbidgee River conveys approximately
4300 gigalitres of water annually, of which around 65 per cent, or 2800 gigalitres, is
licensed for diversion for irrigation and other human uses (Pratt Water 2004). The
Murrumbidgee River catchment is well regarded for the diversity of native flora and
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