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| Topics in Water & Environment |
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Environmental Flow Assessment
Overview, Types of Instream Values
Instream flows are minimum amounts of water necessary to preserve a river or stream's instream functions and values. This section provides a framework for defining and addressing instream flow requirements, types of instream values, methodologies for quantifying instream flow needs as well as balancing the needs with other water demands and the process for developing instream flows requirements.
Rivers are not only sources of freshwater but also very dynamic, complex and diverse ecosystems. Many socio-economic and ecological values directly depend on the proper functioning of the river ecosystems and the productive water resource base is dependent on a healthy ecosystem. Flows are required to maintain aquatic and terrestrial wildlife habitat for fisheries and riparian vegetation, recharge of downstream groundwater resources, maintaining freshwater/saltwater balance, transportation as well to preserve aesthetic value and recreation activities.
Methods for Quantifying Instream Flow Needs
There are a number of different approaches used worldwide for quantifying instream flows and these can be grouped in four main categories:
- historical flow record methods
- hydraulic rating methods
- habitat rating methods
- holistic methods.
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The first category consists of approaches where historical flow records are used to develop instream flow recommendations. Hydraulic and habitat rating methods utilize relationships between habitat condition and discharge to develop instream flow recommendations. The Instream Flow Incremental Methodology (IFIM) represents a type of hydraulic-habitat method and it is widely used in the USA as well as in Australia, New Zealand, Britain and South Africa. The final and more complex method is the holistic approach to the assessment of instream flows, in which all components or attributes of the ecosystem and their interrelationships are addressed. Examples of such approaches are the Building Block Methodology (BBM), Holistic Approach and Expert Panel Assessment Method.
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