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Methods for Environmental Flow Assessment  

Setting Objectives for Environmental Flows - A Negotiated Trade-off

The condition, in which aquatic ecosystems and their services are maintained, is essentially a socio-political decision. The desired ecosystem condition may be set (e.g. by legislation or international conventions), and the environmental flow requirement is the water regime needed to maintain the ecosystems in that desired condition. Alternatively, the environmental flow allocated to a river system may be a negotiated trade-off between water users. In this case, the resulting ecosystem condition is determined by that negotiated and ‘desired’ environmental flow.

Setting environmental flows requirements thus may take two fundamentally different approaches depending on the objective in question:

  • What ecosystem condition must be maintained - and how much water is needed for this?
  • How much water does society allocate ecosystems - and what is the resulting ecosystem condition maintained by this given water allocation? – and is this condition desirable and sufficient?

In the context of IWRM, the latter approach is the most relevant, since it enables an optimal allocation of the entire water resource among all uses and also allows for adaptive management. The former approach is more rigid, and in this case societal optimisation of water allocation does not include environmental flows, which is fixed.

Source: adopted from eFlowNet (Global Environmental Flows Network)

International Methods for Environmental Flow Assessment

Environmental flow assessment, whereby the quantity and quality of water and flow variability required for ecosystem conservation and resource protection are determined, has emerged from the recognition that hydrological alteration of running waters has an environmental consequence. There are some 207 distinctly separate environmental flow methodologies practiced in 44 countries within 6 world regions (Tharme et al., 2008). These can be differentiated into hydrological, hydraulic rating, habitat simulation and holistic methodologies, with a further two categories representing combination-type and other approaches.

Environmental flows to maintain ecosystem functions.
Source: Khayat 2008
( click to enlarge )

In broad terms, these methods can be classified into four categories:

  1. Look-up tables: Worldwide the most commonly applied methods to define target river flows are rules of thumb based on simple indices given in look-up tables. The most widely employed indices are purely hydrological, but some methods employing ecological data were developed in the 1970s.
  2. Desk top analysis: Desk top analysis methods use existing data such as river flows from gauging stations and/or fish data from regular surveys. If needed, some data may be collected at particular sites on a river to supplement existing information. Desk-top analysis methods can be sub-divided into those based purely on hydrological data, those that use hydraulic information (such as channel form) and those that employ ecological data.
  3. Functional analysis: Methods include those that build an understanding of the functional links between all aspects of the hydrology and ecology of the river system. Such methods, which include the Building Block Methodology (BBM) developed in South Africa, take a broad view and cover many aspects of the river ecosystem, using hydrological analysis, hydraulic rating information and biological data.
  4. Habitat modelling: Methods have been developed that use data on habitat for target species to determine ecological flow requirements. Within the environmental conditions required by a specific freshwater species, it is the physical aspects that are most heavily impacted by changes to the flow regime. The relationship between flow, habitat and species can be described by linking the physical properties of river stretches, e.g. depth and flow velocity, at different measured or modelled flows (for example WRYM IMS), with the physical conditions that key animal or plant species require. Once functional relationships between physical habitat and flow have been defined, they can be linked to scenarios of river flow.

Within many countries a lot of data has already been collected, that desk top methods and look-up tables (see Table below) can be used in many cases. For many wetlands in Namibia and Angola this data is not yet available and the only option is to collect these basic data.

Comparison of Different Methods and Charac­teristics of Setting Environmental Flows

    Sub-type     Advantages     Disadvantages  
Look-up table



Inexpensive, rapid to use once calculated Not site-specific. Hydrological indices are not ecologically valid. Ecological indices need region specific data to be calculated.
Desk top




Site specific

An extended time series is required. No explicit use of ecological data. Ecological data are time consuming to collect with limited provision for updating of new data.

Functional analysis - Flexible, robust, more focused on whole ecosystem Expensive to collect all relevant data. Requires experts from wide range of disciplines. Consensus of experts may not be achieved.
Habitat modelling - Objective, replicable, predictive Expensive to collect hydraulic and ecological data. Socio-environmental studies can be subjective.
Source: Babu and Kumara, 2009 (adapted)




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