Development of Drift, a Scenario-Based Methodology for Environmental Flow Assessments
Report No 1159/1/04

Dec 2004



DRIFT (Downstream Response to Imposed Flow Transformations) is a new methodology under development for assessing the flow requirements for maintenance of rivers (environmental flows – EF) that are subject to water developments. It has six important attributes. First, it provides an holistic approach to EF assessments, in that it addresses all parts of the intra-annual and inter-annual flow regime, and all living and non-living parts of the river ecosystem from source to sea. Second, it is a scenario-based approach, combining data, experience from a multi-disciplinary team of specialist river scientists, and any other local knowledge on the river of concern, to provide predictions of how the river could change with flow manipulations. Third, it further predicts the social and economic impacts of these river changes on common-property subsistence users of the river’s resources. Fourth, its outputs comply with the requirements of the South African Department of Water Affairs and Forestry for use in its management of aquatic ecosystems (Resource Directed Measures - RDM). Fifth, all the data and knowledge used in compilation of the scenarios are stored in a database that can be used to create any number of scenarios and that also acts as a resource in its own right on flow-related responses of rivers. Finally, it is grounded in a growing range of custom-built software that allows much of the application of DRIFT to be automated. This report documents the most recent progress in the development of and conceptual basis for DRIFT.

The project’s final objectives (Chapter 1), after amendment in steering committee meetings in October 2000 and October 2001, were to:

The agreed products of the project were:

The two scientific papers

The fundamental nature and concepts of DRIFT are introduced in a paper that is presently in press with the international journal Rivers Research and Applications (Chapter 2). The four modules of DRIFT are explained: 1) biophysical: data on the river are gathered in order to develop predictive capacity of how it would change in response to flow changes 2) subsistence: data on subsistence users of the river are gathered, in order to develop predictive capacity on how they would be affected if the river changed 3) scenario-building: scenarios (options) are compiled, based on potential future changes in river flow, illustrating a range of ways in which the river could change and the range of impacts this could have on subsistence users 4) socio-economic: the compensation and mitigation costs of these impacts are computed. In parallel with these activities, but outside the DRIFT process, continual consultation with, and participation of, all other stakeholders is recommended and outlined, as is a macro-economic assessment of the scenarios in terms of their regional/national implications.

The development of the DRIFT database and scenario-building facility, DRIFT-SOLVER, is described in a paper presently in press with Water SA (Chapter 3). DRIFTSOLVER contains an integer linear programming MCA (multi-criteria analysis) method, which generates optimally distributed flow scenarios for different total annual volumes of water. Also described is DRIFT CATEGORY, which facilitates evaluation of these scenarios in terms of river condition.

Generic lists

Generic lists (Chapter 4) were proposed as an aid for guiding the multi-disciplinary team in its considerations, and to facilitate the orderly input of information into DRIFT-SOLVER. The lists consist of river variables that are important in ecosystem functioning, and sensitive to changes in flow by increasing or decreasing in some way (e.g. in number, area or concentration). In a DRIFT workshop, each team member first compiles her/his list and then, for all flow changes under discussion, describes the consequences for all items on their list. The lists are thus essentially memory guides. It is anticipated that initially the generic list for each discipline will be relevant to the river being considered, but might eventually, after use and adaptation with many rivers, become truly generic in that it could contain all aspects of that discipline that might be important in flow management of any river.

Later development emanating from these generic lists is envisaged. An expert system could evolve that provides some degree of structure on linkages between lists for different disciplines. The structure could possibly take the form of items on generic lists linked by processes that could cause them to change. For example, an increase in the abundance of sand (Sedimentology generic list item), due to a flow change, could lead to a process of sedimentation, which in turn could cause a reduction in the number of interstitial spaces (Geomorphology generic list item). This leads to a process of habitat change, causing a decreased abundance in the riffle-dwelling species assemblage (Macro-invertebrate generic-list item). Such logic pathways could be recorded, to allow re-assessment as knowledge increases. A possible final stage of development might be the ultimate complex link-up of lists/disciplines in a kind of holistic model that predicts how the whole ecosystem could react to flow changes.

Chapter 4 provides a draft generic list for each main discipline involved in an EF assessment: sedimentology, fluvial geomorphology, water quality, vegetation, aquatic invertebrates, fish, and subsistence use.

Subsistence use

Balancing the utilisation (development) of aquatic ecosystems for various human needs with the protection of these ecosystems, so that they can continue to be used by present and future generations, is one of the central challenges in water-resource management. EF assessments are increasingly being recognized as a valuable tool in water-resource management for this purpose. They provide the other side of the picture: the ecological and social implications linked to the benefits provided by the water-resource development. In Chapter 5, the effects of such developments in terms of their impacts on subsistence users of the rivers, and particularly those impacts associated with large dams, are reviewed. Six case studies are then provided of EF assessments that have included social studies, and the lessons learnt are summarised.

It is concluded that the impact of large water-resource developments on riparian communities is little understood, but emerging evidence suggests that the existing and potential impacts of the loss of aquatic resources are high. It is thus crucial that the downstream impacts of a water-resource development be clearly documented or, in the case of a proposed development, predicted. Without an understanding of the actual or potential impacts, and their long-term social implications, the information supporting, and rationale behind, decisions relating to such developments, are essentially flawed. The EF methods now under development in South Africa allow prediction of both the biophysical and socio-subsistence impacts of such projects, which is a first step toward a more holistic approach to decision making.

As yet, there are relatively few examples of sociological studies contributing to EFAs in this manner. There are even fewer where the decision-making process is sufficiently transparent to enable an assessment of the extent to which the results of the EFAs assisted in, or influenced, the final decision on the operation or construction of a water-resource development. The ones that have been completed have attracted a disproportionate amount of attention and the trend is probably toward their more extensive application.

A relatively standard set of procedures and protocols could facilitate incorporation of sociological data into EFAs, and a first attempt at compiling such a list is provided in Section 5.5. This is intended to stimulate discussion on the possibilities for and approach to the development of a set of procedural guidelines for the sociological module of an EFA.

Parallel application of three EF assessment methods

Three EF methods, the Building Block Methodology (King & Louw 1998), DRIFT, and the Flow-stressor Response (O’Keeffe et al. 2002) were applied to the same river system (the Breede River), using the same specialists and data (Chapter 6). The objectives were to:

The methods were evaluated in terms of the following attributes:

The conclusion was that DRIFT and the BBM have similar costs and produce similar results in terms of percentage of Mean Annual Runoff required for a specific River Management Class. The BBM is poorly suited to scenario development, however, and for this reason and others is falling into disuse as per its manual. A Desktop Rapid method has emerged from it that is now used extensively. FSR is not a full holistic approach, but addresses only low flows, working through excising that part of the BBM and inserting new concepts and routines. All three methods meet the requirements of the RDM Directorate of DWAF in terms of output, and FSR and DRIFT are designed to produce multiple scenarios. DRIFT scored most highly for most criteria by which the methods were judged, emerging as a rigorous approach to EF assessments with similar costs to the other methods but with highly structured and transparent data collation and scenario-development phases.

A suggested way forward is outlined. This includes strong support for the following:


DRIFT meets all the requirements of the RDM Directorate of DWAF in terms of outputs from a comprehensive assessment of the Ecological Reserve (Chapter 7). It has approximately the same time and personnel costs as other methods presently used in Comprehensive Reserve determinations, with the added advantage that it contributes to a growing knowledge base through its detailed database of flow-ecosystem links for every river to which it is applied. The database also acts as an implementation tool, through its ability to quickly create many detailed scenarios of flow-change/ecosystem-response. Several additional attributes are listed in Chapter 7.


The following recommendations are made.

  1. Continue development of DRIFT and its knowledge base, through applications in comprehensive assessments of the Ecological Reserve. After about 15 applications of the BBM in real water-resource projects it was felt that South Africa had a method that worked and for which a manual could be written (King et al. 2000). DRIFT sources the same kinds of information as the BBM but uses the data in a different and more sophisticated way. This, and the various sets of DRIFT software, need further development through application as a part of real water-resource projects.
  2. Transfer the DRIFT database from Excel to a Delphi software package and further develop it, to provide the following features:
  3. Develop further the database, to include the socio-economic aspect of DRIFT applications.
  4. Develop further the generic lists to provide lists that can be used in any South African river, and explore their potential as the basis for an expert system.
  5. Liaise with current DWAF work on river management classes to ensure compatibility between these and DRIFT river classes.
  6. Initiate exploration of the possibilities and limitations of applying DRIFT to other types of rivers, such as ephemeral or intermittent systems and/or large floodplain rivers.
  7. Finally, South Africa’s expertise in the field of EF assessments is potentially highly marketable. It is strongly urged that the country act now to take advantage of this whilst other countries are still learning their way into this field.

Capacity building

The parallel application of three methods to the Breede system provided a major training opportunity. In a mentoring approach, most major disciplines (hydraulics, vegetation, sedimentology/geomorphology, invertebrates) were represented by a senior specialist experienced in EF assessments and a junior specialist. In addition, two scientists were introduced to, and helped run, the DRIFT process, and the water-quality discipline was represented by a large group working in a team-building exercise.

Rodney February and Bruce Paxton received additional training and experience in two aspects of EF assessments: subsistence use and generic lists respectively.

Capacity building using a mentoring system is an excellent means of speeding up the training of new specialists. It is most effective if recognition is given that productivity of the mentor will decline in other areas whilst aiding in such training. At present, capacity building is often simply added as an extra requirement without any reduction in expectation of other outputs. This can result in unstructured and inadequate training by a mentor and consequent disillusion of the trainee; or ‘capacity-building fatigue’ from a mentor attempting to provide excellent training whilst still meeting all other commitments; or unease in a mentor who is focussing on capacity building and is aware that the other commitments are not being met at previous standards. There is a real danger of the quality of scientific research being threatened because senior researchers are becoming overwhelmed by ever-increasing demands from a range of funder, university (or equivalent institution), and management sources, and yet the country needs their original thinking as well as their mentoring skills. It is suggested that a more structured approach to capacity building is crucial in the aquatic sciences, in order to increase the number of trained people whilst also providing their mentors with time to proceed with in-depth research.