A scenario-based holistic approach to environmental flow assessments for rivers

Sustainable use of river ecosystems requires that they be managed holistically. This paper describes an holistic methodology, DRIFT (Downstream Response to Imposed Flow Transformation), for advising on environmental flows for rivers targetted for water-management activities. DRIFT's basic philosophy is that all major abiotic and biotic components constitute the ecosystem to be managed; and within that, the full spectrum of flows, and their temporal and spatial variability, constitute the flows to be managed. The methodology employs experienced scientists from the following biophysical disciplines: hydrology, hydraulics, fluvial geomorphology, sedimentology, chemistry, botany and zoology. Where there are subsistence users of the river, the following socio-economic disciplines are also employed: sociology, anthropology, water supply, public health, livestock health and resource economics. DRIFT is a structured process for combining data and knowledge from all the disciplines to produce flow-related scenarios for water managers to consider. It consists of four modules. In the first, or biophysical module, the river ecosystem is described and predictive capacity developed on how it would change with flow changes. In the second, or socio-economic module, links are described between riparian people who are subsistence users of river resources, the resources they use, and their health. The objective is to develop predictive capacity of how river changes would impact their lives. In the third module, scenarios are built of potential future flows and the impacts of these on the river and the riparian people. The fourth, or economic module, lists compensation and mitigation costs. DRIFT should run in parallel with two other exercises which are external to it: a macro-economic assessment of the wider implications of each scenario, and a Public Participation Process whereby people other than subsistence users can indicate the level of acceptability of each scenario. DRIFT has been developed in a semi-arid, developing region, where water-supply problems are pressing, and uncertainties about river-linked ecological and social processes high. The use of DRIFT in this context is discussed. Copyright (C) 2003 John Wiley Sons, Ltd.