A global perspective on environmental flow assessment: Emerging trends in the development and application of environmental flow methodologies for rivers
Recognition of the escalating hydrological alteration of rivers on a global scale and resultant environmental degradation, has led to the establishment of the science of environmental ¯ow assessment whereby the quantity and quality of water required for ecosystem conservation and resource protection are determined. A global review of the present status of environmental ¯ow methodologies revealed the existence of some 207 individual methodologies, recorded for 44 countries within six world regions. These could be differentiated into hydrological, hydraulic rating, habitat simulation and holistic methodologies, with a further two categories representing combination-type and other approaches. Although historically, the United States has been at the forefront of the development and application of methodologies for prescribing environmental ¯ows, using 37 % of the global pool of techniques, parallel initiatives in other parts of the world have increasingly provided the impetus for signi®cant advances in the ®eld. Application of methodologies is typically at two or more levels. (1) Reconnaissance-level initiatives relying on hydrological methodologies are the largest group (30 % of the global total), applied in all world regions. Commonly, a modi®ed Tennant method or arbitrary low ¯ow indices is adopted, but efforts to enhance the ecological relevance and transferability of techniques across different regions and river types are underway. (2) At more comprehensive scales of assessment, two avenues of applica
