Abstract: Ubiquitous hydrological alterations--dam construction and associated water diversion, exploitation of groundwater aquifers, stream channelization, and intercatchment water transfer--are producing global-scale effects on the environment. The articles in this special issue of BioScience highlight the cumulative effects of hydrological alterations associated with dam and reservoirs development. Such information is critical for deciding whether, when, and where the next major hydrological project will be built; it can also warn us about impending environmental impacts.
The study of the cumulative effects of hydrological alterations is a recent endeavor, compared with the study of individual dam and reservoir developments (e.g., Hall 1971, Hecky et al. 1984). The issue of greenhouse gas emissions from reservoirs, for example, is less than a decade old (Rudd et al, 1993). The global significance of reservoirs as sources of green house gases depends on the total surface area of reservoirs and the flux rates from the major types of reservoirs in different geographical locations (Rosenberg et al. 1997). Neither of these quantities is well known, but flux rates have now been measured in 21 locations, enabling the first reasonable estimate of global greenhouse gas emissions from reservoirs (St. Louis et al. 2000).
Other recent examples include attempts to determine cumulative environmental effects at hemispheric or global scales. Chao (1991, 1995) reported that worldwide impoundment of water has reduced sea levels; moreover, the concentration of water in reservoirs at high latitudes has increased, albeit minutely, the speed of the earth's rotational and changed the planet's axis. Vorosmarty et al. (1997) demonstrated a dramatic aging in river runoff, leading to biophysical changes in river systems, caused by the global population of large dams. Dynesius and Nilsson (1994) determined that 77% of the total discharge of the 139 largest river systems in the northern third of the world is affected by river channel fragmentation caused by dams, reservoirs, intercatchment diversions, and irrigation. This fragmentation could profoundly affect biological populations over a substantial area of the world.
The emerging field of study of global-scale, cumulative, environmental impacts was the focus of a special symposium held in 1998 at a national conference--The Land-Water Interface: Science for a Sustainable Biosphere--sponsored by the Ecological Society of America and the American Society of Limnology and Oceanography. At the symposium, entitled "Global-scale Effects of Hydrological Alterations: What We Know and What We Need to Know," participants were asked to synthesize information in their area of interest, working at the largest possible spatial and temporal scales, and to identify knowledge gaps that inhibit work at global scales.
The articles in this issue are based on presentations at the 1998 symposium. The series has the following objectives: to synthesize as much information as possible on large-scale environmental effects of dams and reservoirs, to identify knowledge gaps and research needs to improve our understanding of global-scale effects, and to produce currently available information in a form that is readily accessible to policy makers.
Rosenberg, D. McCully, P., and C. M. Pringle. 2000. Introduction. Global effects of hydrological alterations: What we know and what we need to know. BioScience 746-751.
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