Global physical effects of anthropogenic hydrological alterations: sea level and water redistribution

Human influence on the Earth System and hydrologic cycle has reached the point where it affects the hydrologic balance between ocean and continental storage reservoirs. The anthropogenic redistribution of water mass at a planetary scale even has an effect on Earth rotation parameters. Land use changes associated with expanding agriculture to support an increasing human population have already had a profound influence on basin-scale hydrology, and in extreme cases, on regional climate. Major human activities which lead to hydrologic alterations include irrigation (from ground water mining and surface water diversion), deforestation, wetland filling or drainage, and new dam construction. With the exception of the latter, these all contribute to the transfer of water from the continents to the ocean and a reduction of continental water resources. However, water impoundment behind dams may partially or completely counteract the cumulative effect of the others. Present compilations of reservoirs impounded by dams include only the results of major engineering projects. Smaller impoundments have largely been ignored. The cumulative volume of the literally millions of small reservoirs such as farm ponds and rice paddies may approach that of the larger documented reservoirs. Unfortunately it is not practical to make a global inventory of millions of small and unregistered reservoirs, so their volume may never be known precisely. The quantity of water stored in artifically raised water tables behind dams has also not yet been addressed. The issue of water impoundment or release from continental drainage basins affects global sea level. Recent estimates based solely on major dammed reservoirs suggest that if new dam construction is not maintained at the rates of the 1960s through 1980s, the rate of sea level rise could increase by about half a millimeter per year. If small impoundments are taken into account, this figure could be much greater.