Human Intervention in the Global Water Cycle

On the other hand, as discussed above in Ponding the Waters: Impacts of Dams, the effects of large-scale alteration of river systems and the hydrologic cycle have become increasingly clear in the past few decades. The scale of human intervention in the global water cycle is also becoming apparent, including restriction of river flows such that they no longer reach the ocean in many years, associated two- or three-fold increases in the residence time of runoff, decreased sediment delivery to the oceans, and a long-term measurable effect on global sea level caused by the impoundment of thousands of cubic km of water (Vörösmarty et al., 2004). As a result, new large dam projects have been heavily scrutinized and faced political and environmental opposition. At the same time, the efficiency and economics of energy production, and the net offset of greenhouse gas emissions from increased hydropower generation have been increasingly questioned (World Commission on Dams, 2000). One way to minimize environmental impacts is to design "run of the river" systems, in which no reservoir is created and instead the natural flow of the river in its channel is harnessed to generate power. However, these systems have several drawbacks: they rely on natural flows, so the power generating capacity fluctuates dramatically as a function of seasonal rainfall patterns and climate change; and there are no added benefits of flood control or water supply.