Understanding Hydrological Alteration

Abstract Water enters the terrestrial phase of the water cycle when precipitation exceeds evapotranspiration at the land surface. Downstream drainage of this water via surface and subsurface flow paths including transient storage in snowpack, soils, aquifers, and lakes combine to produce the water regimes of freshwater ecosystems. Humans have altered these processes through changes in climate and atmospheric CO 2 concentrations, modifications to the land surface including tree clearing and urbanization, construction of surface water impoundments that are generally operated for water supply, flood control or hydropower generation, diversion of surface water, extraction of groundwater, draining of wetlands, agricultural irrigation and drainage practices, and flood alleviation works in floodplain rivers. Most aspects of the terrestrial water cycle have been modified, leading to widespread changes in the hydrology of freshwater ecosystems. Many of these human impacts are expected to increase in the future, which underlines the urgency of implementing effective environmental water practices. Much of the environmental water literature has focused on environmental water requirements downstream of large dams. In the future, comprehensive environmental water planning should address the broader range of hydrological alterations that affect river ecosystems.