Changes in land use have potentially large impacts on water resources, yet quantifying these impacts remains among the more challenging problems in hydrology. Water, food, energy, and climate are linked through complex webs of direct and indirect effects and feedbacks. Land use is undergoing major changes due not only to pressures for more efficient food, feed, and fiber production to support growing populations but also due to policy shifts that are creating markets for biofuel and agricultural carbon sequestration. Hydrologic systems embody flows of water, solutes, sediments, and energy that vary even in the absence of human activity. Understanding land use impacts thus necessitates integrated scientific approaches. Field measurements, remote sensing, and modeling studies are shedding new light on the modes and mechanisms by which land use changes impact water resources. Such studies can help deconflate the interconnected influences of human actions and natural variations on the quantity and quality of soil water, surface water, and groundwater, past, present, and future.
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 Land-use impacts are not limited to irrigated areas. Dryland agriculture can have large effects on water resources, increasing recharge and flushing accumulated salts to rivers [Cook et al., 2001; Scanlon et al., 2005]. Impacts from dryland agricultural areas, which currently produce about 60% of the world's food supply, are likely to increase as (1) irrigation becomes increasingly constrained by water availability, (2) newly developed dryland cultivars replace their less drought tolerant counterparts, and (3) dryland production of biofuels and replanting of formerly irrigated land for carbon sequestration both undergo expansion [NRC, 1996; 2008]. Urbanization affects water resources locally, impacting water quality [Randhir, 2003; Sickman et al., 2007], storm discharge [Hollis, 1975], and groundwater recharge [Ku and Simmons, 1985; Filippone and Leake, 2005].
2. Challenges, Approaches, and Progress
 Isolating the impacts of land use change on water resources is problematic. Reasons for the difficulty include the wide range in time scales over which impacts from land use changes propagate through hydrologic systems, the confounding effects of climate and weather, and the fact that large-scale observational field studies often lack controls, making it difficult to ascribe temporal changes to causal mechanisms. Biophysical responses such as changes in evapotranspiration occur over time scales of minutes to hours, whereas plant growth and species succession occur over days to decades. Large groundwater systems respond to perturbations in land use and climate more slowly. Studies using stable isotopes and other tracers from unsaturated-zone profiles have shown that groundwater systems in arid regions are still responding to climatic shifts that occurred at the end of the Pleistocene [Walvoord et al., 2004].
 The studies presented in this special section apply a variety of observational, modeling, and conceptual techniques to evaluate water resource impacts of land use change at sites located on six continents (Figures 1 and 2) . Several studies combine vegetation and water balance models to simulate watershed dynamics for past and projected climates and land use scenarios. Other studies examine specific linkages, such as sulfur dynamics in viticultural systems and changes in soil and stream chemistry due to afforestation. The largest cluster of studies considers watershed dynamics (streamflow and sediment transport) in portions of northern China that have experienced large changes in land use, together with periods of severe drought. Such studies help explain the mechanisms and interactions that led to increasingly frequent zero-flow conditions along the Yellow River during the latter half of the twentieth century. Several studies are global in scope, for example developing models that can help manage “green water” (plant-transpirable water) in addition to “blue water” (surface water and groundwater) to meet increasingly pressing demands and constraints [cf. Smil, 2008].
 Land use and climate change are inexorably linked through the hydrologic cycle. The papers in this special section apply a variety of tools to past, present, and future time frames to understand the water resources impacts of land use change from watershed to global scales. All share a common thread in addressing one of the most fundamental questions about human sustainability.
 We thank the editors of Water Resources Research for their support of the special section, as well as authors and reviewers of its constituent papers. Noah Knowles (USGS, Menlo Park, California), John Vaccaro (USGS, Tacoma, Washington), and Thomas Torgersen (coordinating editor, WRR) provided helpful comments on drafts of the overview. Support was provided by the authors' respective institutions (USGS via the National Research Program, Toxic Substances Hydrology Program, and Water Resources Program), as well as the USEPA and the Texas Commission on Environmental Quality. Finally, we acknowledge with regret the death on 14 May 2009 of special section author Ian Calder, a pioneer in land use research.