Our planet is in a never-ending natural state of flux, with perhaps the life-sustaining hydrologic system being the prime example. This system displays a seasonal rhythm, year-to-year variations, and slower changes at time scales of decades, centuries, and even longer periods. Since the dawn of civilization, humans have contributed to and influenced this natural flux. During early human settlement and the beginning of agriculture, impacts were minimal, local, and scattered, mainly in the form of rudimentary field-scale irrigation and limited water withdrawals for human consumption. Over time, as population increased and societies and technologies evolved, humans began to impact hydrology, land use, and microclimate at the watershed, basin, and regional scale. For example, during the time of Greek civilization and the Roman Empire, large portions of the Mediterranean basin were irreversibly deforested for fuel and construction material, trans-basin water diversions were made to supply growing cities with water, and agriculture was expanded to meet the demand for food (Hughes, 1975). All these actions contributed further to uncontrolled erosion and the sedimentation problems already caused by deforestation. In the 20th century such human impacts affected most continents, often at larger scales due to widespread mechanization, rapidly increasing population, and expanding metropolitan area. In addition to impacts on water supply and sedimentation, more recent impacts included emissions from industrialization, energy production, and transportation, as well as agrichemical runoff from agricultural croplands. Towards the end of the 20th century, there was little doubt that the hydrologic system, land use, and climate were changing at continental and global scales, and action was needed to reduce or slow cumulative negative impacts of human activity on our life-sustaining environment. Today, global climate change, society's unsustainable consumption of energy, increasing greenhouse gases emissions, continued deforestation, and mitigating actions to regain control of these runaway cumulative impacts have become popular topics of discussion. Within this broad framework, specific concerns in the United States that must be recognized and addressed include current and anticipated impacts of changing climate, increased bio-energy demands, and population growth on agricultural landscapes. Climate change, only recently viewed by many as a far-away projection 30, 50 or 100 years from now, is already affecting us. Studies have shown that large regions of the United States are experiencing more frequent and more severe storm events (e.g. Groisman et al., 1999; Kunkel et al., 1999) and corresponding increases in runoff and streamflow (e.g. Lins and Slack, 1999; Miles et al., 2000), leading to higher soil and agricultural chemical movement and transport (e.g. Phillips et al., 1993; Pruski and Nearing, 2002), as well as more stream erosion, stream instability, and gully formation in susceptible areas. At the same time, climbing global energy prices and the push for energy independence and biofuels in the United States are increasing pressures to expand and intensify the production of biofuel feedstocks