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The Southwest Ecosystem Service Program (SwESP) is part of the U.S. Environmental Protection Agency (EPA)s newly undertaken Ecological Service Research Program (ESRP) to examine the variety of ways in which landscapes including crop lands, conservation areas, wetlands, lakes, and streams contribute to wildlife and human well-being. The goal of the SwESP is particularly to examine the landscape of the Southwest area to quantify the current magnitude of those contributions from the landscape, and to examine how ecosystem services in the Southwest could change over the next decade. Given the growing demand for water due to population growth and potentially decreased snow and precipitation due to climate change, water availability has become a dominant issue in arid and semi-arid ecosystems. As a part of this effort, quantifying past climate changes and how these changes affect quantity and quality is very important. Thus, the objectives of this study are to examine historical climate and streamflow changes and estimate its impact on future available water resources and management. To achieve the overall objective of this study, climate data, U.S. Geological Survey daily stream flow and peak discharge were analyzed from a southwestern U.S. watershed. It was found that the annual precipitation has a decreasing trend from 1960 to 2008. While the monthly average minimum temperature has a tendency of increasing, the monthly average maximum temperature does not show the same phenomena. As annual precipitation decreases, the annual stream flow has a tendency of decreasing although annual stream flow does not correspond with annual precipitation very closely. This is because the unique storm characteristics such as rainfall intensity and duration also impact the runoff generation in addition to the total amount of rainfall as demonstrated by many other studies (Critchley et al., 1991). Although the increased urban landcover (Homer et al., 2004; Fry et al., 2009) would result in increased stream flow (Franczyk and Chang, 2009), the increased temperature seems to override the effects of increased urban landcover. Since there is little sediment data available from the watershed, peak discharge was analyzed to provide an indication of potential soil erosion and sediment transport. The highest peak discharge is probably a result of the combination of higher intensity rainfall and increased urban landcover. Therefore, the watershed is potentially more vulnerable to flooding risk and degraded water quality due to potentially increased soil erosion and sediment transport.