Impacts of climate variability and changes on groundwater recharge in the semi-arid southwestern United States Sarah Davidson Physical climatology Dr. Liang Yang November 20, 20071abstract An understanding of groundwater recharge processes in the semi-arid southwestern United States is essential for the creation of sustainable water and land resource management. Recharge is a function of soil texture, vegetation, the type and locations of preferential flowpaths, and water availability. The spatial and temporal variability of precipitation in the area is influenced by several climate cycles including glacial periods, the Atlantic Multidecadal Oscillation, the Pacific Decadal Oscillation, the North American Monsoon System, and the El Niño-Southern Oscillation. The current semi-arid climate developed about 12,000 years ago, after the end of the most recent ice age. Field and modeling studies show that recharge in the region occurs primarily in drainage areas and is more likely in coarse soils, in unvegetated areas, and when precipitation events are large and occur during periods of cooler temperatures. Variations in groundwater recharge have been correlated by different researchers with cycles of the Pacific Decadal Oscillation and the El Niño-Southern Oscillation. Current modeling of future climate in the southwestern United States projects increased temperatures and decreased precipitation. These changes are likely to result in reduced groundwater recharge in the region, however in the distribution of changed climate parameters, indirect effects of these, and local anthropogenic effects, which are less certain, might largely influence future recharge.2introduction This paper presents an overview of literature that describes the relationship between groundwater recharge and climate variability in the semi-arid portions of the southwestern United States. I first introduce the region and the general understanding of recharge processes in the area. Next, I define several climate cycles that influence temperature and precipitation in the region and summarize their impact on precipitation. Following this, I describe a number of studies that help to characterize how climate variability influences recharge in the area. Finally, I discuss current projections of climate change over the next century and the implications of these changes on future groundwater recharge. Throughout, I attempt to tie together research findings and past, current, and future trends, with a focus on future groundwater availability. past and current recharge Much of the southwestern United States has a semi-arid climate, characterized by the Köppen climate classification system as having low rainfall (between 250 and 500 millimeters per year) and grass or scrub vegetation. The lack of rainfall in these areas typically precludes the development of significant local flow systems and result in thick unsaturated zones through which water must travel to reach the water table. Understanding recharge processes in semi-arid regions of the southwestern United States and around the world is essential to estimating groundwater availability and to assessing the risk of groundwater contamination. Further, these processes can vary significantly in time and space, which has important implications for water and land use planners interested in sustainable resource management and development. Calculations of groundwater systems for modeling and resource management have often3used an estimate of long-term average annual recharge as the input to the system. However, this seems increasingly an inadequate measure, both because it does not accommodate our full understanding of recharge processes and because it does not allow for an assessment of how climate and land use changes might affect groundwater availability (Hanson and others 2004, Pool 2005). Recharge can be defined by a water-balance equation R = P + I - ET - RO (1) where R is recharge, P is precipitation (a function of climate parameters), I is irrigation water (a function of land use, crop type, climate, and irrigation technique), ET is evapotranspiration (a function of climate, vegetation, and soil parameters), and RO is runoff (a function of slope, soil, and vegetation parameters). These variables are functions of climate and weather parameters, soil, vegetation, irrigation techniques, land use, slope, and other factors. For the purposes of this discussion, I equate recharge to infiltration below the root zone. This definition assumes that once water has infiltrated below the area where it can be evaporated or transpired, it will eventually reach the water table. In addition, recharge can be characterized as diffuse—infiltrating through the soil matrix in interdrainage areas—or focused—infiltrating through drainage areas or other preferential flow paths. In addition to effects of climate variability, the primary focus of this review, previous studies have identified recharge-controlling mechanisms in the semi-arid southwestern United States. They have found that recharge is more likely to occur  in topographic depressions where precipitation is likely to collect, such as playas and ephemeral streams (Scanlon and Goldsmith 1997, Pool 2005)  in areas where soils at the surface are coarse-grained (Gee and others 1994, Scanlon and others42003)  through preferential flow paths, such as cracks or along roots (Scanlon and Goldsmith 1997, Scanlon and others 2003)  where rooting depths are shallow, or, even more likely, where there is no vegetation (Gee and others 1994, Scanlon and others 2005a)  in irrigated areas (Scanlon and others 2005b) Study results indicate that in general, interdrainage areas in the southwestern United States commonly have no recharge or even upward fluxes of water. While the characteristics described above help define whether a given area is likely to allow downward infiltration of water, the primary limiting characteristic in this region (except for in irrigated areas) is the availability of precipitation to infiltrate. Therefore, it is important to identify variations in precipitation, the climate patterns that drive these variations, and how these patterns change over time. For water and land use planners, the ability to predict likely future changes in precipitation on short and long time scales could significantly increase the sustainability of management policies. past and current