The primary goal of this research is to gain a better understanding of the spatial and temporal relationships between atmospheric circulation features and winter climate variability in the southwestern United States, and to investigate the variations in these relationships over the past three centuries. A set of six circulation indices is compiled that describes circulation features important to winter climate variability in this region. This set includes pre-existing indices such as the SOI and a modified PNA index, as well as regionally-tailored indices. A network of 88 tree-ring chronologies is then used to reconstruct the indices and the regional winter climate variables: numbers of rainy days (a variable not previously reconstructed with tree rings) and mean maximum temperature. Analyses suggest that three types of circulation features have influenced winter climate in the Southwest over the past three centuries. Although ENSO-related circulation patterns have been an important factor, especially in the 20th century, circulation patterns featuring a southwestern low appear to be as important if not more important to climate in some time periods. Results suggest that low frequency variations in atmospheric circulation patterns have occurred over the past three centuries and have had spatially and temporally varying impacts on winter climate in the Southwest.

Interdisciplinary and unconventional research methods offer important insights into geomorphic, hydrologic and hydroclimatologic characteristics of large floods that are often difficult or impossible to resolve in the framework of conventional flood analysis. Four detailed studies of modern floods, historical floods, and paleofloods in western and central Arizona demonstrate the benefits of analyzing recent and historical extreme floods within the conceptual framework of paleoflood hydrology and flood hydroclimatology. Analysis of the hydroclimatological and paleohydrological context of extreme flooding in Arizona during the winter of 1993 provides a detailed analog to the likely climatic, meteorologic, and hydrologic conditions associated with the largest events in the regional paleoflood record. Investigation of the distribution of relict high-water evidence from extreme floods on the lower Verde River in 1993 improves the accuracy of the river's paleoflood record and reveals interesting hydrological phenomena of extreme floods in the Verde River Basin. A multidisciplinary study of the extraordinarily large Bronco Creek, Arizona, flood of August 1971, shows the original estimate to be significantly overestimated because of complex flow behavior of an extreme flood and the related dynamic morphological response of a high-gradient alluvial channel. The approach to this study is a template for similar analyses of extreme floods and extraordinary flood discharge estimates. A similar, more comprehensive application of paleoflood research methods is demonstrated by the compilation of a detailed regional chronology of flash-flooding in small desert drainage basins (7-70 km²) in western Arizona. The occurrences of large, recent and historical floods were documented with nearly annual resolution, and a 1200-year regional paleoflood record was compiled. Comparison of these records to conventional regional flood-frequency relations indicates that the regional equations are probably inaccurate because of data limitations. The study presents a viable approach to developing a quantitative assessment of regional flood frequency in areas with no conventional data on real floods. The results of each of these studies extend the spatial and temporal scope of the paleoflood and historical flood record of the lower Colorado River Basin and provide further support for the concept of a regional upper limit to flood peak magnitudes.

Watershed ecosystems consist of numerous resources which have important environmental, social, cultural, and economic values. The mutual existence and interaction among different resources within the watershed ecosystem calls for a multiobjective watershed resources management analysis. These objectives are often uncertain since they are based on estimation and/or measurement data. Probabilistic methods or fuzzification are usually the methods used in modeling these uncertainties. Selection of the best decision alternative is based on using some Multiple Criterion Decision Making (MCDM) technique. Through simulation in this dissertation, we examine the probabilistic model to address the watershed management problem. In particular, the distance-based methods, which are the most frequently used MCDM techniques, are employed in the problem analysis. In most cases, several interest groups with conflicting preferences are willing to influence the final decision. In our study, a new method is suggested to incorporate their preference orders into the DM's final preference. The application of MCDM techniques combined with stochastic simulation and conflicting preference orders is new in the watershed management literature. Detailed analysis and comparison of the numerical results will help to decide on the suitability of the MCDM technique in watershed resources management. In particular, our numerical results indicate that in practical applications the best alternative selection is significantly influenced by the uncertainties in the payoff values. Hence, in situations where suitable data are available, our methodology is highly recommended.

At the 7.6 km spatial scale in which remotely-sensed satellite imagery is used in many studies of subcontinental-scale vegetation vigor and dynamics, the information acquired has yet to be fully understood and integrated with ground-level reality. This dissertation reports results and analysis from ground-truth-sampling in the arid lands of West Africa's Sudano-Sahelian zones. The geographical locations of the transects investigated were obtained from areas exhibiting steep gradients in the interannual (1980-1994) coefficients of variation (CoV) of the mean annual monthly maximum composite of the Global Area Coverage's (GAC) Normalized Difference Vegetation Index (NDV1) from the Advanced Very High Resolution Radiometer (AVHRR) series of satellites. I begin this work by disaggregating the term "food security" into its various components, then continue by exploring what is generally understood by the concept of "desertification" and what this actually translates to in terms of land degradation. I then discuss how an error in NASA's method for calculating interannual NDVI CoVs impacted both my own work and our concepts of the Sahel's boundaries. Field data I gathered in the central and northern Sahel indicate that cogent, simple explanations of latitudinal variations in CoV do exist, albeit not everywhere. The Gourma region of Mali provides an excellent example of how complexity confounds any neat quantization of information. For the more southerly agropastoral zone, high CoV variability flags rapid, dynamic desertification processes. Results of village- and household-level profiles along a transect in the heart of Hausaland confirm that rapid, dynamic land degradation corresponds with a high interannual CoV. Climate, especially rainfall and potential evaporation, form the basis of an analysis the outcome of which explains how and why the Malian Gourma shows a nonlinear, "anomalous" NDVI response to rainfall. I also explore the strong correlation between rainfall and NDVI in the southern Sahel, but conclude that if there is a link between NDVI and crop yields, it is very weak indeed. Finally, my research highlights several policy measures that may retard desertification and enhance food security.

The release of ionic solute from melting seasonal snow produces an influx of ion laden water into hydrologic systems at the start of spring snowmelt. The spatial and temporal variability of meltwater and solute release from melting snow was investigated at different spatial scales to assess the magnitude and variability of this process. Four laboratory experiments were performed where an 0.4 m³ volume of snow was placed in a plexiglass box and melted from above. NaCl and dye tracer experiments revealed contemporaneous areas of concentrated dye and dilute meltwater in flow fingers, indicating that meltwater in preferential flow paths is diluted by low concentration water from the top of the snowpack. Meltwater discharge and meltwater electrical conductivity were measured in snow lysimeters, and snow accumulation and electrical conductivity of samples from snowpits were measured over four snowmelt seasons at an alpine field site. Peak snow-water equivalent ranged from 0.57 to 2.92 m, and lysimeter discharges ranged from 20 to 205% of the mean flow; however mean lysimeter flow was representative of snow ablation observed in snow pits. The electrical conductivity in snowpit samples and lysimeter meltwater averaged 2-3 μS cm⁻¹. Peak meltwater electrical conductivity ranged from 6 to 14 times that of the bulk premelt snowpack. The highest conductivities were observed during the first few days following the onset of flow, and the lysimeters that began flowing earliest tended to have the highest conductivities at the onset of flow. A mathematical model for solute transport in snow was developed that includes the effects of mass transfer between mobile and immobile liquid phases, advection, hydrodynamic dispersion, and melt—freeze episodes. The ability of the model to accurately simulate solute movement and release depends on the validity of the assumption of one—dimensional flow and on the accuracy of modeling the snowpack energy balance. This model is preferable to the empirical models of solute elution currently in use for investigations of watershed hydrogeochemical response because it has the ability to respond directly to changes in snow accumulation or meteorlogical conditions.

The goal of this research was to develop a watershed-scale model for predicting changes in plant species distribution and abundance (phytogeography) that might occur as a result of changes in climatic factors with global warming. The model was designed: 1) to be spatially explicit and applicable across the entire watershed; 2) to apply to a number of particular species rather than general vegetation types; 3) to predict abundance as well as presence/absence; and 4) to work with simple environmental data, but reflect a biological rationale. Correlations were sought between current phytogeography in the watershed and the synoptic climate variables mean annual temperature, total annual precipitation and cool-/warm-season precipitation ratio. The contribution of edaphic and topographic variables to correlative models was examined and found to be negligible. The correlations established for current conditions were extended to hypothetical future conditions of changed climate in which the values of the variables were manipulated and the model run to produce predictions of altered future phytogeographies. Twenty-seven different hypothetical climate scenarios were modeled, incorporating a 1°C or 2°C rise in temperature with as much as a 10% increase or decrease in seasonal precipitation. Spatial articulation of the model was achieved through raster analysis of gridcell based data layers in a geographic information system. Primary input layers were a series of high-resolution (360x360m) interpolated climate-variable surfaces and a geographically referenced database of plant species presence and abundance derived from an aerial videography sample of the watershed. Logistic regression analysis was used to calculate, for a given set of conditions, the most probable state (present/absent) and abundance class for ten plant species at each grid-cell location in the watershed. Fragmentation of species' distributions before and after change was examined. Results for all studied species showed marked changes in distribution and abundance with temperature rise. Desert species will likely increase in abundance and occupiable area as forest and woodland species decrease, but much depends on the interaction of precipitation with temperature. Model predictions are conservative compared with paleoecological evidence of past changes.

Over the last decade, luminescence measurements have been used primarily to detect and quantify specific organic pollutants and heavy metals, and in a few cases for monitoring microbial activity. In this study, a fiber-optic luminescence detection system was developed to examine the relationship between microbial activity and the resultant impact on biodegradation and transport of substrate in porous media. This system allows rapid, real-time, and non-destructive measurements of in-situ luminescence from a specific lux reporter microbial population in porous media. An understanding of the formation and dynamics of bioactive zones is very important for in-situ bioremediation applications because it is in these zones that the remediation process is optimal. This study also examined the location and size of a biologically active zone in response to changes in local substrate and electron acceptor availability. Results show that when DO was not a limiting factor, the bioactive zone encompassed the entire system. However, as the availability of DO became limiting for the higher-00 experiments, the size of the bioactive zone shrank and was ultimately limited to the proximity of the substrate source. Furthermore, a decrease in the size of the bioactive zone enhanced the rate of substrate biodegradation per unit area. This study also investigated the impact of several coupled factors including substrate concentration, pore-water velocity, and initial cell density on solute biodegradation and transport behavior for a system influenced by three stressors, microbial lag, microbial growth, and cell transport. Results showed that temporal changes in biodegradation potential, and therefore attendant substrate transport behavior, were influenced by microbial lag, growth, dissolved oxygen limitations, and cell elution. As a result, substrate transport behavior was non-steady except for relatively short residencetime conditions wherein substrate degradation exhibited quasi first-order behavior. Cell transport and elution was important, especially under significant growth conditions. Under such conditions, the majority of the cells in the system (60 to 90%) was distributed in the solution phase where most of the biodegradation took place. This study illustrates the complex behavior that can be associated with microbially mediated processes, and which should be included in solute transport models to accurately predict the fate of contaminants in the subsurface environment.

Two piñon (Pinus edulus) tree-ring chronologies developed on each of three substrates (sandstone, shale, and alluvial fan deposits) in southern Utah for the period 1702 to 1997 demonstrate that geologic substrate affects dendrochronologic streamflow reconstructions. Chronologies from alluvial fan deposits explain the most variance of cool-season (October 1 to May 31) flow with an adjusted coefficient of determination (Rₐ²) equal to 0.59. Chronologies from sandstone deposits account for 52 percent of the variance, while those on shale deposits account for 45 percent. Correlation coefficients among the three substrates are significantly different at the 95% confidence level. The highest single-site annual discharge reconstruction (October 1 to September 30), Rₐ² = 0.25, is provided by chronologies from shale deposits. The highest substrate-pair annual discharge reconstruction, Rₐ² = 0.27, is provided by chronologies from alluvial fan deposits. The highest summer discharge reconstruction(July 4 to September 3), Rₐ² = 0.14, is provided by chronologies from sandstone. Over 90 percent of the summer reconstructions are below Rₐ² = 0.10.

Rillito Creek, near Tucson, Arizona is like many ephemeral streams in the Southwest in that flows within the creek are an important source of recharge to the regional groundwater system. Increasing demands on groundwater resources in the area have prompted proposals for artificial recharge along the 20-km-long channel. The U.S. Geological Survey, along with the Arizona Department of Water Resources, is developing a regional groundwater-flow model to determine recharge effectiveness. This study provides the geometry and a detailed characterization of recent alluvium and basin-fill deposits. The approach used to determine the geometry and physical and hydraulic properties of the deposits included the integration of surface and borehole geological data and geophysical data. Data were derived from electromagnetic, resistivity and seismic-refraction surveys, pre-existing geologic logs and analysis of cores and cuttings for grain-size distribution, particle and bulk density, porosity, saturated hydraulic conductivity and volumetric water content. A fine-grained layer in the lower reach of Rillito Creek may impede infiltration and decrease recharge rates in this area.

This work contains the results of studies of three fluid projects that investigated aspects of groundwater contaminant transport and remediation. The first project performed at Hill Air Force Base in Utah evaluated the performance of a vertical water flushing system for the remediation of a multi-component non-aqueous phase liquid. This project also encompassed determining contaminant distribution through soil core analysis and partitioning tracer studies. The work determined that the limited aqueous solubility of the primary contaminants led to the lack of efficient removal by the vertical water flushing system. The second remediation test evaluated the performance of the solubility enhancing agent cyclodextrin in restoring the contaminated aquifer at Air Force Plant 44 in Tucson, Arizona. The results concluded that this advanced remediation technique was efficient in removing trichloroethene. Further, the project demonstrated the ability to separate TCE from the extracted solution through treatment with an air-stripping system and re-inject it for another multiple flushing of the aquifer. Additional site assessment including determination of lithological and contaminant distribution through well-bore sampling and system optimization by conducting a series of vertical tracer studies. To evaluate the impact of groundwater chemistry and travel distances on the transport behavior of enteric virus, experiments were conducted in the unconfined aquifer at the USGS Cape Cod Research Site. Separate experiments examined the transport behavior of bromide (Br-) and the bacteriophage PRD-1 in the effluent plume and the shallower uncontaminated groundwater. Results indicated the vast majority of the bacteriophages were lost from solution upon injection. The results further showed that this initial loss occurred within the first meter for the uncontaminated zone, whereas it occurred over a 4-meter distance in the contaminated zone. The greater distance required for the contaminated zone to defect similar mass loss is attributed to anion-exchange competition by organic matter, phosphate, and other anions present in higher concentrations in the contaminated zone. The results of this study indicate that a small, but infectious fraction of viable virus particles can persist and travel significant distances in sedimentary aquifers, despite variability in water chemistry.