<p> Droughts reduce water resources necessary for human survival, economic development, and to sustain healthy ecosystems. Our ability to monitor and forecast droughts has grown dramatically in the past decades due to improved hydrological modeling made possible by satellite data and high computing power. However, there is still a large gap of knowledge regarding the mechanisms behind drought onset, development, and recovery. This gap prevents us from being able to forecast every severe drought and from being more confident about the effects of climate change. This thesis proposes a paradigm shift from droughts as local events to droughts as dynamic hazards that can travel in space. In this framework, droughts become the frame of reference, opening new possibilities for drought assessment and forecasting. Here, droughts are shown to have traveled across continents between 1979&mdash;2009. Patterns of frequent and common directions of displacement are identified. Precipitation recycling is proposed as an important mechanism behind these observed dynamics, and a detailed study of moisture sources over North America from 1980&mdash;2016 is carried out. This work shows that drought conditions can propagate downwind, especially from the U.S. Southwest to the U.S. Midwest, and from the northwest of Mexico and Central America to the center and south of Mexico. The effect of local precipitation recycling on drought intensification is quantified and shown to be highest in the north of Mexico and the U.S. Southwest. In a study of climate change's impacts on droughts, large biases are found in the climate models' representation of the hydrologic cycle and land-atmospheric coupling. This is shown to affect the models' drought projections by the end of the twenty-first century. Finally, this thesis includes a study of drought impacts on electricity generation and on <i>CO</i>₂, <i> SO</i>₂, and <i>NO_x</i> emissions from the power sector in the American West under current and future climates. This work advances the understanding of how droughts propagate through the hydrologic cycle locally and across continents, opening new opportunities for seasonal forecasting. It also includes a rigorous drought impact study on the electricity sector that provides useful information to stakeholders and decision makers.</p><p>

Maintaining flows and quality of water resources is critical to support ecosystem services and consumptive needs. Understanding impacts of changes in climate and land use on ecohydrologic processes in a watershed is vital to sustaining water resources for multiple uses. This study completes a continental and regional scale assessment using statistical and simulation modeling to investigate ecohydrologic impacts within watershed systems. Watersheds across the continental United States have diverse hydrogeomorphic characters, mean temperatures, soil moistures, precipitation and evaporation patterns that influence runoff processes. Changes in climate affect runoff by impacting available soil moisture, evaporation, precipitation and vegetative patterns. A one percent increase in annual soil moisture may cause a five percent increase in runoff in watersheds across the continent. Low soil moisture and high temperatures influence runoff patterns in specific regions. Spring runoff is increased by the influence Spring soil moisture, Winter and Spring evaporation, and Winter and Spring evaporation. Spring runoff is decreased by increases in Winter and Spring temperatures and increases in the vegetation index. Winter runoff is affected by maximum vegetative index, temperature, soil moisture, evaporation and precipitation. Contributing factors to runoff are influenced by geomorphic and seasonal variations requiring strategies that are site-specific and use system-wide information. Regional scale watershed analysis investigates the influence of landscape metrics on temporal streamflow processes in multiple gauged watersheds in Massachusetts, U.S.A. Time of concentration, recession coefficient, base flow index, and peak flow are hydrologic metrics used to relate to landscape metrics derived using FRAGSTAT software. Peak flow increases with increasing perimeter-area fractal dimensions, and Contagion index and decreases as Landscape Shape Index increases. There was an increasing trend in the fractal dimension over time indicative of more complex shape of patches in watershed. Base flow index and recession coefficient fluctuated from low to high decreasing recently. This could be indicative of open space legislation, conservation efforts and reforestation within the state in the last ten years. Coastal systems provide valuable ecosystem services and are vulnerable to impacts of changes in climate and continental land use patterns. Effects of land use and climate change on runoff, suspended sediments, total nitrogen and total phosphorus are simulated for coastal watersheds around the Boston Bay ecosystem. The SWAT (Soil and Water Assessment Tool) model, a continuous-time, semi distributed, process-based model, is used to simulate the watershed ecohydrologic process affecting coastal bodies. Urbanization in watersheds increased runoff by as much as 80% from the baseline. Land use change poses a major threat to water quality impacts affecting coastal ecosystems. Total nitrogen increased average of 53.8% with conservative changes in climate and land use. Total phosphorus increased an average of 57.3% with conservative changes in land use and climate change. Climate change alone causes up to 40% increase in runoff and when combined with a 3.25% increase in urban development runoff increased an average of 114%. Coastal ecosystems are impacted by nutrient runoff from watersheds. Continued urbanization and changes in climate will increase total nitrogen, total phosphorus and suspended sediments in coastal ecosystems. Continental scale runoff is affected by soil moisture and vegetative cover. Cover crops, low tillage farm practices and natural vegetation contribute to less runoff. Developing policies that encourage protection of soil structure could minimize runoff and aid in maintaining sustainable water resources. Best Management Practices and Low impact development at the national level with continued stormwater legislation directed towards sustainable land use policy will improve water quantity and quality. Fragmentation observed in Massachusetts increases the number of urban parcels and decreases the size of forested areas. Faster runoff patterns are observed but recent land management may be changing this runoff pattern. Municipal and state zoning ordinance to preserve open space and large forest patches will restrict urban growth to specific regions of a watershed. This could improve quantities of water available to ecosystems. Increases in total nitrogen, phosphorus and suspended sediments to coastal ecosystems can be minimized with use of riparian buffers and Best Management Practices within coastal watersheds. Urbanization and climate change threatens coastal ecosystems and national policy to preserve and restrict development of coastal areas will preserve coastal ecosystem services.

Uncertainty in climate change plays a major role in watershed systems. The increase in variability and intensity in temperature and precipitation affects hydrologic cycle in spatial and temporal dimensions. Predicting uncertainty in climate change impacts on watershed systems can help to understand future climate-induced risk on watershed systems and is essential for designing policies for mitigation and adaptation. Modeling the temporal patterns of uncertainties is assessed in the New England region for temperature and precipitation patterns over a long term. The regional uncertainty is modeled using Python scripting and GIS to analyze spatial patterns of climate change uncertainties over space and time. The results show that the regional uncertainty is significant in variation for changes in location and climatic scenarios. Watershed response to climate change under future scenarios is assessed using hydrologic simulation modeling for the Connecticut River watershed. Changes in water budgets are assessed for each of the subbasins using spatial analysis and process modeling using GIS and Soil and Water Assessment tool (SWAT). The results show that climate change uncertainty in precipitation and temperature can lead to uncertainty in both quantity and quality in the watershed system. A spatiotemporal, dynamic model was applied to subbasins within the Chicopee River Watershed to estimate climate change uncertainty impacts at a micro scale. These changes were assessed relative to changes in land use and climatic change. The results show that there is a significant potential for climate change to increase evaporation, watershed runoff and soil erosion rates and this varied with climate change uncertainty. Finally, water sustainability gradient analysis was applied to the Volga River watershed in Russia to assess potential climate change impacts by combining with downscaled Global Circulation Model estimates and spatial assessment. Results show that runoff and evapotranspiration are projected to increase with potential for more localized floods and drought events effecting both water resources and food supply. Overall results show that climate change uncertainty can impact watershed systems and spatial and temporal assessments is important for developing strategies for adaptation to climatic change conditions at local and regional scales.

Synthetic unit hydrographs, which assume uniform rainfall excess distribution and static watershed conditions, are frequently used to estimate hydrograph characteristics when observed data are unavailable. The objective of this research was to develop a spatially distributed unit hydrograph (SDUH) model that directly reflects spatial variation in the watershed in generating runoff hydrographs. The SDUH model is a time-area unit hydrograph technique that uses a geographic information system (GIS) to develop a cumulative travel time map of the watershed based on cell by cell estimates of overland and channel flow velocities. The model considers slope, land use, watershed position, channel characteristics, and rainfall excess intensity in determining flow velocities. The cumulative travel time map is divided into isochrones which are used to generate a time-area curve and the resulting unit hydrograph. Predictions of the SDUH model along with the Snyder, SCS, and Clark synthetic unit hydrographs were compared with forty observed storm events from an 1153-ha Virginia Piedmont watershed. The SDUH model predictions were comparable or slightly better than those from the other models, with the lowest relative error in the peak flow rate prediction for 12 of the 40 storms, and a model efficiency of at least 0.90 for 21 of the storms. Despite the good predictions of the hydrograph peak flow rate and shape, the time to peak was underpredicted for 34 of the 40 storms. Runoff from the 40 storms was also generated for two subwatersheds (C: 462 ha; D: 328 ha) in Owl Run to assess the effect of scale on the SDUH model. Peak flow rate predictions were more accurate for the entire watershed than for either subwatershed. The time to peak prediction and model efficiency statistics were comparable for the entire watershed and subwatershed D. Subwatershed C had poorer predictions, which were attributed to a large pond in the main channel, rather than to scale effects. The SDUH model provides a framework for predicting runoff hydrographs for ungauged watersheds that can reflect the spatially distributed nature of the rainfall-runoff process. Predictions were comparable to the other synthetic unit hydrograph techniques. Because the time to peak and model efficiency statistics were similar for the 1153-ha watershed and a 328-ha subwatershed, scale does not have a major impact on the accuracy of the SDUH model. / Master of Science

Headwater watersheds initiate material export to downstream environments. A nested headwater study examined the flux and source of dissolved constituents and water from a perennial stream and four ephemeral/intermittent streams in the Upper Gulf Coastal Plain, Mississippi. Water was collected during storm and baseflow conditions. Multiple linear regression was used to model constituent concentration and calculate flux. Source of water was determined using principle components analysis and end-member mixing analysis. Rain was the major source of water discharged from the ephemeral and intermittent streams, while groundwater was the major source for water discharged by the perennial stream during events. Baseflow from both stream types was dominated by groundwater sources. The perennial stream had an area weighted average yields of 10.1, 0.01, 1.0, 0.6, and 0.03 kg-1 ha-1 yr-1 of DON, NO3--N, NH4+-N, PO4-3, and DOC,respectively. This research highlights the interaction of source water and dissolved constituent flux.

Effective prediction of localized flash flood regions for an approaching rainfall event requires an in-depth knowledge of the land surface and stream characteristics of the forecast area. Flash Flood Guidance (FFG) is currently formulated once or twice a day at the county level by River Forecast Centers (RFC) in the U.S. using modeling systems that contain coarse, generalized land and stream characteristics and hydrologic runoff techniques that often are not calibrated for the forecast region of a given National Weather Service (NWS) office. This research investigates the application of embedded geographic information systems (GIS) modeling techniques to generate a localized flash flood model for individual small watersheds at a five minute scale and tests the model using historical case storms to determine its accuracy in the FFG process. This model applies the Soil Conservation Service (SCS) curve number (CN) method and synthetic dimensionless unit hydrograph (UH), and Muskingum stream routing modeling technique to formulate flood characteristics and rapid update FFG for the study area of interest. The end result of this study is a GIS-based Flash Flood Forecasting system for ungaged small watersheds within a study area of the Blacksburg NWS forecast region. This system can then be used by forecasters to assess which watersheds are at higher risk for flooding, how much additional rainfall would be needed to initiate flooding, and when the streams of that region will overflow their banks. Results show that embedding these procedures into GIS is possible and utilizing the GIS interface can be helpful in FFG analysis, but uncertainty in CN and soil moisture can be problematic in effectively simulating the rainfall-runoff process at this greatly enhanced spatial and temporal scale. / Master of Science

Precipitation and surface runoff producing mechanisms are inherently spatially variable. Many hydrologic runoff models do not account for this spatial variability and instead use "lumped" or spatially averaged parameters. Lumped model parameters often must be developed empirically or through optimization rather than be calculated from field measurements or existing data. Recent advances in geographic information systems (GIS) remote sensing (RS), radar measurement of precipitation, and desktop computing have made it easier for the hydrologist to account for the spatial variability of the hydrologic cycle using distributed models, theoretically improving hydrologic model accuracy. Grid based distributed models assume homogeneity of model parameters within each grid cell, raising the question of optimum grid scale to adequately and efficiently model the process in question. For a grid or raster based hydrologic model, as grid cell size decreases, modeling accuracy typically increases, but data and computational requirements increase as well. There is great interest in determining the optimal grid resolution for hydrologic models as well as the sensitivity of hydrologic model outputs to grid resolution. This research involves the application of a grid based hydrologic runoff model to the Upper Roanoke River watershed (1480km2) to investigate the effects of precipitation resolution and grid cell size on modeled peak flow, time to peak and runoff volume. The gridded NRCS curve number (CN) rainfall excess determination and ModClark runoff transformation of HEC-HMS is used in this modeling study. Model results are evaluated against observed streamflow at seven USGS stream gage locations throughout the watershed. Runoff model inputs and parameters are developed from public domain digital datasets using commonly available GIS tools and public domain modeling software. Watersheds and stream networks are delineated from a USGS DEM using GIS tools. Topographic parameters describing these watersheds and stream channel networks are also derived from the GIS. A gridded representation of the NRCS CN is calculated from the soil survey geographic database of the NRCS and national land cover dataset of the USGS. Spatially distributed precipitation depths derived from WSR-88D next generation radar (NEXRAD) products are used as precipitation inputs. Archives of NEXRAD Stage III data are decoded, spatially and temporally registered, and verified against archived IFLOWS rain gage data. Stage III data are systematically degraded to coarser resolutions to examine model sensitivity to gridded rainfall resolution. The effects of precipitation resolution and grid cell size on model outputs are examined. The performance of the grid based distributed model is compared to a similarly specified and parameterized lumped watershed model. The applicability of public domain digital datasets to hydrologic modeling is also investigated. The HEC-HMS gridded SCS CN rainfall excess calculation and ModClark runoff transformation, as applied to the Upper Roanoke watershed and for the storm events chosen in this study, does not exhibit significant sensitivity to precipitation resolution, grid scale, or spatial distribution of parameters and inputs. Expected trends in peak flow, time to peak and overall runoff volume are observed with changes in precipitation resolution, however the changes in these outputs are small compared with their magnitudes and compared to the discrepancies between modeled and observed values. Significant sensitivity of runoff volume and consequently peak flow, to CN choices and antecedent moisture condition (AMC) was observed. The changes in model outputs between the distributed and lumped versions of the model were also small compared to the magnitudes of model outputs. / Master of Science

Protection of downstream channels and reduction in flooding can potentially be improved by evaluating alternative site stormwater management (SWM) strategies at a watershed scale and selecting the optimal strategy for a subject watershed. Tailoring a management strategy for a specific watershed may be worthwhile to minimize development costs and maximize downstream benefit. A hydrologic/hydraulic model for a watershed in Blacksburg, Virginia, is used to evaluate downstream results based on implementation of several alternative SWM strategies currently practiced within the United States. Results show none of the strategies meet the goal of maintaining the baseline goal at the watershed POI for the full range of design storms. Modification to the strategy that performs best at the watershed scale did meet the watershed goal for all design storms except the 1-year. For smaller storm events, it appears that increasing the volume of an initial capture and the drawdown time to release that volume does not increase performance downstream. This is potentially significant as extra dollars spent on site would not provide extra benefit downstream. When post-development peak runoff rates are detained to the predevelopment rate for larger storm events, whether based on a site or watershed focused strategy, the watershed goal can be met. A volume reduction strategy performs well, but implementation is hindered by soils with poor infiltration and the presence of karst. Other insight to watershed based management strategies, the role of regional facilities and predevelopment condition assumptions at the site scale to maintain a baseline condition downstream are discussed. / Master of Science

The development of a second generation Electrically Scanned Thinned Array Radiometer, ESTAR-B, is presented. The design of ESTAR-B and how it relates to ESTAR-A is described, including the use of vertically polarized arrays, the correlator redesign, changes in the null feedback receiver and the calibration network. An examination of antenna effects on the ESTAR inversion is presented paying particular attention to the Fourier inversion and polarization effects in the generalized matrix inversion. The thermal and calibration stabilities of the two ESTARs are compared and improvements in reliability are discussed. Experimental results from antenna chamber measurements and a flight experiment are presented: the first measures the ESTAR-B spatial impulse response; the flight experiment demonstrates ESTAR-B's imaging ability over land and water.

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