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Railroad bridges and culverts in the United States are often subject to extreme floods, which have been known to washout sections of track and ultimately lead to derailments. The potential for these events is particularly high in the western U.S. due to the lack of data, inadequate radar coverage, and the high spatial and temporal variability of storm events and terrain. In this work, a hydrologic model is developed that is capable of effectively describing the rainfall-runoff relationship of extreme thunderstorms in arid and semi-arid regions. The model was calibrated and validated using data from ten storms at the semi-arid Walnut Gulch Experimental Watershed. A methodology is also proposed for reducing the amount of raingages required to provide acceptable inputs to the hydrologic model, and also determining the most appropriate placement location for these gages. Results show that the model is capable of reproducing peak discharges, peak timings, and total volumes to within 22.1%, 12 min, and 32.8%, respectively. Results of the gage reduction procedure show that a decrease in the amount of raingages used to drive the model results in a disproportionally smaller decrease in model accuracy. Results also indicate that choosing gages using the minimization of correlation approach that is described herein will lead to an increase in model accuracy as opposed to selecting gages on a random basis.

Arid and semi-arid basins in the Western United States (US) have been significantly impacted by human alterations to the water cycle and are among the most susceptible to water stress from urbanization and climate change. The climate of the Western US is projected to change in response to rising greenhouse gas concentrations. Combined with land use/land cover (LULC) change, it can influence both surface and groundwater resources, both of which are a significant source of water in the US. Responding to this challenge requires an improved understanding of how we are vulnerable and the development of strategies for managing future risk. In this dissertation, I explored how hydrology of semi-arid regions responds to LULC and climate change and how hydrologic projections are influenced by the choice and calibration of models. The three main questions I addressed with this dissertation are: 1. Is it important to calibrate models for forecasting absolute/relative changes in streamflow from LULC and climate changes? 2. Do LSMs make reasonable estimates of groundwater recharge in the western US? 3. How might recharge change under projected climate change in the western US? Results from this study suggested that it is important to calibrate the model spatially to analyze the effect of LULC change but not as important for analyzing the relative change in streamflow due to climate change. Our results also highlighted that LSMs have the potential to capture the spatial and temporal patterns as well as seasonality of recharge at large scales. Therefore, LSMs (specifically VIC and Noah) can be used as a tool for estimating current and future recharge in data limited regions. Average annual recharge is projected to increase in about 62% of the region and decrease in about 38% of the western US in future and varies significantly based on location (-50% - +94 for near future and -90% to >100% for far future). Recharge is expected to decrease significantly (-13%) in the South region in the far future. The Northern Rockies region is expected to get more recharge in both in the near (+5.1%) and far (+9.0%) future. Overall, this study suggested that land use/land cover (LULC) change and climate change significantly impacts hydrology in semi-arid regions. Model choice and model calibrations also influence the hydrological predictions. Hydrological projections from models have associated uncertainty, but still provide valuable information for water managers with long term water management planning.

abstract: This study performs numerical modeling for the climate of semi-arid regions by running a high-resolution atmospheric model constrained by large-scale climatic boundary conditions, a practice commonly called climate downscaling. These investigations focus especially on precipitation and temperature, quantities that are critical to life in semi-arid regions. Using the Weather Research and Forecast (WRF) model, a non-hydrostatic geophysical fluid dynamical model with a full suite of physical parameterization, a series of numerical sensitivity experiments are conducted to test how the intensity and spatial/temporal distribution of precipitation change with grid resolution, time step size, the resolution of lower boundary topography and surface characteristics. Two regions, Arizona in U.S. and Aral Sea region in Central Asia, are chosen as the test-beds for the numerical experiments: The former for its complex terrain and the latter for the dramatic man-made changes in its lower boundary conditions (the shrinkage of Aral Sea). Sensitivity tests show that the parameterization schemes for rainfall are not resolution-independent, thus a refinement of resolution is no guarantee of a better result. But, simulations (at all resolutions) do capture the inter-annual variability of rainfall over Arizona. Nevertheless, temperature is simulated more accurately with refinement in resolution. Results show that both seasonal mean rainfall and frequency of extreme rainfall events increase with resolution. For Aral Sea, sensitivity tests indicate that while the shrinkage of Aral Sea has a dramatic impact on the precipitation over the confine of (former) Aral Sea itself, its effect on the precipitation over greater Central Asia is not necessarily greater than the inter-annual variability induced by the lateral boundary conditions in the model and large scale warming in the region. The numerical simulations in the study are cross validated with observations to address the realism of the regional climate model. The findings of this sensitivity study are useful for water resource management in semi-arid regions. Such high spatio-temporal resolution gridded-data can be used as an input for hydrological models for regions such as Arizona with complex terrain and sparse observations. Results from simulations of Aral Sea region are expected to contribute to ecosystems management for Central Asia. / Dissertation/Thesis / Ph.D. Aerospace Engineering 2012

The current water resource management in the Brazilian Semi-arid area is scarce in information on methods that seek the optimization of the use of its water, which is fundamentally important having in view the serious panorama experienced in relation to the water availability. Searching to explore and understand the behavior of hydrosedimentology variables in the semi-arid region is increasingly common the study in experimental basins. In this context, this paper presents the instrumentation of an experimental basin in the semi-arid regions of the State of Alagoas, with emphasis on hydrosedimentology monitoring processes. This study was entirety developed from subsidies from hydrosedimentology monitoring processes in the field, having as the general objective to quantify the production of sediments in two different scales of experimental units: plots and microbasin. The selected microbasin has contributing area of 0.183 ha, and an average declivity of 6.5% and being constructed in the outlet a sediment pit for collecting the superficial discharge. In another area, three plots were installed, with 3 m² each, in a land with average declivity of 13%. The monitoring in the plots happened as of simulated rainfall, the simulator being calibrated to produce heavy rain for each of the plots. In relation to soil cover, the microbasin was studied in uncovered soil conditions, while as in the plots was done the plantation of cultivated corn hill under which is quite usual in the region. The production of sediment in the microbasin and in the plots was calculated from samples of the superficially drained material. The data observed made it possible to correlate the production of sediments with variables related to the total and effective rainfall. A growth tendency in the production of sediments due to the increase in superficial draining for both the monitored events in the microbasin as well as the monitoring done in the plots was observed. In the plots, the protection of the soil exerted by the corn cultivation provided a reduction in the production of sediments for all the simulated pluviometric intensities. / A atual gestão dos recursos hídricos no semi-árido brasileiro é carente em informações acerca de métodos que busquem a otimização do uso de suas águas, sendo estas de fundamental importância tendo em vista o grave panorama vivenciado em relação à disponibilidade hídrica. Buscando analisar e compreender o comportamento das variáveis hidrossedimentológicas na região semi-árida, é cada vez mais comum o estudo em bacias experimentais. Neste contexto, o trabalho apresenta a instrumentação de uma bacia experimental no semi-árido alagoano, com ênfase no monitoramento dos processos hidrossedimentológicos. O estudo foi desenvolvido em sua totalidade a partir de subsídios provenientes de monitoramento dos processos hidrossedimentológicos em campo, tendo como objetivo geral quantificar a produção de sedimentos em duas diferentes escalas de unidades experimentais: parcelas e microbacia. A microbacia selecionada possui área contribuinte de 0,183 ha, e declividade média de 6,5%, sendo construída no exutório uma fossa de sedimentos para coleta do deflúvio superficial. Em outra área foram instaladas três parcelas, possuindo 3 m² cada, em um terreno com declividade média de 13%. O monitoramento nas parcelas se deu a partir de chuva simulada, sendo o simulador calibrado para produzir chuvas intensas específicas para cada uma das parcelas. Em relação à cobertura do solo, a microbacia foi estudada em condição de solo descoberto, enquanto que nas parcelas foi realizado o plantio de milho cultivado morro abaixo por ser bastante usual na região. A produção de sedimentos na microbacia e nas parcelas foi calculada a partir de coletas do material escoado superficialmente. Os dados observados possibilitaram correlacionar a produção de sedimentos com variáveis relacionadas com a chuva total e efetiva. Foi observada uma tendência de crescimento da produção de sedimentos em função do aumento do escoamento superficial tanto para os eventos monitorados na microbacia quanto para o monitoramento realizado nas parcelas. Nas parcelas, a proteção do solo exercida pelo cultivo do milho proporcionou uma redução na produção de sedimentos para todas as intensidades pluviométricas simuladas.

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