The Gravity Recovery and Climate Experiment (GRACE) has been orbiting the Earth and determining its gravity field since 2002. Throughout the course of the mission, the orbital elements occasionally change such that the satellites enter a repeat ground track configuration. Repeat ground tracks result in reduced spatial resolution of the satellites, which poses problems in the context of gravity recovery. The monthly gravity solutions during these periods are examined and shown to have lower quality than usual. The characteristics of these repeat period solutions are identified and compared to a period of uniform coverage to illustrate the ways in which the solutions are degraded. An investigation into the underlying physical and computational sources of these errors is also presented. / text

La sequía agrícola ocurre cuando la cantidad de precipitación, su distribución, las reservas de agua en el suelo y las pérdidas producidas por la evapotranspiración se combinan para causar una disminución considerable de los rendimientos del cultivo y el ganado. Desde el punto de vista hidrogeofísico, la sequía se manifiesta como una disminución de las reservas de agua en la zona afectada por este evento. Durante el año 2009, la provincia de Buenos Aires se vió afectada por una sequía que se estima ha sido el peor evento de este tipo que atravesó el país en más de medio siglo y cuyos efectos climáticos repercutieron en un descenso significativo en los cuerpos de agua superficiales, una disminución de los niveles freáticos en toda la región y significativas pérdidas en la economía de la principal región agrícola-ganadera del país. El presente trabajo de Tesis analiza la potencialidad de la misión satelital Gravity Recovery and Climate Experiment (GRACE) para detectar y estudiar esta impactante sequía. La misión espacial GRACE tiene como principal objetivo la medición de variaciones espacio-temporales del campo de gravedad terrestre, siendo estas variaciones de gravedad originadas principalmente por las variaciones de las reservas de agua. Para estudiar los efectos de la sequía en las reservas de agua subterráneas durante el año 2009 se tomó como referencia el año 2008 que puede considerarse a los fines de este análisis como un año hidrológico normal. Los datos utilizados consisten en alturas de agua equivalente calculados cada 10 días a partir de los datos de la misión GRACE por el Grupo de Investigación de Geodesia Espacial del Centro Nacional de Estudios Espaciales de Francia (CNES/GRGS). Estos datos satelitales que poseen una cobertura regional muy amplia, han sido comparados con datos puntuales de dos estaciones de medición de niveles freáticos ubicadas en las ciudades de Azul y La Plata. A partir de los datos de alturas de agua equivalente se realizó un seguimiento de la evolución de la sequía en la provincia de Buenos Aires que pudo ser validado por mediciones de los niveles freáticos. A partir de la correlación de los datos satelitales y de campo se calcularon propiedades de la zona de estudio, como la capacidad específica y las variaciones de humedad del suelo en la zona no saturada. El carácter regional del fenómeno de la sequía observado a partir de los datos de GRACE también queda de manifiesto en los descensos del los niveles freáticos observados en las estaciones de Azul y La Plata. Los resultados obtenidos muestran la utilidad de los datos de GRACE para cuantificar las reservas de agua subterráneas y realizar estudios regionales en zonas donde no se cuenta con datos de campo o resultan de difícil acceso.

The Gravity Recovery and Climate Experiment (GRACE) is a joint National Aeronautics and Space Administration / Deutsches Zentrum für Luftund Raumfahrt (NASA/DLR) mission to map the time-variable and mean gravity field of the Earth, and was launched on March 17, 2002. The nature of the gravity field inverse problem amplifies the noise in the data that creeps into the mid and high degree and order harmonic coefficients of the earth's gravity fields for monthly variability, making the GRACE estimation problem ill-posed. These errors, due to the use of imperfect models and data noise, are manifested as peculiar errors in the gravity estimates as north-south striping in the monthly global maps of equivalent water heights. In order to reduce these errors, this study develops a methodology based on Tikhonov regularization technique using the L-curve method in combination with orthogonal transformation method. L-curve is a popular aid for determining a suitable value of the regularization parameter when solving linear discrete ill-posed problems using Tikhonov regularization. However, the computational effort required to determine the L-curve can be prohibitive for a large scale problem like GRACE. This study implements a parameter-choice method, using Lanczos bidiagonalization that is a computationally inexpensive approximation to L-curve called L-ribbon. This method projects a large estimation problem on a problem of size of about two orders of magnitude smaller. Using the knowledge of the characteristics of the systematic errors in the GRACE solutions, this study designs a new regularization matrix that reduces the systematic errors without attenuating the signal. The regularization matrix provides a constraint on the geopotential coefficients as a function of its degree and order. The regularization algorithms are implemented in a parallel computing environment for this study. A five year time-series of the candidate regularized solutions show markedly reduced systematic errors without any reduction in the variability signal compared to the unconstrained solutions. The variability signals in the regularized series show good agreement with the hydrological models in the small and medium sized river basins and also show non-seasonal signals in the oceans without the need for post-processing. / text

Satellite geodesy plays an important role in earth observation. This dissertation presents three applications of satellite geodesy in environmental and climate change. Three satellite geodesy techniques are used: high-precision Global Positioning System (GPS), the Gravity Recovery and Climate Experiment (GRACE) and Interferometric Synthetic Aperture Radar (InSAR). In the first study, I use coastal uplift observed by GPS to study the annual changes in mass loss of the Greenland ice sheet. The data show both spatial and temporal variations of coastal ice mass loss and suggest that a combination of warm atmospheric and oceanic condition drove these variations. In the second study, I use GRACE monthly gravity change estimates to constrain recent freshwater flux from Greenland. The data show that Arctic freshwater flux started to increase rapidly in the mid-late 1990s, coincident with a decrease in the formation of dense Labrador Sea Water, a key component of the deep southward return flow od the Atlantic Meridional Overturning Circulation (AMOC). Recent freshening of the polar oceans may be reducing formation of Labrador Sea Water and hence may be weakening the AMOC. In the third study, I use InSAR to monitor ground deformation caused by CO2 injection at an enhanced oil recovery site in west Texas. Carbon capture and storage can reduce CO2 emitted from power plants, and is a promising way to mitigate anthropogenic warming. From 2007 to 2011, ~24 million tons of CO2 were sequestered in this field, causing up to 10 MPa pressure buildup in a reservoir at depth, and surface uplift up to 10 cm. This study suggests that surface displacement observed by InSAR is a cost-effective way to estimate reservoir pressure change and monitor the fate of injected fluids at waste disposal and CO2 injection sites.

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