| Estimating terrestrial evaporation and vegetation transpiration (collectively referred to in the literature as evapotranspiration (ET)) at global scales remains a challenge for the climate community. The only feasible approach for consistent estimates is through satellite remote sensing, which itself posses significant challenges that are addressed in this dissertation. Chapter 1 presents the motivation for the research, presents the current status of using satellite measurements for the estimation of terrestrial ET and the challenges facing the estimation of this critical climate variable. The following two chapters present an analysis of two critically important issues. In Chapter 2, an evaluation is carried out for newly available satellite-based surface shortwave downward radiation (SSDR) retrievals, since solar radiation is a primary variable that controls terrestrial ET. In Chapter 3, an analysis of the scaling behavior of satellite based SSDR data is presented. Since current global solar radiation products are spatially coarser than the variability of vegetation and topography that also controls ET, understanding how radiation scales is fundamental to understanding ET at small-scales where in-situ measurements are available. Chapter 4 and 5 presents an evaluation of the Surface Energy Budget System (SEBS) retrieval model, which uses inputs of radiation, vegetation and surface meteorology to estimate ET. In Chapter 4, ET retrievals are compared to measurements using data from the July 2002 Soil Moisture, Atmospheric Coupling Experiment (SMACEX'02). ET was retrieved at the flux tower scale and over the SMACEX study region, with the latter offering a framework for using satellite measurements. In Chapter 5, the SEBS is further evaluated for a diverse set vegetation types (cover) and climate zones, which is an important step to do ET retrievals globally. In the final chapter, the contribution of this dissertation is summarized and the future directions in ET retrieval from remote sensing are suggested. |
