Estimation of Large-Scale Evaporation Using Remote Sensing II
Presiding: M Jasinski, NASA Goddard Space Flight Center; B Kustas, USDA Hydrology and Remote Sensing Laboratory
H31A-01 INVITED 08:30h
SEAFLUX: Status and Prospects
Determination of the ocean evaporative flux is critical for the global water and energy cycle and the partitioning of heat transport between the ocean and atmosphere. Further, high-resolution surface fluxes over the global ocean are needed to evaluate coupled atmosphere-ocean models and weather forecasting models, provide surface forcing for ocean models, understand the regional and temporal variations of the exchange of heat between the atmosphere and ocean, and provide a large-scale context for field experiments. Under the auspices of the World Climate Research Programme (WCRP) Global Energy and Water Experiment (GEWEX) Radiation Panel, the SEAFLUX Project has guiding the production and evaluation of high-resolution satellite-based data sets of surface turbulent fluxes over the global oceans. An overview will be given of progress to date, including outstanding challenges. Prospects for improvements using new (exisiting and planned) satellite sensors will be presented. Proposed strategies for evaluating new flux products will be described.
H31A-02 INVITED 08:50h
A Climatological Study of Surface Fluxes over the Continental U.S. using Thermal Remote Sensing
The Atmosphere-Land Exchange Inverse (ALEXI) model was designed for routine monitoring of surface water, energy, and carbon fluxes at continental scales based on thermal imagery from a geostationary satellite platform. Operational processing of fluxes over the continental U.S. at 10-km resolution commenced in 2002; to date, three consecutive years of flux data have been archived. Simple gap-filling methodologies have been implemented to predict fluxes under cloud cover conditions, when the thermal data are unavailable, and to extrapolate instantaneous fluxes to full hourly and daily coverage. Quantitative validation of the regional ALEXI algorithm has been accomplished by spatially disaggregating the 5-10km flux predictions down to the scale of the measurement sensor footprint, yielding good agreement with tower and aircraft-based eddy-covariance data. Results from a climatological study of continental-scale flux patterns will be presented, along with an analysis of model capabilities in assessing regional moisture stress conditions.
http://www.soils.wisc.edu/alexi
H31A-03 INVITED 09:10h
Estimation of Surface Evaporation Based on Assimilation of Sequential Images of Remotely Sensed Land Surface Temperature
Evapotranspiration does not have a unique signature that can be detected by remotely deployed instruments. In order to retrieve the fluxes, the measurements need to be merged into models that infer fluxes from their space and time patterns. We here discuss an approach based on variational assimilation of Land Surface Temperature (LST) into a surface energy balance model. It does not require empirical relations such as those relating evaporation to vegetation indices (or LST) nor does it require closure assumptions such as those that assume ground heat flux is a given fraction of net radiation. There are two major unknown parameters in the estimation of evapotranspiration (near-surface air turbulent conductivity and evaporative fraction) when following an energy-balance criteria. The air turbulent conductivity scales the magnitude of the turbulent fluxes and the evaporative fraction partitions the total turbulent flux into latent and sensible heat fluxes. Both are non-dimensional parameters and uncertainties about the values drive the errors in the calculation of turbulent fluxes. The mapping capability comes from the use of land surface temperature sensed from several environmental satellites. The data may relate to different times of day and apply at different and often overlapping resolutions. Thus the assimilation system has to be multiscale and capable of constraining the estimation at varying resolutions. The surface composition can be treated either as a `combined' soil-vegetation medium, or with a `two-source' formulation where the contributions from the canopy and the exposed soil surface are singled out. A number of applications are discussed, which demonstrate the robustness with respect to environmental and satellite availability conditions. These include an application on the Southern Great Plains site using a combination of three different LST sources (where hourly evapotranspiration is mapped with a resolution of the order of few kilometers) over large continental area and a case study in a Mediterranean small watershed using data with high refresh rate from the geostationary MSG. In the framework of the latter application, the potential for using the retrieval approach in combination with a high resolution (tens to hundreds meters) distributed hydrological model is also discussed.
H31A-04 INVITED 09:30h
Landsat-based METRIC/SEBAL Energy Balances in Western Water Resources Management
Since about 2000, there have been an increasing number of METRIC/SEBAL applications in the western U.S. to spatially quantify actual ET for focused land areas. SEBAL, the Surface Energy Balance Algorithm for Land, by Bastiaanssen and METRIC, Mapping Evapotranspiration at High Resolution using Internalized Calibration, are a suite of pragmatic energy balance solutions that rely heavily on high quality satellite imagery. METRIC additionally uses ground-based weather data and Penman-Monteith estimates of ET from full-cover, well-watered vegetation to assist in calibrating the energy balance surface and to extrapolate to 24hrs and beyond. Results indicate why applications with high resolution Landsat or ASTER will remain extremely relevant in applications to irrigated systems where knowlege of ET consumption by individual fields and farms is critical. Comparisons with some MODIS-derived ET for similar areas and periods are made.
http://www.kimberly.uidaho.edu/metric