A. Olioso

Evaluation of the Surface Energy Balance System (SEBS) applied to ASTER imagery with flux-measurements at the SPARC 2004 site (Barrax, Spain)

Accurate quantification of the amount and spatial variation of evapotranspiration is important in a wide range of disciplines. Remote sensing based surface energy balance models have been developed to estimate turbulent surface energy fluxes at different scales. The objective of this study is to evaluate the Surface Energy Balance System (SEBS) model on a landscape scale, using tower-based flux measurements at different land cover units during an overpass of the ASTER sensor over the SPARC 2004 experimental site in Barrax (Spain). A sensitivity analysis has been performed in order to investigate to which variable the sensible heat flux is most sensitive. Taking into account their estimation errors, the aerodynamic parameters ( hc, z0M andd0) can cause large deviations in the modelling of sensible heat flux. The effect of replacement of empirical derivation of these aerodynamic parameters in the model by field estimates or literature values is investigated by testing two scenarios: the Empirical Scenario in which empirical equations are used to derive aerodynamic parameters and the Field Scenario in which values from field measurements or literature are used to replace the empirical calculations of the Empirical Scenario. In the case of a homogeneous land cover in the footprints of the measurements, the Field Scenario only resulted in a small improvement, compared to the Empirical Scenario. The Field Scenario can even worsen the result in the case of heterogeneous footprints, by creating sharp borders related to the land cover map. In both scenarios modelled fluxes correspond Correspondence to: J. van der Kwast (hans.vanderkwast@vito.be) better with flux measurements over uniform land cover compared to cases where different land covers are mixed in the measurement footprint. Furthermore SEBS underestimates sensible heat flux especially over dry and sparsely vegetated areas, which is common in single-source models.

Simulating the relationship between thermal emissivity and the Normalized Difference Vegetation Index

Abstract An experimental relation between the thermal ermssrvity and the Normalized Difference Vegetation Index of vegetation canopies has been found by van de Griend and Owe. This relationship is investigated by means of aradiative transfer model. The relationship was highly sensitive to soil and leaf emissivities, canopy structure, optical properties of leaves and soil in the solar domain and Sun geometry, but insensitive to viewing geometry.

Quantification of land-atmosphere exchanges of water, energy and carbon dioxide in space and time over the heterogeneous Barrax site

To advance our understanding of land–atmosphere exchanges of water, energy and carbon dioxide (CO2) in space and time over heterogeneous land surfaces, two intensive field campaigns were carried out at the Barrax agricultural test site in Spain during 12–21 July 2004 (SPARC 2004) and 8–14 July 2005 (SEN2FLEX 2005) involving multiple field, satellite and airborne instruments for characterizing the state of the atmosphere, the vegetation and the soil from the visible to the microwave range of the spectrum. Part of the experimental area is a core site of area 25 km2, within which numerous crops are grown, on both irrigated and dry land, alongside fields of bare soil. The campaigns were carried out in the framework of the Earth Observation Envelope Programme of the European Space Agency (ESA) with the aim of supporting geophysical algorithm development, calibration/validation and the simulation of future spaceborne Earth Observation missions. Both campaigns were also contributions to the EU 6FP EAGLE Project. The emphasis of this contribution is on the in situ measurements of land–atmosphere exchanges of water, energy and CO2 as well as the thermal dynamic states of the atmosphere, the soil and the vegetation. Preliminary analysis and interpretation of the measurements are presented. These two data sets are open to the scientific community for collaborative investigations.

Accounting for atmospheric boundary layer variability on flux estimation from RS observations

A Large Eddy Simulation (LES) model is coupled to a remote‐sensing‐based SVAT that accounts for soil and vegetation in order to study the feedback effects between surface state and spatial variability in fluxes, through the induction of spatial variability in the lower atmosphere. As such, this study focuses on sensible heat flux exchange. The simulations indicated that changes are introduced to the flux distributions mainly at higher surface temperatures. A scale‐dependent method is presented to account for feedback effects. This showed the most significant correlation between surface and air temperature at scales from 500 to 1000 m and modulated the spatial variance of sensible heat flux. This suggests that feedback effects act to limit the spatial variability in fluxes, and ignoring them will cause the largest errors at the extremes.

Dynamic change of CO2 flux over bare soil field and its relationship with remotely sensed surface temperature

This study investigated the potential of thermal remote sensing for estimating ecosystem surface CO2 flux. Ecosystem surface CO2 flux was measured by an eddy covariance method for more than 3 years, in conjunction with thermal and optical remote sensing measurements as well as micrometeorological, soil and plant measurements. The soil was Andisol (Hydric Hapludands), a humic volcanic ash soil, which is the major cultivated soil for upland crops in Japan. The soil surface CO2 flux under bare soil conditions was best correlated with the remotely sensed surface temperature, while air temperature was less well correlated and soil temperature and soil water content were poorly correlated. The relationship was well expressed by an exponential Q 10 function (r 2=0.66, RMSE=0.098). The value of Q 10 and the threshold temperature at which the CO2 flux approached zero were estimated to be 1.47 and 10.0°C, respectively. Results suggested that the soil surface temperature had the dominant effect on the microbial respiration as well as on the physical processes determining the CO2 gas transfer at the soil–atmosphere interface. Remotely sensed surface temperature will provide useful information for investigation of CO2 transfer processes near the soil surface, as well as for quantitative assessment of ecosystem surface CO2 flux.

Retrieval Of Daily Evapotranspiration From Remote Sensing Images Of High And Low Spatial Resolution. Application To The Iberian Peninsula

Evapotranspiration is an important parameter to optimize water use, especially in non‐irrigated and arid or semiarid rangelands. A simplified method to derive the daily evapotranspiration using remote sensing images of high and low spatial resolution is presented in this paper. The method is based on the evaporative concept employed in the S‐SEBI (Simplified Surface Energy Balance Index) model, and it is applicable under the assumption of constant atmospheric conditions and sufficient wet and dry pixels over the image. The method, applied to DAIS and NOAA‐AVHRR images, has been tested using ET values measured over the agricultural area of Barrax (Castilla‐La Mancha, Spain), and an accuracy for the daily evapotranspiration lower than 1 mmd−1 has been obtained..