Sanchez, JM, Caselles, V, Niclos, R, Valor, E, Coll, C, Laurila, T (2007). Evaluation of the B-method for determining actual evapotranspiration in a boreal forest from MODIS data. INTERNATIONAL JOURNAL OF REMOTE SENSING, 28(6), 1231-1250.
Boreal forests occupy about 11% of the terrestrial surface and represent an important contribution to global energy balance. The ground measurement of daily evapotranspiration (LEd) is very difficult due to the limitations on experiments. The objective of this paper is to present and explore the applicability of the B-method for monitoring actual LEd in these ecosystems. The method shown in this paper allows us to determine the surface fluxes over boreal forests on a daily basis from instantaneous information registered in a conventional meteorological tower, as well as the canopy temperature (T-c) retrieved by satellite. Images collected by the MODIS ( moderate resolution imaging spectroradiometer) on board EOS-Terra have been used for this study. The parameters of the model were calibrated from the SIFLEX-2002 ( Solar Induced Fluorescence Experiment 2002) campaign dataset in a northern boreal forest in Finland. A study of these parameters was made on an hourly basis in order to make the method applicable, not only at midday but within an interval of 7 h around it. This is an important advance with respect to the original formulation of this approach since the overpass time of satellites can be very variable. The comparison between T-c ground measured with a thermal infrared radiometer, and T-c retrieved from land surface temperature (LST) MODIS data, showed an estimation error of +/- 1.4 degrees C for viewing angles from 5 to 60 degrees. A complete sensitivity analysis was carried out and an estimation error of about +/- 35%, corresponding to the interval 10.00-11.00 h UTC, was shown as the lowest in LEd retrieval. Finally, the method was validated over the study site using 21 MODIS images for 2002 and 2003. The results were compared with eddy-correlation ground measurements. An accuracy of +/- 1.0 mm/day and an overestimation of 0.3 mm/day were shown in the LEd retrieval.