Estimating evapotranspiration of European forests from NOAA-imagery at satellite overpass time: Towards an operational processing chain for integrated optical and thermal sensor data products

Abstract

Evapotranspiration (ET) using the Integral NOAA-imagery processing Chain (iNOAA-Chain) is quantified by implementing visible and thermal satellite information on a regional scale. ET is calculated based on the energy balance closure principle. The combination of evaporative fraction (EF), soil heat flux and instantaneous net radiation, results in an instantaneous spatial distribution of ET values. Surface broadband albedo and land surface temperature (LST) serve to determine EF. EF is derived using four methods based on NOAA/AVHRR satellite imagery. Instantaneous evapotranspiration, i.e. at time of satellite overpass, on European continental scale with emphasis on forest stands is estimated using the iNOAA-Chain. Finally, the estimated net radiation ( R n), soil heat fluxes ( G 0) and evaporative fraction and evapotranspiration at time of satellite overpass are validated against EUROFLUX site data for the growing season of 1997 (March–October). The regression line for the pooled R n (iNOAA-Chain versus EUROFLUX) has a slope, intercept, Pearson product moment correlation coefficient ( R 2) and relative root mean square error (RRMSE) of respectively 0.943, 17.120, 0.926 and 5.5%. The soil heat fluxes, calculated with two approaches are not-well modelled with slopes smaller than − 3.000 and a R 2 in the order of zero. We observe a slight underestimation of the iNOAA Chain estimated EF. The regression line for pooled EF data for the best performing method (SPLIT-method) has a slope of 0.935, an intercept of 0.041 and the R 2 is 0.847. A pooled RRMSE EF value of 12.3% is found. The pooled slope, intercept, R 2 and RRMSE for EF derived with SORT-method 1 are respectively 0.449, 0.251, 0.043 and 65.1%, with SORT-method 2, 0.567, 0.203, 0.174 and 39.1%, and with SORT-method 3, 0.568, 0.254, 0.288, and 32.8%. Also instantaneous evapotranspiration is underestimated with a pooled RRMSE on ET of 23.4%. The regression curve of pooled ET data for the best performing method has a slope of 0.889 an intercept of 15.880 and a Pearson product moment correlation coefficient of 0.771. The other method gives a slope of 0.781, an intercept of 17.541 and a R 2 of 0.776. Error propagation analysis reveals that the relative error on evapotranspiration at satellite overpass time is at least 27%.