Multi-dimensional Evaluation of SEVIRI-MSG ET Products across Europe

This study validates the evapotranspiration (ET) products derived from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensor onboard the Meteosat Second Generation (MSG) satellites, along five key dimensions, i.e., ecosystem, ecoregion, climate, intra-annual (within the year) and inter-annual (between years). This five-dimensional assessment was conducted to evaluate the accuracy of SEVIRI products for actual and reference ET (SEVIRI-ETa and SEVIRI-ET0) using in situ measurements collected at 55 eddy covariance (EC) sites across Europe at daily and monthly increments between 2011 and 2018. First, a spatial analysis was performed to determine the homogeneity of the SEVIRI pixels across all 55 EC sites in Europe, using higher resolution land cover data. A balanced optimization of product bias, variability, and the temporal fit was adopted through Kling-Gupta efficiency (KGE) to quantify the error. The results showed a reasonable accuracy for SEVIRI-ETa and SEVIRI-ET0 products. Considering SEVIRI-ET0, the highest (lowest) accuracy was obtained in grassland (forest) ecosystems, Baltic mixed forest (Iberian sclerophyllous and semi-deciduous forest) ecoregion, and the alpine polar tundra (warm temperate steppe hot summer) climate zone with KGE values of 0.83 (0.76), 0.9 (0.6) and 0.88 (0.61), respectively. Accuracy was low at the beginning of the year from January to March, then increased to mid-year from April to September, and then decreased from October to December. Accuracy was relatively stable in the middle of the year (0.3<KGE<0.5) but fluctuated significantly at the beginning (-2.8<KGE<0) and end of the year (-3.2<KGE<-1.4). Considering SEVIRI- ETa, the highest (lowest) accuracy was obtained in peatland (forest) ecosystem, Carpathian montane coniferous forests (Iberian sclerophyllous and semi-deciduous forest) ecoregion, and the warm temperate fully humid warm summer (warm temperate steppe hot summer) climate zone with KGE values of 0.82 (0.67), 0.85 (0.48) and 0.88 (0.47), respectively. Similar to SEVIRI- ET0, accuracy was low from January to March, increased from April to September, and then began to decline from October to December. Although accuracy remained relatively stable during the middle of the year (0.3<KGE<0.5), it varied considerably at the beginning (-3.8<KGE<-1) and end of the year (-4<KGE<-1). This study provides the first comprehensive evaluation of the accuracy of SEVIRI-MSG ET products across Europe, which may serve as a stimulus for further optimized selection of these products for various applications.