Evaluation of a LUE Model and Various Water Scalars Based on Eddy Covariance Data from 13 Forest Sites Across Europe

Light use efficiency (LUE) models are widely used to estimate gross primary productivity (GPP) because they provide strong accuracy while maintaining low complexity. The aim of this study is (a) to evaluate the performance of a LUE model (sCASE) and (b) to compare the performance of several alternative water scalars. The analyses are done using GPP measurements from thirteen eddy covariance sites across Europe, corresponding to different forest types. Daily GPP estimates produced by sCASE were highly accurate for most sites (average R2 = 0.750 and average RMSE = 2.317 g C m−2 d−1), matching the performance of other widely used LUE models in the literature. All three scalars were essential for maintaining model accuracy, although their relative importance varied among sites. The developmental scalar, which is not incorporated in most productivity models, was particularly important for accurately estimating GPP in deciduous species. Among the ten water scalars tested, those based on simple water balance calculations performed best in water-limited sites, whereas the VPD-based scalar performed better in sites without water limitation. The EF (evaporative fraction) scalar showed high accuracy at some sites across both water status categories but very low accuracy at others. For large-scale applications, water scalars based on MODIS indices offer the advantage of global coverage, which can outweigh their lower accuracy relative to other scalars.