Mean annual GPP of Europe derived from its water balance

On local scale, the eddy covariance technique is suited to estimate gross primary production (GPP). Scaling up such observations to the regional and continental level, however, remains a challenge. Here, we show that there is a surprisingly robust stoichiometric relationship between vegetation CO2 and H2O fluxes, mediated by vapor pressure deficit (VPD), across many different forest vegetation types. This relationship is used to provide a data-driven estimate of Europe's GPP from its water balance. Namely, watershed-wide evapotranspiration (ET), as derived from precipitation (P) and river runoff (R), is multiplied by the ratio of GPP to ET as derived from eddy covariance measurements (water-use efficiency, WUE). In doing so, GPP of Europe is estimated to range between 3.9 and 5.8 PgC/a (median 5 PgC/a). Such GPP estimate is an important independent benchmark for large-scale ecosystem models and may be extended to global scale when relevant data becomes available.