Development of a coupled carbon and water model for estimating global gross primary productivity and evapotranspiration based on eddy flux and remote sensing data
Terrestrial gross primary productivity (GPP) and evapotranspiration (ET) are two key ecosystem fluxesin the global carbon and water cycles. As carbon and water fluxes are inherently linked, knowing oneprovides information for the other. However, tightly coupled and easy to use ecosystem models arerare and there are still large uncertainties in global carbon and water flux estimates. In this study, wedeveloped a new monthly coupled carbon and water (CCW) model. GPP was estimated based on thelight-use efficiency (LUE) theory that considered the effect of diffuse radiation, while ET was modeledbased on GPP and water-use efficiency (WUE). We evaluated the non-linear effect of single (GPPOR) orcombined (GPPAND) limitations of temperature and vapor pressure deficit on GPP. We further comparedthe effects of three types of WUE (i.e., WUE, inherent WUE, and underlying WUE) on ET (i.e., ETWUE,ETIWUEand ETUWUE). CCW was calibrated and validated using global eddy covariance measurement fromFLUXNET and remote sensing data from Moderate Resolution Imaging Spectroradiometer (MODIS) from2000 to 2007. Modeled GPPANDand GPPORexplained 67.3% and 66.8% of variations of tower-derived GPP,respectively, while ETUWUE, ETIWUEand ETWUEexplained 65.7%, 59.9% and 58.1% of tower-measured ET,respectively. Consequently, we chose GPPANDand ETUWUEas the best modeling framework for CCW, andestimated global GPP as 134.2 Pg C yr−1and ET as 57.0×103km3for vegetated areas in 2001. Global ETestimated by CCW compared favorably with MODIS ET (60.5×103km3) and ET derived from global pre-cipitation (56.5×103km3). However, global GPP estimated by CCW was about 19% higher than MODISGPP (109.0 Pg C yr−1). The mean global WUE value estimated by CCW (2.35 g C kg−1H2O) was close to themean tower-based WUE (2.60 g C kg−1H2O), but was much higher than the WUE derived from MODISproducts (1.80 g C kg−1H2O). We concluded that the new simple CCW model provided improved esti-mates of GPP and ET. The biome-specific parameters derived in this study allow CCW to be further linkedwith land use change models to project human impacts on terrestrial ecosystem functions.