Comparing global models of terrestrial net primary productivity (NPP): theimportance of water availability

Given that neither absolute measures nor direct model validations of global terrestrial net primary productivity (NPP) are feasible, intercomparison o f global NPP models provides an effective tool to check model consistency. For this study, we tested the assumption that water availability is the pri mary limiting factor of NPP in global terrestrial biospheric models. We com pared a water balance coefficient (WBC), calculated as the difference of me an annual precipitation and potential evapotranspiration to NPP for each gr id cell (0.5 degrees x 0.5 degrees longitude/latitude) in each of 14 models . We also evaluated different approaches used for introducing water budget limitations on NPP: (1) direct physiological control on evapotranspiration through canopy conductance; (2) climatological computation of constraints f rom supply/demand for ecosystem productivity; and (3) water limitation infe rred from satellite data alone. Plots of NPP vs. WBC showed comparable patt erns for the models using the same method for water balance Limitation on N PP. While correlation plots revealed similar patterns for most global model s, other environmental controls on NPP introduced substantial variability.