Assessing estimates of evaporative demand in climate models using observed pan evaporation over China

Here we assess estimates of atmospheric evaporative demand over China in 12 state-of-the-art global climate models (GCMs) against observed D20 pan evaporation (E-pan) over the period of 1961-2000. To do that, we use an energy-relevant and physical-based approach, namely, PenPan model, to comprehensively evaluate GCM performance with respect to their ability to simulate annual, seasonal, and monthly statistics of E-pan (and its radiative and aerodynamic components, E-p,E-R and E-p,E-A). The results indicated that most GCMs generally captured the spatial pattern and seasonal cycle of E-pan, E-p,E-R, and E-p,E-A. However, regional means of annual and monthly E-pan, E-p,E-R, and E-p,E-A were underestimated by most GCMs mainly due to negatively biased surface air temperature (T-a) and vapor pressure deficit (vpd) outputted/simulated by the GCMs. Overall, the discrepancies among GCMs in estimating the regional statistics (regional means and seasonal cycles) of E-p,E-A were relatively larger than that of E-p,E-R, which indicates considerable uncertainties in the calculation of the aerodynamic component of evaporation based on the GCM outputs. Moreover, a few GCMs captured negative trends of regional mean annual and seasonal E-pan, E-p,E-R, and E-p,E-A well over the period of 1961- 2000, but most showed positive trends. The underestimation of net radiation (R-n) and overestimation of wind speed at 2m (u(2)) in most GCMs may, to some extent, accentuate/compensate the negative biases in GCM-estimated annual and seasonal E-pan, E-p,E-R, and E-p,E-A. The results demonstrate the importance of incorporating observation of pan evaporation and well-validated PenPan model to evaluate GCM performance on atmospheric evaporative demand that is relevant to projections of future drought and regional water-energy budgets.