A calibrated advection‐aridity evaporation model requiring no humidity data

A modified advection‐aridity model was tested with 24 h averaged data from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment‐87 and the Cooperative Atmospheric‐Surface Exchange Study (CASES) ‐97 experiments. Two modifications were made. First, the need for humidity measurements in the “drying power” term of the equation was circumvented by assuming the daily average specific humidity for a 24 h day is equal to the specific humidity at the minimum temperature during the day. When compared to measured latent heat fluxes, this modification resulted in no deterioration in advection‐aridity estimates compared to versions using measured humidity. A second modification followed previous researchers by formulating the drying power in the Penman equation using Monin‐Obukhov similarity (MOS) theory. However, this version calibrated kB−1 (≡ln(zo/zov) by setting the Priestley‐Taylor evapotranspiration rate equal (on average) to the Penman evapotranspiration on several moist days and solving for the value of kB−1 as the only unknown. The calibration involved no latent heat flux measurements. The results suggest that the advection‐aridity model performs modestly better in the calibrated MOS version than with Penman's original wind function. Further investigation is recommended because MOS theory accounts for varying momentum roughness lengths, and the calibration was not done under ideal (well‐watered or nearly saturated) conditions that deteriorated the results somewhat with the CASES data set.