The Influence of Atmospheric Stability on Potential Evaporation

The Penman relationship for potential evaporation is modified to simply include the influence of atmospheric stability on turbulent transport of water vapor. Explicit expressions for the stability-dependent, surface exchange coefficient developed by Louis are used. The diurnal variation of potential evaporation is computed for the stability-dependent and original Penman relationships using Wangara data. The influence of afternoon instability increases the aerodynamic term of the modified Penman relationship by 50% or more on days with moderate instability. However, the unmodified Penman relationship predicts values of daily potential evaporation close to that of the stability-dependent relationship. This agreement is partly due to compensating overestimation during nighttime hours. Errors due to use of daily-averaged variables are examined in detail by evaluating the nonlinear interactions between the diurnal variation of the variables in the Penman relationship. A simpler method for estimating the exchange coefficient is constructed from an empirical relationship between the radiation Richardson number and the Obukhov length. This method is less accurate, but it allows estimation of the stability-dependent exchange coefficient using only parameters already required for evaluation of the Penman relationship. Finally, the diurnal variation of the atmospheric resistance coefficient appearing in the Penman-Monteith relationship is presented.