Comment on Two Papers About the Generalized Complementary Evaporation Relationships by Crago et al

Crago et al. published two papers in Water Resources Research in 2016 and 2018, respectively, which discussed recent generalized complementary evaporation relationships; they recommended that the lower boundary condition of the Generalized Nonlinear Advection‐Aridity (GNAA) model proposed by Brutsaert (2015, https://doi.org/10.1002/2015WR017720) should be modified to y = 0 at x = xmin, where xmin∈[0,1]. First, it is shown herein that the rescaled complementary relationship (CR) y = X, proposed by Crago et al., does not advance the analysis forward, but that instead the rescaling is tantamount to a reversion to an earlier linear and more traditional asymmetric complementary model, in which the parameter b is a function of xmin. In fact, at xmin = 0, y = X the rescaled formulation becomes physically unrealistic. The second part of this comment deals with the extended quartic form of GNAA model. The nature of the adjustable parameter c in this extended formulation is further explored, and it is shown that while its lower limit is −1, calibration with actual data can result in values in excess of 2. Such calibration allows then the estimation of xmin as a function of c, that is, xmin = f (c). This way, in addition to satisfying the original four boundary conditions, the extended GNAA model also satisfies the boundary condition of y = 0 at x = xmin. The physical variables affecting the nature of the parameter c and its relationship with Epo/Epmax will require further investigation. Key Points We raise concerns about Crago's study of the rescaled complementary relationship (CR) and the modified lower boundary condition The rescaled CR y = X is a linear asymmetric AA model with the parameter b as a function of xmin, and dy/dx depends on both xmin and x There is a relationship between xmin and parameter c of GNAA at xmin > 0; calibration with actual data can result in values of c in excess of 2