An important scaling consideration is introduced into the formulation of the complementary relationship (CR) of land surface evapotranspiration (ET) by specifying the maximum possible evaporation rate (Epmax) of a small water body (or wet patch) as a result of adiabatic drying from the prevailing near‐neutral atmospheric conditions. In dimensionless form the CR therefore becomes yB = f( Epmax−EpEpmax−EwxB) = f(X) = 2X2 − X3, where yB = ET/Ep, xB = Ew/Ep. Ew is the wet‐environment evaporation rate as given by the Priestley‐Taylor equation, Ep is the evaporation rate of the same small wet surface for which Epmax is specified and estimated by the Penman equation. With the help of North American Regional Reanalysis data, the CR this way yields better continental‐scale performance than earlier, calibrated versions of it and is on par with current land surface model results, the latter requiring vegetation, soil information and soil moisture bookkeeping. Validation has been performed by Parameter‐Elevation Regressions on Independent Slopes Model precipitation and United States Geological Survey runoff data. A novel approach is also introduced to calculate the value of the Priestley‐Taylor parameter to be used with continental‐scale data, making the new formulation of the CR completely calibration free.

No abstract

A novel approach has been found to estimate the equilibrium surface temperature (T e ) of wet environment evaporation (E w ) on a daily basis. Employing this temperature in the Priestley-Taylor equation as well as in the calculation of the slope of the saturation vapor pressure curve with pan measurements improved the accuracy of longterm mean evaporation (E) estimation of the Advection-Aridity (AA) model when validated by Morton's approach. Complementarity of the potential evaporation (E p ) and E terms was considered both on a daily and a monthly basis with the involved terms always calculated daily from 30 yr of hourly meteorological measurements of the 1961-1990 period at 210 SAMSON stations across the contiguous US. The followings were found: (a) only the original Rome wind function of Penman yields a truly symmetric Complementary Relationship between E and E p which makes the so-obtained E p estimates true potential evaporation values; (b) the symmetric version of the modified AA model requires no additional parameters to be optimized; (c) for a long-term mean value of evaporation the modified AA model becomes on a par with Morton's approach not only in practical applicability but also in its improved accuracy, especially in arid environments with possible strong convection; (d) the latter two models yielded long-term mean annual evaporation estimates with an R 2 of 0.95 for the 210 stations, which is all the more remarkable since they employ very different approaches for their E p calculations; (e) with identical apparent E p values the two models yielded practically identical long-term mean annual evaporation rates; (f) with the proper choice of the wind function to estimate apparent E p the long-term mean annual E estimates of the modified AA model are still very close (R 2 = 0.93) to those of the Morton approach. ª