A COMPARISON OF SEVERAL EVAPORATION EQUATIONS

Several well-known evaporation equations were tested with daily data averaged over six stations in a 15 x 15 km region in Kansas. The equilibrium evaporation and advection-aridity evaporation equations produced acceptable results, under the generally moist conditions encountered, with the former performing slightly better. Three methods tested also required soil moisture (SM) availability data. These were the Penman and equilibrium evaporation equations adjusted with SM-dependent coefficients, and the Penman-Monteith formulation with surface resistance. All three gave significantly better performances than the unadjusted equilibrium equation. In general, it appeared that the use of simple temporal (daily) and spatial averages may not be appropriate, especially under conditions of variable SM. The Penman, equilibrium, and advection-aridity evaporation equations are linear combinations of the same two terms, with different coefficients. The optimal values of these coefficients, determined for various ranges of SM, were consistent with the underlying physical models.