Diurnal Variation of Surface Fluxes During Thorough Drying (or Severe Drought) of Natural Prairie

Abstract

Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment‐1987 showed (1) that the total daily values of evaporation exhibited a kind of second stage of drying behavior with a t−½ dependency at the daily timescale and (2) that this day‐to‐day evolution was modulated by the available energy at the surface, that is, the hourly radiation input. This allowed a simple description of the phenomenon by combining a desorptive diffusion‐type parameterization for the total daily evaporation or for its dimensionless counterpart (such as Priestley and Taylor's α, the evaporative fraction, and a few others), with an assumption of self‐preservation in the surface energy budget during the daytime hours. The resulting formulation, which involves two timescales, a daily and an hourly, was able to reproduce daytime hourly flux values over a 2‐week period of intensive drying. The method can also be useful in the disaggregation of daily, or even weekly, evaporation into hourly values.