Diurnal variation of surface fluxes during thorough drying (or severe drought) of natural prairie

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(-1/2) 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 ct, 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.