DIURNAL-VARIATION OF SURFACE FLUXES DURING THOROUGH DRYING (OR SEVEREDROUGHT) OF NATURAL PRAIRIE

Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surf ace Climatology Project (ISLSCP) Field Experiment-1987 showed (1) that the total daily values of evaporation exhibited a kind of second stag e 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 allow ed a simple description of the phenomenon by combining a desorptive di ffusion-type parameterization for the total daily evaporation or for i ts dimensionless counterpart (such as Priestley and Taylor's ct, the e vaporative fraction, and a few others), with an assumption of self-pre servation in the surface energy budget during the daytime hours. The r esulting formulation, which involves two timescales, a daily and an ho urly, was able to reproduce daytime hourly flux values over a 2-week p eriod of intensive drying. The method can also be useful in the disagg regation of daily, or even weekly, evaporation into hourly values.