DESORPTION AND THE 2 STAGES OF DRYING OF NATURAL TALLGRASS PRAIRIE

During the First ISLSCP (International Satellite Land Surface Climatology Project) Field experiment (FIFE) the first stage of drying (with the atmosphere controlling) in these natural grasslands occurred when the soil was moist with a volumetric soil moisture content SM >27% after rainfall. Both at the local scale and at the regional scale, once SM in the top 10 cm of the soil dropped below about 17% and its vertical gradient started to exceed about 1.1%/cm at 5 cm, the cumulative daily evaporation-rate could be taken to be proportional to the square root of time, t(1/2), which is not unlike a second stage of drying (with the soil moisture content controlling). Between these two drying stages, there was a transition period which lasted from several days to 2 weeks depending on the soil moisture conditions and on the season. The longer transition periods were observed under conditions of lower net radiation and of higher soil moisture content at depths in excess of, say, 50 cm. The present findings of gradual transitions from the first stage to a desorptive second stage of drying for a surface covered with grass (which can extract water from greater depths) are in contrast to earlier findings of relatively abrupt transitions for bare soil.