Temporal variability of the controls on evapotranspiration from grazed pastureland during extreme drought

Using eddy covariance flux, soil property, and vegetation measurements collected in the panhandle of Oklahoma as a part of the International H2O Project, this research explains how environmental factors controlling evapotranspiration (ET) from a grazed pastureland varied over a four-week period beginning at the start of the growing season (May 19 through June 16, 2002). Since the study area was in a region that was experiencing extreme drought conditions, it was anticipated that the relative role of such environmental factors as wind speed and water availability would be of key importance. Statistical analysis, however, indicated that the importance these, as well as other, environmental factors changed significantly over time and depended on the time scale chosen for the investigation. For example, on a diurnal basis net radiation was consistently the most important factor driving ET; yet, on longer time scales, such as several days or weeks, water availability in the form of soil moisture was the strongest control on ET and net radiation became a minor consideration. It is suggested that this temporal variability in the controls on ET was due to changes in the surface resistance as environmental conditions changed. For example, a strong relationship between soil moisture content and soil resistance, one component of the surface resistance, was evident. By clarifying relationships such as this, this research has important implications not only for improving our ability to accurately model land surface-atmosphere interactions but also for strengthening our understanding of those relationships in drought-prone environments.