A comparison of summertime water and CO2 fluxes over rangeland for well watered and drought conditions

Continuous measurements of the surface energy balance components (net radiation R-n, sensible heat flux H, latent heat flux LE, ground heat flux G, and CO2 fluxes began in early June of 1995 at the Little Washita Watershed, near Chickasha, Oklahoma. A severe drought during 1998 provided a unique opportunity to evaluate the range of fluxes that can be expected during the summer period. Data obtained during four continuous summer periods were used to evaluate the year-to-year variability in summertime energy and CO2 fluxes. During the summer period (day 150-240), total evapotranspiration for non-drought years ranged from 234 to 273 mm with a mean and standard error of 253 +/- 12 mm. The mean and standard error of the net ecosystem exchange (NEE) rate of carbon dioxide for the same summer period was - 120 +/- 36 g C/m(2). In a year with severe drought (1998) total evapotranspiration for the summertime period was 145 mm. The lack of precipitation during this time resulted in total losses to the atmosphere of 155 g C/m(2) from soil respiration. (C) 2001 Elsevier Science R.V. All rights reserved.