Agricultural and Forest Meteorology 106 2001 205–214
A comparison of summertime water and CO
2
fluxes over rangeland for well watered and drought conditions
Tilden P. Meyers
∗
NOAAARLAtmospheric Turbulence and Diffusion Division, 456 South Illinois Avenue, PO Box 2456, Oak Ridge, TN 37831-2456, USA Received 27 January 2000; received in revised form 21 August 2000; accepted 23 August 2000
Abstract
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 CO
2
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 CO
2
fluxes. During the summer period day 150–240, total evapotranspiration for non-drought years ranged from 224 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 Cm
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 Cm
2
from soil respiration. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Sensible heat flux; Latent heat flux; Ground heat flux; Drought conditions; Carbon flux
1. Introduction
The sensitivity of regional and global scale i.e., general circulation, or GCM numerical models to the
surface layer parameterizations of soil moisture and the surface energy balance components is well docu-
mented Troen and Mahrt, 1986; Meehl and Washing- ton, 1988; Sato et al., 1989; Atlas et al., 1993; Garratt,
1993. Characterization of the surface energy balance is necessary for determining the overall surface tem-
perature, the surface flux of water vapor into the atmo- sphere, and the overall atmospheric heating rate. On
short time scales, these processes determine the atmo- spheric stability, the height of the planetary boundary
∗
Tel.: +1-423-576-1245; fax: +1-423-576-1327. E-mail address: meyersatdd.noaa.gov T.P. Meyers.
layer PBL Pan and Mahrt, 1987, and play a role in governing cloud formation and convective precipita-
tion processes. On longer time scales, the local water balance could play a significant role in determining
the capacity of the surface to fix carbon. For example, an accumulated deficit in the local water budget can
potentially alter the sign and magnitude of the sea- sonal carbon budget, which can then have feedbacks
into the evaporative processes. Ultimately, the short- and long-term carbon balance has the potential to be
intimately linked to the local water balance.
To evaluate the impact of water deficits on the local surface energy balance and carbon fluxes on seasonal
and annual time scales, long-term continuous mea- surements of the surface energy balance components
and CO
2
fluxes were initiated in June of 1995 in the Little Washita Watershed in south central Oklahoma
0168-192301 – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 1 3 - 6
206 T.P. Meyers Agricultural and Forest Meteorology 106 2001 205–214
as a National Oceanic and Atmospheric Administra- tion NOAA contribution to the GEWEX Continental
Intercomparison Project GCIP; see Lawford, 1999. Data collected during the last 4 years have provided a
unique opportunity to investigate the inter-annual vari- ability in summertime water and carbon balances for
a rangeland location. In particular, a drought during the summer of 1998 believed to be associated with
the El Niño event provided an opportunity to quan- tify extreme values for parameters related to the local
water and carbon budgets.
2. Methodology
