A comparison of summertime water and CO2 fluxes over rangeland for well watered and drought conditions
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 documented (Troen and Mahrt, 1986; Meehl and Washington, 1988; Sato et al., 1989; Atlas et al., 1993; Garratt, 1993). Characterization of the surface energy balance is necessary for determining the overall surface temperature, the surface flux of water vapor into the atmosphere, and the overall atmospheric heating rate. On short time scales, these processes determine the atmospheric stability, the height of the planetary boundary layer (PBL) (Pan and Mahrt, 1987), and play a role in governing cloud formation and convective precipitation 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 seasonal 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 measurements of the surface energy balance components and CO2 fluxes were initiated in June of 1995 in the Little Washita Watershed in south central Oklahoma as a National Oceanic and Atmospheric Administration (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 variability 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 quantify extreme values for parameters related to the local water and carbon budgets.
Section snippets
Methodology
The turbulent fluxes of water vapor, sensible heat, and CO2 were measured using the eddy covariance technique. Historically, the use of the eddy covariance method (Businger, 1986; Baldocchi et al., 1988) has been constrained to mainly short-term intensive field campaigns. However, improvements in instrument design, ruggedness, and stability over the past decade now allow for nearly continuous measurements of sensible and latent energy fluxes using the eddy covariance technique (Goulden et al.,
Little Washita Watershed, Oklahoma
The instrumentation is located within the Little Washita Watershed, a tributary of the Washita River in southwest Oklahoma. Located in the southern part of the Great Plains of the United States (Fig. 1), this area has a climate classification as moist and subhumid. Annual rainfall is about 75 cm. Summers are typically long, hot and dry with an average daily high temperature in July of 34°C. The tower (34°58′N, 97°77′W) was placed about one quarter mile north of State Road 19 within a grazed
Results and discussion
The results presented and discussed here are from data collected during the summer periods beginning 1 June and ending 31 August for the 4 years 1995–1998 (hereafter denoted as Y95, Y96, Y97, and Y98). Data recovery rates for the summer periods Y96, Y97, and Y98 were 85, 99, and 94%, respectively. However, during the first year of operation (Y95), several periods during the summer had gaps in the eddy covariance data resulting is a data recovery rate of 65%. Gap periods were typically 2–3 days.
Conclusions
Summertime water and carbon fluxes over grassland were determined from an eddy covariance system were measured over a 4-year time period from 1995 to 1998 with 1998 being a drought year with only 67 mm of precipitation for the 90 day summer period. For the non-drought years the average water and net carbon fluxes (net fixation) were and , respectively. The evaporative fraction (LE/Rn−G) was nearly constant (0.46) and showed little dependence on vapor pressure deficit during
Acknowledgements
I would like to acknowledge the support of Dr. Rick Lawford and Dr. John Leese of the NOAA Office of Global Programs for their support of this work.
References (26)
- et al.
On using eco-physiological, micrometeorological and biogeochemical theory to evaluate carbon dioxide, water vapor, and trace gas fluxes over vegetation: a perspective
Agric. Forest. Meteorol.
(1998) - et al.
Carbon dioxide fluxes over bermudagrass, native prairie, and sorghum
Agric. Forest. Meteorol.
(1999) - et al.
Variability in surface energy flux partitioning during Washita’92: resulting effects of Penman–Monteith and Priestly–Taylor parameters
Agric. Forest. Meteorol.
(1996) - et al.
The effect of SST and soil moisture anomalies on GLA model simulations of the 1988 US summer drought
J. Clim
(1993) - et al.
An open path, fast response infrared absorption gas analyzer for H2O and CO2
Boundary-Layer Meteorol.
(1992) - et al.
Measuring biosphere–atmosphere exchanges of biologically related gases with micrometeorological methods
Ecology
(1988) - et al.
Application of self-preservation in the diurnal evolution of the surface energy budget to determine daily evaporation
J. Geophys. Res.
(1992) Evaluation of the accuracy with which dry deposition can be measured with current micrometeorological techniques
J. Clim. Appl. Meteorol.
(1986)- et al.
Spatial and temporal variability of heat, water vapor, carbon dioxide and momentum air–sea exchange in a coastal environment
J. Geophys. Res.
(1993) - et al.
The boardman regional flux experiment
Bull. Am. Meteorol. Soc.
(1992)
Sensitivity of climate simulations to land–surface and atmospheric boundary layer treatments — a review
J. Clim.
Measurements of carbon sequestration by long-term eddy covariance: methods and a critical evaluation of accuracy
Global Change Biol.
Flow distortion by a Solent Sonic anemometer: wind tunnel calibration and its assessment for flux measurements over forest and field
J. Atmos. Oceanic Technol.
Cited by (185)
Energy flux observation in a shrub ecosystem of a gully region of the Chinese Loess Plateau
2022, Ecohydrology and HydrobiologyEnergy balance and partitioning over grasslands on the Mongolian Plateau
2022, Ecological IndicatorsEffects of agricultural management regimes on rotating cropland ecosystem respiration and its components in Southeast China
2021, Agricultural and Forest MeteorologyCarbon exchange of a dryland cotton field and its relationship with PlanetScope remote sensing data
2020, Agricultural and Forest MeteorologyImpact of rainfall extremes on energy exchange and surface temperature anomalies across biomes in the Horn of Africa
2020, Agricultural and Forest MeteorologyEast Asian summer monsoon substantially affects the inter-annual variation of carbon dioxide exchange in semi-arid grassland ecosystem in Loess Plateau
2019, Agriculture, Ecosystems and Environment