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

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Abstract

Continuous measurements of the surface energy balance components (net radiation Rn, 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 224 to 273 mm with a mean and standard error of 253±12mm. The mean and standard error of the net ecosystem exchange (NEE) rate of carbon dioxide for the same summer period was −120±36gC/m2. 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/m2 from soil respiration.

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 253±12mm and −118±36gC/m2, respectively. The evaporative fraction (LE/RnG) 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.

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