Energy, water and CO2 flux over a semi-arid grassland associated with North American monsoon

Continuous eddy covariance measurements were made above a semi-arid grassland located on the Audubon Research Ranch in south western Arizona (32°35'25" N, 110°30'26"W, elevation=864 m), USA during 2005-2006 as a contribution to the global energy and water cycle experiment (GEWEX). The mean annual temperature and precipitation at this site were ~15 deg C and 450 mm, respectively. More than 70% of the annual precipitation was received during the North American monsoon period, which typically extends from July to September. In this study, we examined the influence of precipitation cycles associated with North American monsoon on the energy, water and CO2 fluxes over this site. Onset of monsoon in July resulted in an increase in leaf area index, calculated using estimated normalized difference vegetation index, from ~0.2 to 2 m2m-2. Associated with the onset of precipitation period in July of 2006, daily maximum values of air temperature varied from 36 to 28 °C, sensible heat flux from 400 to 150 Wm-2, latent heat flux from 100 to 300 Wm-2 and ground heat flux from 130 to 90 Wm-2. Associated with this, daily evapotranspiration increased from <0.5 to 2 mm d-1 by the end of August resulting in an annual evaporation of about ~310 mm, where July-September evapotranspiration was ~200 mm. Our analysis suggested that soil evaporation at this site contribute to 10-15% of the total evapotranspiration. Energy balance closure based on daytime fluxes were ~0.91 for pre and post monsoon periods whereas it was ~0.87 for monsoon period. The ecosystem was a carbon sink following the onset of monsoon and associated increase in LAI from July to September. This analysis suggested a strong link between energy, water and CO2 fluxes and precipitation cycle associated with the monsoon at this site.