|Krishnan, P. -|
|Meyers, T.P. -|
|Kennedy, L. -|
|Heuer, M. -|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 21, 2011
Publication Date: January 24, 2012
Citation: Krishnan, P., Meyers, T., Scott, R.L., Kennedy, L., Heuer, M. 2012. Energy exchange and evapotranspiration over two temperate semi-arid grasslands in North America. Agricultural and Forest Meteorology. 153: 31-44. Interpretive Summary: The exchange of energy and water between the land surface and the atmosphere is a key driver of the Earth’s climate system. Understanding the relative roles of climate versus vegetation or land cover on energy exchange processes is critical for predicting how ecosystems will respond to future physical and biological changes, especially to predictions of vegetative and climatic changes. The seasonal and year-to-year change in surface energy and water exchange of two semiarid grasslands in southern Arizona, USA were investigated using measurements collected from 2004 through 2007. One of the grasslands was a post-fire site and the other was an unburned site. Although there was large variation between yearly rain totals and differences in site characteristics like vegetation amount, both sites responded similarly to changes in environmental conditions. The exchanges of energy and evaporation were mainly governed by the amount of rainfall and vegetation growth.
Technical Abstract: The seasonal and interannual variability in surface energy exchange and evapotranspiration (E) of two temperate semi-arid grasslands in southern Arizona, USA were investigated using continuous eddy covariance measurements from 2004 to 2007, including two drought years (2004 and 2005). One of the grasslands was a post-fire site (AG) and the other was an unburned site (KG). 2004 was the driest year at AG with annual precipitation (P) 27% lower than the four year (2004-2007) mean (379 ± 81 mm) whereas 2005 was the driest year at KG with annual precipitation 34% lower than the three year (2005-2007) mean (250 ± 78 mm). Irrespective of the difference in annual P, temperature and albedo, both sites responded similarly to changes in environmental conditions. Seasonal surface energy partitioning at both sites were determined by soil water availability and vegetation growth. Seasonally, all components of net radiation, except downwelling longwave radiation peaked in May or June at both sites but net radiation was highest in August. Drastic changes in albedo, vegetation growth, upwelling radiation components, energy fluxes, surface conductance (gs), decoupling coefficient (E) and Priestly Taylor coefficient (a) occurred following the onset of monsoon in July. During dry or cold periods of autumn, winter and spring, sensible heat flux was the major component of energy balance whereas latent heat flux dominated during warm and wet periods of summer. Irrespective of the soil water conditions a scaled consistently with gs. E was substantially reduced when the soil water content in the 0-15cm layer of the soil profile dropped below 0.08 m3m-3 at AG and 0.09 m3m-3 at KG, respectively. Annual E varied from 264 mm to 338 mm at AG and from 196 to 284 mm at KG with the lowest value during the severe drought years. July-September E was positively correlated with total P and the broadband normalized vegetation index (NDVI) during that period. The study suggested strong coupling between soil water conditions and vegetation on energy exchange and E.