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United States Department of Agriculture

Agricultural Research Service

Title: Changes in Vegetation Condition and Surface Fluxes During Name 2004 1794

Authors
item Watts, C. - UNIV. OF SONORA, MEXICO
item Scott, Russell
item Garatuza-Payan, J. - ITSON, SONORA, MEXICO
item Rodriguez, J. - IMADES
item Prueger, John
item Kustas, William
item Douglas, M. - NOAA

Submitted to: Journal of Climate
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 15, 2006
Publication Date: May 15, 2007
Citation: Watts, C.J., Scott, R.L., Garatuza-Payan, J., Rodriguez, J.C., Prueger, J.H., Kustas, W.P., Douglas, M. 2007. Changes in vegetation condition and surface fluxes during name 2004. Journal of Climate. 20: 1810-1820.

Interpretive Summary: The North American Monsoon (NAM) controls warm season climate and is responsible to providing the majority of rainfall over much of southwestern North America. Knowing how ecosystems respond to this input of rainfall is critical to understanding how surface vegetation response may affect monsoon intensity. Unfortunately, little has been documented about this vegetation response across much of the NAM region. In 2004 US and Mexican scientists embarked on a comprehensive study of NAM. As a part of this work, surface energy balance measurements were made over seven different ecosystem types in Sonora, Mexico and Arizona, USA to better understand how land surface vegetation change alters the way that energy is exchanged between the atmosphere and land surface. Large changes in vegetation, monitored by satellites, were observed, especially for the subtropical sites in Sonora. The surface net radiation was consistent with the previous observations, being largest for surfaces that are transpiring and cool, and smallest for surfaces that are dry and hot. The largest evaporation rates were observed for the subtropical and riparian vegetation. These results suggest site-to-site variability in the surface fluxes was large and highly depended upon local rainfall. Nevertheless, the vegetation response was larger for the sites that were closer to the core region of NAM in Mexico and more monitoring is needed to better quantify surface fluxes in this region.

Technical Abstract: The vegetation in the core region of the North American Monsoon (NAME) system changes dramatically after the onset of the summer rains so that large changes may be expected in the surface fluxes of radiation, heat and moisture. Most of this region lies in the rugged terrain of western Mexico and very few measurements of these fluxes have been made in the past. Surface energy balance measurements were made at seven sites in Sonora, Mexico and Arizona, USA during the Enhanced Observation Period (EOP) of the NAM Field Experiment in summer 2004 to better understand how land surface vegetation change alters energy flux partitioning. The 2004 monsoon rainfall pattern was somewhat erratic and there was almost no rainfall in the first three weeks of August. Satellite data was used to obtain time series for vegetation indices and land surface temperature for these sites. The results were analyzed to contrast conditions before the onset of the Monsoon with those afterwards. As expected, large changes in vegetation index were observed, especially for the subtropical sites in Sonora. However, the changes in the broadband albedo were very small, which was rather surprising. The surface net radiation was consistent with the previous observations, being largest for surfaces that are transpiring and cool, and smallest for surfaces that are dry and hot. The largest evaporation rates were observed for the subtropical and riparian vegetation. The evaporative fraction for the forest site was highly correlated with its vegetation index, except during the dry spell in August. This period was clearly detected in the land surface temperature data which rose steadily in this period to a maximum at its end.

Last Modified: 8/21/2014
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