Vegetation Change in the Rio Grande River Basin

LA-UR-07-3471
February 2006
Amanda White and Everett Springer

Changes in vegetation due to drought, wildfires, and urban encroachment are important issues that have the potential to influence future water resources in the Rio Grande River Basin.  This watershed is a central water source for the southwestern U.S., where the current water demand already exceeds the supply.  The goal of this project is to investigate the impact of vegetation change on the water availability within the Rio Grande River Basin using remote sensing techniques and a detailed, fully-distributed hydrologic model.

Each aforementioned aspect of vegetation change will be explored, beginning with drought impacts on vegetation and the recent piñon pine mortality that occurred due to a long-term drought from 2000 to 2003.  Drought, combined with high temperatures and bark beetle infestations, caused regional-scale tree mortality throughout the southwestern U.S.  Remote sensing, aerial surveys, and ground-based data are used to examine the spatio-temporal nature of the mortality.  The vegetation index NDVI (normalized difference vegetation index), as measured by the AVHRR sensor, is the longest remote sensing time series of vegetation photosynthetic activity available, and is used in this analysis.

In order to examine change, a baseline average NDVI is established from 1989 to 1999 (prior to the drought), and anomalies are computed for each year during the drought, during the mortality, and after the drought/mortality (2000 to 2006).  The figure below (Figure 1) shows NDVI anomalies for the Rio Grande River Basin for the second half of May 2003 in the Carson and Santa Fe National Forests.  The red scale indicates less photosynthetic activity than the baseline and the green scale indicates more.  The gray areas are aerially surveyed locations of piñon mortality.  May is shown because a stronger relationship exists between the mortality and the NDVI anomalies during this robust photosynthetically active period, the beginning of the growing season.  However, as can be seen in the figure, the areas of mortality are not always correlated with the NDVI anomalies.  This issue is under current investigation.  Once the relationship between drought-induced tree mortality and remote sensing data is solidified, the impact of the drought and mortality on evapotranspiration and stream flow will be examined using remote sensing and the complex hydrologic model.

Figure 1.  NDVI anomalies for the second half of May 2003 for a portion of the Rio Grande River Basin.  Gray areas indicate locations of piñon mortality.

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