DECLINING VEGETATION INDICES in The UPPER SAN PEDRO:
Climate change or Groundwater pumping?
Uyen Nguyen*, Edward P. Glenn*, Pamela L. Nagler** and Russell L. Scott***
*The *The
*Environmental Research Laboratory of the University of Arizona, Tucson, AZ
**U.S. Geological Survey, Southwest Biological Science Center, Tucson, AZ
***Southwest Watershed Research Center, USDA, Agricultural Research Station, Tucson, AZ
Service,
Tucson,
AZ
Tucson,
AZ
Abstract
Health of vegetation has become one of the highest concerns of climate change.
Monitoring vegetation at multi temporal and spatial scales provides the most informative
approach for detecting changes and inferring the causal agents of change. The study
focused on the response of vegetation in the riparian ecosystem that have been impacted
by water resources and climate change and address the correlation between riparian
vegetation and hydro-climatic variables during the past three decades in one of the last
remaining undammed rivers in the southwestern U.S, the Upper San Pedro River in
Arizona. The riparian forest of the Upper San Pedro is threatened by diminishing base
flows attributed to either: (1) increases in evapotranspiration (ET) due to conversion of
grasslands to mesquite scrublands in the adjacent uplands, or (2) to increased regional
groundwater pumping that serves growing populations in surrounding urban areas.
Changes in the hydrology of the Upper San Pedro allows for drought-tolerant invasive
species to invade the floodplain, resulting in a narrowing of the riparian zone. To
determine vegetation change along the San Pedro River, long-term satellite data of
vegetation indices were acquired yearly from 1984 to 2012 for the month of June when
riparian trees had leafed out but before the arrival of summer monsoon rains in July.
Results showed Normalized Difference Vegetation Index (NDVI) values from both
Landsat and Moderate Resolution Imaging Spectrometer (MODIS) significantly
decreased from 1984-2012. This Vegetation Index values are positively correlated to
river flows, which decreased over the study period, and negatively correlated with air
temperatures, which have increased by about 1.4oC from 1904 to the present. There was a
significant relationship between the temporal correlations of riparian patterns with runoff
generation under climate change and spatial contrast of perennial and intermittent
reaches. Therefore, increased regional groundwater pumping appears to be reasonable
alternative explanation for decreased flows and deterioration of the San Pedro riparian
forest.