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Research Project: Ecohydrological Processes, Scale, Climate Variability, and Watershed Management

Location: Southwest Watershed Research Center

Title: Long-term decrease in satellite vegetation indices in response to environmental variables in an iconic desert riparian ecosystem: the Upper San Pedro, Arizona, USA

Author
item NGUYEN, U. - University Of Arizona
item GLENN, E. - University Of Arizona
item NAGLER, P.L. - Us Geological Survey (USGS)
item Scott, Russell - Russ

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2014
Publication Date: 8/1/2014
Citation: Nguyen, U., Glenn, E., Nagler, P., Scott, R.L. 2014. Long-term decrease in satellite vegetation indices in response to environmental variables in an iconic desert riparian ecosystem: the Upper San Pedro, Arizona, USA. Ecohydrology. 8:610-625. https://doi.org/10.1002/eco.1529.
DOI: https://doi.org/10.1002/eco.1529

Interpretive Summary: The Upper San Pedro River is one of the few remaining undammed rivers that maintains a vibrant riparian ecosystem in the southwest U.S., but its riparian forest is threatened by diminishing groundwater and surface water inputs. To assess the temporal and spatial trends in San Pedro riparian health, we used satellite measures of vegetation abundance and vigor from 1984 to 2012. The river was divided into a northern reach, which has mainly intermittent and ephemeral flow reaches, and a mainly perennial flow, southern reach. The satellite measures of riparian greenness showed a 20% drop for the northern reach and no net change for the southern reach. Vegetation abundance and vigor were positively related to river flows, which decreased over the study period in the northern reach, and negatively related to increasing trends in reach air temperatures, which have increased by about 1.4 degrees Celsius from 1904 to 2012. Hypothesized increases in riparian and upland evaporation could not explain the reduction in river flows because the satellite measures of vegetation health decreased in both the riparian corridor and in the uplands over the 1984-2012 period. Regional groundwater pumping is a feasible alternative explanation for decreased flows and deterioration of the riparian forest in the northern reach.

Technical Abstract: 1. The Upper San Pedro River is one of the few remaining undammed rivers that maintains a vibrant riparian ecosystem in the southwest U.S. Its riparian forest is threatened by diminishing groundwater and surface water inputs, attributed by different studies to either (1) changes in watershed characteristics such as changes in riparian and upland vegetation, or (2) human activities such as regional groundwater pumping. 2. To assess the temporal and spatial trends in San Pedro riparian vegetation vigor, we used Landsat and Moderate Resolution Imaging Spectrometer (MODIS) satellite data to quantify the green leaf density for the groundwater-dependent riparian forest with vegetation indices from 1984 to 2012. One cloud-free Landsat 5 image was acquired for June of each year, when riparian trees had leafed out but before the arrival of summer monsoon rains in July. The river was divided into a northern reach, which has mainly intermittent and ephemeral flow reaches, and a mainly perennial flow southern (upstream) reach. 3. Normalized Difference Vegetation Index (NDVI) values showed a 20% drop for the northern reach (P < 0.001) and no net change for the southern reach. Landsat and MODIS imagery showed that NDVI and Enhanced Vegetation Index (EVI) values were positively correlated (P < 0.05) with river flows, which decreased over the study period in the northern reach, and negatively correlated (P < 0.05) with air temperatures in both reaches, which have increased by about 1.4oC from 1904 to 2012. 4. Hypothesized increased riparian and upland ET could not explain the reduction in river flows because foliage density as assayed by NDVI and EVI decreased in both the riparian corridor and in the uplands over the 1984-2012 period (P < 0.001). 5. Regional groundwater pumping is a feasible alternative explanation for decreased flows and deterioration of the riparian forest in the northern reach.