Remote Sensing Time-Series Analysis of Plant Cover Change in Restoration Areas in the Riparian Zone of the Colorado River Delta; Pamela Nagler

 

¹ US Geological Survey, Southwest Biological Science Center, 520 N Park Ave, Tucson, AZ 85719, USA

² Biosystems Engineering of The University of Arizona, 1177 E. 4^th St., Tucson, AZ 85719

 

 

The 1944 Water Treaty allocates the Colorado River flows through seven basin states (97% of the basin is in the U.S. and is comprised of Wyoming, Colorado, Utah, New Mexico, Nevada, California, Arizona) and Mexico. Under the 1944 Water Treaty, the United States is required to provide Mexico with 1.5 million acre-feet (AF) of Colorado River water annually. Under the Minute 319 Agreement, 130 million cubic meters of water was released during the spring of 2014 from Morelos Dam near Yuma, AZ on the lower Colorado River, to the river’s delta in Mexico, allowing water to reach the Gulf of California for the first time in 13 years. In 2018, the Minute 323 Agreement went into effect and allows for science monitoring of restoration areas in the delta. Our study examined the multi-year effects of the 2014 binational pulse-flow on vegetative response along the riparian corridor with particular focus on the restoration areas within the riparian zone. We used the years 2000-2013 before the 2014 water release to look at pre-pulse flow greenness and evapotranspiration (ET), then we used the last five years (2014-2018) to assess the post-pulse changes in these variables. We used 250 m Moderate Resolution Imaging Spectroradiometer (MODIS) and 30 m Landsat 8 satellite imagery to track ET and the scaled normalized difference vegetation index (a measure of greenness). Our analysis showed an overall increase in NDVI and ET in 2014 (year of the pulse), which reversed a decline in those metrics since the last major flood in 2000. NDVI and ET levels decreased in 2015, but were still significantly higher than pre-pulse (2013) levels. This declining trend has persisted into 2018 with lower than pre-pulse values in all reaches MODIS EVI time-series data for these reaches also shows a declining trend after 2015. We illustrate our results with NDVI change maps developed from Landsat data for seven river reaches in the delta. We also analyze results for restoration areas for both short term (2013-2018) and long-term (2000-2018) changes in NDVI and ET. Our results support the conclusion that the Minute 319 pulse had a positive but short-lived impact on vegetation growth in the delta and we will continue monitoring of the restoration zones over the future years that Minute 323 is in effect.