Vegetation indices trends in restoration and non-restored riparian vegetation sites: Exploring the potential to detect changes in foliar cover through Landsat and UAV vegetation indices in the Colorado River Delta, Mexico.

Minute 319 (2014-2017) and Minute 323 (2018-2026) binational agreements under the 1944 Water Treaty between the U.S. and Mexico provided a framework to allocate Colorado River water to restore riparian vegetation in the Colorado River delta. During the release of water over Morelos Dam in 2014, 195 million cubic meters (mcm) were delivered as a pulse flow and base flow. Minute 323 allocated 259 mcm for the environment. In plant communities dominated by Tamarix spp. (saltcedar) active restoration efforts are crucial for the success of native vegetation establishment. Specifically, the removal of saltcedar and arrowweed (Pluchea sericea), to create bare ground for new planting, was among the approaches used in conjunction with different irrigation and planting methods in restoration sites. The goal of this study was to investigate the trends in Vegetation Indices (VI´s) in restoration sites where native riparian species were planted as well as in the control sites, the plant communities dominated by saltcedar without any habitat manipulation. We estimated both the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) from both a drone and Landsat from 2013 to 2018 to analyze the plant community response and determine the feasibility of using these platforms to detect changes in vegetation cover. Results indicate that Landsat NDVI and EVI decreased in control sites (8% to 50%), in contrast to restoration plots, which depicted NDVI and EVI increases (10% to 270%) in the last five years. NDVI trends in both restoration and control sites are related to land clearing, fire effects, water table changes, age of the forest as well as drought stress. NDVI derived from a UAV multispectral sensor was higher compared to Landsat NDVI and showed R² that ranged from 0.21 to 0.90, indicating the use of UAV's NDVI need to be evaluated by site.