Trends in Evapotranspiration and Drought in a Dozen Riparian Restoration Sites in the Colorado River Delta in Mexico
Pamela Nagler1*, Ibrahima Sall2,3, Armando Barreto-Muñoz3, Hamideh Nouri4, Sattar Chavoshi Borujeni5,6, Martha Gómez-Sapiens7, and Kamel Didan8
1U.S. Geological Survey, Southwest Biological Science Center, 520 North Park Avenue, Tucson, AZ 85719 USA; pnagler@usgs.gov
2Department of Agricultural and Resource Economics, University of Arizona Tucson, AZ 85721, USA
3Biosystems Engineering, University of Arizona, Tucson, AZ, 85721 USA
4Division of Agronomy, University of Göttingen, Von-Siebold-Strasse 8, 37075, Göttingen, Germany
5School of Life Sciences, Faculty of Science, University of Technology Sydney, Sydney, NSW, 2007, Australia
6Soil Conservation and Watershed Management Research Department, Isfahan Agricultural and Natural Resources Research and Education Centre, AREEO, Isfahan, Iran
7Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
The Colorado River delta riparian vegetation has been declining in greenness and water use since 2000 as measured by Landsat. Restoration activities in two reaches in the delta (Reach 2 and Reach 4) have been expanding and now total 13 sites (“All Restoration Sites”), with initial planting dates between 2010 and 2018. Due to more available water in recent years, restoration has been very successful. Three sources of water have been provided to the region, first in 2014 as the Minute 319 Pulse Flow and as directed flows to the restoration sites under Minute 323, which allocated water for restoration starting in 2018, plus any excess flows such as from the MODE canal in 2019. We assess if the restoration, which comprised only 7.5% of the area of the reaches, had impact on reach-level health (“All Reaches 1-4,” for the area not restored) by measuring greenness and water use and comparing the values between the restored and adjacent unrestored areas during three periods in our study: the 21-years from 2000-2020, the recent decade (2011-2020) and since the Minute 319 Pulse Flow (2014-2020). We use the two-band Enhanced Vegetation Index (EVI2) and evapotranspiration (ET, mmd-1) using EVI2 and ground-based meteorological potential evapotranspiration (ETo) acquired from Yuma. Over 21-years greenness in the unrestored riparian corridor of the delta’s reach 2 and 4 decreased by 23.6% and ET(EVI2) decreased by 32% (0.87 mmd-1), whereas greenness increased by 33.6% in the 13 restored sites and ET(EVI2) increased by 58% (1.29 mmd-1) since 2014. The restored sites showed increases over the last decade (2011-2020) in riparian vegetation greenness (36%) and ET(EVI2) (20%). The 13 restored sites in the delta are much healthier based on greenness and water use than the adjacent unrestored vegetation along the river reach; however, they do not have significant impact on the larger reach-level riparian health. The restoration sites are little patches of success in a declining landscape, but site-scale restoration has not yet stopped or reversed a two-decade decline in vegetation health. For the combined restored sites and unrestored reaches, EVI2 and ET as a function of the 3-month standardized precipitation evapotranspiration index (SPEI03) show declines. These findings can be utilized by decision makers in their quest to mitigate declines in riparian woodlands.