Hydrological and bioclimatic processes that lead to drought may stress plants and wildlife, restructure plant community type and architecture, increase monotypic stands and bare soils, facilitate the invasion of non-native plant species and accelerate soil erosion. Our study focuses on the impact of a paucity of Colorado River surface flows from the United States (U.S.) to Mexico. We measured change in riparian plant greenness and water use over the past two decades using remotely sensed measurements of vegetation index (VI), evapotranspiration (ET) and a new annualized phenology assessment metric (PAM) for ET. We measure these long-term (2000-2019) metrics and their short-term (2014-2019) response to an environmental pulse flow in 2014, as prescribed under Minute 319 of the 1944 Water Treaty between the two nations. In subsequent years, small-directed flows were provided to restoration areas under Minute 323. We use 250 m MODIS and 30 m Landsat imagery to evaluate three vegetation indices (NDVI, EVI, EVI2). We select EVI2 to parameterize an optical-based ET algorithm and test the relationship between ET from Landsat and MODIS by regression approaches. Our analyses show significant decreases in VIs and ET for both the 20-year and post-pulse 5-year periods. Over the last 20 years, EVI Landsat declined 34% (30% by EVI MODIS) and ET Landsat-EVI declined 38% (27% by ET MODIS-EVI), overall ca. 1.61 mm/ day or 476 mm/year drop in ET; using PAM ET Landsat-EVI the drop was from 1130 to 654 mm/year. Over the 5 years since the 2014 pulse flow, EVI Landsat declined 20% (13% by EVI MODIS) and ET Landsat-EVI declined 23% (4% by ET MODIS-EVI) with a 0.77 mm/day or a 209 mm/year 5-year drop in ET; using PAM ET Landsat-EVI the drop was from 863 to 654 mm/year. Data and change maps show the pulse flow contributed enough water to slow the rate of loss, but only for the very short-term (1-2 years). These findings are critically important as they suggest further deterioration of biodiversity, wildlife habitat and key [Correction added on 16 November 2020, after first online publication: Figures 1, 2, 5, 10 and 11 were previously incorrect and have been replaced in this current version.] [Correction added on 16 November 2020, after first online publication: Tables 2, 12 and 13 were previously incorrect and have been replaced in this current version.] This article has been contributed to by US Government employees and their work is in the public domain in the USA.

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