The U.S. portion of the Lower Colorado River between Hoover and Morelos Dams has a critically important riparian ecosystem that is used by avian species as one of the only north-south flyways in this arid region. Landsat data from 2000 through 2020 were used to measure greenness and water use along five reaches of these riparian woodlands to evaluate trends in riparian ecosystem health. Researchers used the two-band Enhanced Vegetation Index (EVI2; a proxy for greenness) and evapotranspiration (ET, millimeter/day) using EVI2 and ground-based meteorological potential evapotranspiration (ETo) from three stations along the river. The team also used an annualized ET based on EVI2 Landsat time series, a new Phenology Assessment Metric (PAM ET). A key finding was that riparian vegetation greenness and its water use has been in decline since 2000 as measured by Landsat, depicting a loss of healthy vegetation over the last two decades. EVI2 results showed a decrease of 14%, while average daily ET(EVI2) between the first and last decade had a decrease of 27%, over -1 millimeter/day. The average PAM ET losses were 18% or -171 mm/yr between these two decades. The difference between the first and last five-year periods, 2000–2005 and 2016–2020, showed the largest decrease in daily ET, a loss of 33% or -1.24 millimeter/day. These declines come from a loss in healthy, green, riparian plant cover, rather than a change in plant water use efficiency or efficient use of managed water resources. Results suggest further deterioration of biodiversity, wildlife habitat, and other key ecosystem services on the U.S. portion of the Lower Colorado River.
ET (mmd-1) averaged across four groups of years in ~5-year periods (2000–2005 = green, 2006–2010 = light green, 2011–2015 = peach, 2016–2020 = red) for five reaches and the weighted average of all reaches along the Lower Colorado River.