Response to Surface Flow: Vegetation Greenness and Water Use in the Colorado River Delta since 2000

Abstract:

We studied the health of riparian reaches in the delta of the Colorado River that have undergone reductions in green vegetation from confounding pressures related to plant drought, precipitation declines, ground-water losses, land clearing and fire. Our study focuses on the impact of a paucity of Colorado River surface flows from the United States to Mexico in this dryland delta. We measured change in riparian plant greenness and their water use over the past two decades using vegetation index (VI), a proxy for greenness, evapotranspiration (ET, mm/day), and the Phenology Assessment Metric (PAM, mm/year). We measured long-term vegetative greenness and ET along the riparian corridor (2000-2019) and its short-term (2014-2019) response to an environmental flow in 2014, as prescribed under Minute 319. We used 250 m Moderate Resolution Imaging Spectroradiometer (MODIS) and 30 m Landsat satellite imagery time-series to evaluate three vegetation indices (NDVI, EVI, EVI2). We selected EVI and EVI2 to parameterize an optical-based ET algorithm and test the relationship between ET from Landsat and MODIS by regression approaches. There was a significant non-linear correlation between EVI _Landsat and EVI_MODIS indicating that both sensors are measuring nearly the same landscape metrics. Our analyses show significant decreases in VIs and ET as measured by Landsat and MODIS for both the 20-year and post-pulse 5-year periods. We capture annual spatial change maps of NDVI*_Landsat and ET_Landsat-EVI (mm/day) for ca. 23,000 ha and 150 km of riparian corridor along the river. Over the last 20 years, the VI signal decline was 34% (EVI_Landsat) and 30% (EVI _MODIS), while ET losses were 1.58 mm/day (38.36%) (ET_Landsat-EVI) and 1.13 mm/day (27.33%) (ET_MODIS-EVI). In a few reaches, greenness declined as much as 43% using both resolutions, with a corresponding ET loss of 1.95 mm/day (51%) since 2000. Our findings are significant because this added in-stream water helped slow the decline in greenness and ET. We conclude that the VIs will continue to decline into the future without ongoing active restoration effort. These findings are critically important as they suggest further deterioration of biodiversity, wildlife habitat and other key ecosystem services due to drought and other hydrological processes.