Global climate change is having considerable impacts on water resources and most components of the hydrological cycle, yet quantifying these impacts is still considerable and an on-going challenge. Recent research suggests extended growing seasons, resulting from earlier onset of growth and delayed onset of fall senescence, is causing plants in water-stressed basins to respond with increased "greening" and demand for water. Riparian extent, foliage density and greenness cycles have a marked impact on riparian water balance through evapotranspiration (ET). Empirical ET algorithms have used both Landsat (30 m) and MODIS (250 m) vegetation indices, but now, a recent method for matching ET estimates in riparian zones at both scales was applied using the Enhanced Vegetation Index 2(EVI2). EVI2 is a more stable form of the index as it does not depend on the less reliable blue band and is an indicator of vegetation greenness. Our objective was to measure changes in riparian zones along the Lower Colorado River (LCR) with satellite-based EVI2 to detect vegetation greenness and ET using EVI2 and local meteorological data. Time-series data of these key riparian ecosystem variables supports identification of the research and management needs to cope with emerging droughts, landcover and other changes.
Results/Conclusions
We measured, using Landsat EVI2, vegetation greenness and ET of riparian-dominated cottonwood/willow forests and tamarisk-monotypic stands over the past two decades for two regions: (1) the U.S. portion of the Lower Colorado River (LCR) from Cibola NWR to the border with Mexico (4 reaches) and (2) the Mexico portion of the LCR and the delta (150 km south of Yuma, Arizona into Mexico) (7 reaches). For the U.S. portion of the LCR, we saw declines in both metrics in 12 of 19 years, with overall losses of 23% in vegetation greenness over nearly two-decades and losses in ET (mm/day) of 17 %. In the Mexico portion of the LCR and the delta, greenness declined 34% and ET declined 38%. Since an environmental flow in 2014 added surface water, greenness declined only 20% and ET declined only 23% in the five years since that flow. Along the border, especially on the Mexico side, there have been decades-long losses of trees, shrubs and grasses; this loss of stand structure and architecture results in reduced function as existing avian habitat. Regardless, ongoing active restoration of native plants and timing of water deliveries has been beneficial.to restoring plant growth.