Remotely sensed observations using Landsat time-series from 2013-2023 to determine water savings for riparian corridor sites in the Lower Colorado River Basin affected by tamarisk defoliation

These spatial and tabular data were compiled to have a decades-long record of the water usage in the riparian corridor for 27 selected sites where tamarisk (Tamarix spp) or saltcedar defoliation was reported by ground observations and/or exhibited in Google Imagery at locations across the Lower Colorado River Basin. Data for these sites was measured remotely for plant health as characterized by vegetation greenness, through the vegetation indices the Enhanced Vegetation Index (EVI) or the two-band EVI2, and its plant water use or actual evapotranspiration (ET) estimation. In 2001, tamarisk leaf beetles (“beetles,” Diorhabda spp) were released to control invasive tamarisk (Tamarix spp), which dominates western United States (US) floodplains and alters riparian ecosystems. The beetles now inhabit thousands of river miles across the western US, radiating far from original release sites and repeatedly defoliating tamarisk stands. While biological control offers an alternative to mechanical or chemical removal, the benefits from tamarisk biological control remain an open question. Some river reaches now have more native shrubs where tamarisk cover has declined due to beetles, but native tree recovery remains rare. The objective of our study was to quantify ET (in millimeters per day) for each of 27 selected areas within Colorado Basin riparian zones. These tabular data represent ET and reference evapotranspiration (ETo) time series between Spring 2013 and December 2023. Landsat spectral bands were used to estimate EVI and EVI2. ET was then calculated based on these VIs as ET(EVI) and ET(EVI2). For its calculation, the Nagler ET(EVI2) empirical model was utilized. These spatial data represent polygons of tamarisk-dominated land cover from which were extracted as time series of EVI and EVI2 and used to estimate ET(EVI2) (in millimeters per day). This ET method requires using nearby Arizona Meteorological Stations. These meteorological data were collected for 27 sites, where the riparian corridor contains tamarisk (Tamarix spp) and was defoliated by beetles (Diorhabda spp). These data were created by the University of Arizona’s Vegetation Index and Phenology Lab, using surface reflectance data collected by the spaceborne mission Landsat-8 Operational Land Imager (OLI) to estimate EVI2. ETo, and consequently ET(EVI2), was estimated from AZMET; however, we used Daymet gridded weather data to produce ETo across the study area, mostly in Arizona. These spatial and tabular data can be used for monitoring the health of riparian vegetation and water use trends. From 2013–2023, 52% of studied sites in the Colorado River Basin showed modest reductions in ET from tamarisk biocontrol, while ET of remaining sites increased slightly, yielding inconsistent water savings. Beetle-impacted tamarisk stands support a depauperate avian community and negatively affect breeding in rare birds, while restored native vegetation provides important replacement habitat.