We studied effects of changes in riparian plant communities dominated by tamarisk (Tamarix spp.) and the beetle, Diorhabda carinulata, which was released as a biocontrol agent, on evapotranspiration (ET) at uranium mill tailings sites. We used an unmanned aerial system (UAS) to acquire high resolution spectral data needed to estimate spatial and temporal variability in ET in riparian ecosystems at uranium mill tailings sites adjacent to the San Juan River near Shiprock, New Mexico, and the Colorado River near Moab, Utah. UAS imagery allowed us to monitor changes in phenology, fractional greenness, ET, and effects on water resources at these sites. We measured leaf area index (LAI) and sampled biomass on tamarisk, cottonwood (Populus spp.), and willow (Salix spp.) within the UAS acquisition areas to scale leaf area on individual branches to LAI of whole trees. UAS cameras included a Sony Alpha A5100 for species-level vegetation mapping and a MicaSense Red Edge five-band multispectral camera to map Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI). The UAS images were correlated with an August 2016 Landsat satellite image to scale plant water use. Following this, MODIS time-series data was used to document long-term trends and relationships of ET.
Our results show that remotely sensed monitoring of remediation efforts related to cleanup of groundwater (GW) contaminated by uranium milling at two sites adjacent to U.S. rivers is improved using high resolution aerial imagery acquired with a UAS (drone). We further provide landscape-scale estimates ET and its impact on GW recharge and discharge and GW phytoremediation using 30m Landsat and 250m MODIS sensors which depict the spatial and temporal variability in ET. We used the three sources of imagery to evaluate our model of GW flow and contaminant transport. This data is useful for improving our development of GW remediation strategies. GW elevation, flow, and contaminant transport appear to vary seasonally and annually in response to changes in tamarisk green cover; therefore, effects of tamarisk and beetle interactions on ET are particularly relevant at these sites. We found that the UAS imagery was critical for testing hypotheses related to these GW observations. We conclude that our methods to estimate the effects of changes in tamarisk cover on water use, GW flow, and contaminant transport was enhanced by the drone imagery at both sites and allowed us to monitor changes in tamarisk phenology, fractional greenness, ET, and effects on water resources.