Restoration Efforts in Riparian Ecosystems in the Colorado River Delta as Measured by Greenness Indices and Evapotranspiration (ET) and using Hydrology, Avian Studies and ET Change Maps

Colorado River water has been allocated through recent Minutes (319 from 2014-2017; 323 from 2018-2026) to the 1944 Water Treaty between the United States and Mexico to support efforts to restore native riparian forests, which provide essential habitat for migratory birds, in the Colorado River delta. Our study was largely conducted in the context of assessing the effects of restoration efforts on riparian corridor health. We processed and analyzed remotely sensed data from 2000 to 2023 to assess large-scale dynamics of vegetation health by measuring satellite vegetation index (VI, a proxy for canopy greenness) and plant water use (actual evapotranspiration, ETa) in the riparian corridor.

Under Minute 323, water deliveries are used primarily to irrigate managed restoration areas. Our study reports the outcomes of restoration actions on variables such as vegetation extent and density through two-band Enhanced Vegetation Index (EVI2) measurements and hydrological processes including ETa. We integrated EVI2 with potential ET from two sources, the Yuma Valley Arizona Meteorological Station “AZMET” ground station and gridded Daymet, to calculate ETa. We quantify ETa in restoration sites compared to the unrestored reaches from 2000-2023. Our findings showed an average increase of 42% in EVI2, an indication of land cover greenness, within the restoration sites in the decade since 2014, when efforts by many non-government organizations collaborated to improve the riparian corridors, with one large effort in Reach 2 and a dozen smaller sites in Reach 4. Conversely, greenness in adjacent, unrestored areas in these reaches declined by 27%. The study also indicates a 22% increase in ETa in the restored areas, compared to a 31% reduction in the unrestored regions. Restored sites in Reach 4, which contains a dozen restoration areas, experienced ETa increases ranging from 9-12%, whereas their unrestored counterparts show a decline of 21%. Restoration efforts focusing on small plots have successfully revitalized habitat, the motivation for this research.

Measurements of VIs and ETa several years after the Minute 323 federal flows were delivered in 2020 and 2021 to the riparian corridor, including to restoration sites in Reaches 2 and 4, do not show any boost to the greenness and ETa in the unrestored riparian reaches in the delta after these federal flows were delivered. However, further downstream, in Reaches 5 and 7, the non-native shrub saltcedar (Tamarisk spp.) has been repeatedly defoliated by saltcedar beetles (Diorhabda spp.). Select regions of these defoliated shrubs in Reaches 5 and 7 were measured using Landsat time series data from 2000-2023 using peak growing season dates of May 1 through October 30. The measured change between the ETa in the first five years (2000-2004), with a mean of 737 mm/year, and latter five years (2019-2023), with a mean of 599 mm/year, showed a decrease of 138 mm/year in ETa, which is a decrease in ETa of 18.7%. Despite the challenges posed by small water deliveries and beetle defoliation for non-native saltcedar shrubs, restoration efforts focusing on small plots have successfully revitalized habitat, the motivation for this research.