Effect of Restoration on Plant Greenness and Water Use in Relation to Drought in the Riparian Corridor of the Colorado River Delta

Abstract

The unrestored delta riparian corridor continues to decline in EVI2 and ET but restoration has positively impacted habitats and contributed to the understanding of the continuing efforts of Minute 323.ABSTRACT: Revitalization of the Colorado River delta riparian corridor and increasing riparian plant greenness and water use may be accomplished by added water and restoration efforts to offset declines measured since 2000 by Landsat. We use the two-band Enhanced Vegetation Index (EVI2; a proxy for greenness) and evapotranspiration (ET, mm/day) using EVI2 and potential ET(ETo) from Yuma Valley. We assess if restoration with only 7.5% landcover had an impact on the unrestored reach-level landcover by measuring these two metrics, EVI2 and ET(EVI2) by comparing restored and unrestored areas. A key finding is that over 21-years EVI2 in the unrestored corridor decreased by 23.6% and ET(EVI2) decreased by 32% (0.87 mm/day). Since 2011, the unrestored reaches lost EVI2 (11%) and −0.73 mm/day ET(EVI2) (28%), but restored sites increased in EVI2 (36%) and 0.58 mm/day ET(EVI2) (20%). Water delivered to restored sites increased EVI2 by 33.6% and 1.29 mm/day ET(EVI2) (58%). EVI2 and ET(EVI2) as a function of the Standardized Precipitation Evapotranspiration Index for drought periods highlight that restored and unrestored sites respond differently. Unrestored reaches are in decline; restored sites show increases in EVI2 and ET(EVI2). Restored sites do not have a significant impact on unrestored adjacent area, but smaller surface flows, a greater reliance on directed agricultural return flows, and deliveries of water to active restoration sites have revitalized habitat and increased ecosystem services in the delta.

Interannual controls on riparian plant health in a dryland river

“…Zamora-Arroyo et al ( 2001) determined that the minimum discharge necessary for the recruitment of willowcottonwood forests along the lower Colorado River was a modest flow of 3 Â 10 9 m 3 over a 3-month period that coincides with seed release timing (i.e., February-April). Like our findings, their results have clear implications for the management of flow releases from upstream dams to support native riparian forests but within dryland riparian systems such as the Santa Clara River and lower Colorado River, competing water uses for agricultural and municipal purposes is intense, making it difficult to balance human needs and conservation goals (Nagler et al, 2021;Nagler et al, 2022;Stromberg et al, 1996).…”

Greenness and Actual Evapotranspiration in the Unrestored Riparian Corridor of the Colorado River Delta in Response to In-Channel Water Deliveries in 2021 and 2022

“…The monitoring of riparian corridors at Landsat spatial scales of 30 m resolution, or finer, is imperative to comprehend the multifaceted impacts of climate variability [1-3], species invasions, habitat fragmentation, and acute disturbances such as wildfires and flooding. This is particularly critical in the arid and semi-arid regions of northwestern Mexico and the southwestern United States, where riparian vegetation is declining due to both anthropogenic pressures and natural stressors, such as increased temperatures and water scarcity [4]. These changes not only threaten biodiversity and ecosystem services but also have economic implications, affecting local economies that benefit from green spaces and recreational activities [5][6][7].…”

“…The hydrological conditions in the study area are demonstrated by the disparity between surface flows and plant water use, creating a niche for predominantly phreatophytic plants which draw water from the aquifer [14]. These species are primarily riparian trees in the narrow Reach 4, where more than a dozen restoration sites are monitored [4,12,24,25], but in Reach 5 where the Delta truly begins to form and in Reach 7 where the river further meanders and spreads, the predominant species are no longer native riparian trees but rather shrubs such as Tamarix spp., with high fractions of low vegetation cover including saltgrass (Distichlis spicata) [24,25]. Minute 323 provided the framework for the restoration site infrastructure, the physical infrastructure for delivering water through weirs at four locations to deliver water (e.g., 2021-2022) into Reach 4, and the monitoring of riparian health outcomes compared with the unrestored riparian corridor [23].…”

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

“…2022) and wetlands health (Nagler et al. 2022)). In still other cases, this new Featured Collection addressed issues not explored in the original articles.…”

“…Some articles in this Featured Collection revisited original analysis of the SSD (e.g., the role and implications of tree-ring research, impacts of drought on water-based recreation). Others explore earlier topics in greater detail (e.g., effects on fish species (Bruckerhoff et al 2022) and wetlands health (Nagler et al 2022)). In still other cases, this new Featured Collection addressed issues not explored in the original articles.…”