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Introduction

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The Colorado River Delta historically provided tremendous environmental and cultural benefits, but agricultural land conversion, river regulation, consumptive water use, and non-native species have decreased and degraded riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian areas. While many organizations are conserving and restoring cottonwood-willow and mesquite forests along the river in the United States and Mexico, overuse and changing climates indicate increasing vulnerability of habitat. Determining the extent of groundwater-supported riparian ecosystems under different scenarios enables land managers and conservation organizations to prioritize restoration areas and identify strategies for mitigating groundwater limitations. 

The Colorado River Delta was once 3,000 square miles of riparian, marsh, and estuarine habitat that supported an extreme diversity of plant, bird, and marine life. Dam construction and water use along the Colorado River and its tributaries reduced flows to the Delta and eliminated seasonal peak flows required to sustain riparian ecosystems. Much of the historic Delta was converted for agricultural use. As a result, the Delta was reduced to less than 10% of its original size. However, floods in the last two decades of the 1900s revived vegetation along the riparian corridor, which is still a very important area for both resident and migratory birds, along with many other vertebrate species. Wetland, cottonwood-willow, and mesquite ecosystems are still prevalent in the central Delta, where surface and subsurface agricultural return flows have maintained shallow groundwater. The riparian corridor has been acknowledged as a conservation priority by U.S. and Mexican government officials, conservation organizations, and scientists. 

In 2008, Sonoran Institute and Pronatura Noroeste secured a 1,400-acre land concession agreement with the Mexican government to set aside a portion of the historic riparian corridor for restoration. This site is called the Laguna Grande Restoration Area. As of 2018, over 700 acres of riparian habitat have been restored at the site. While restoration has been successful, the long-term sustainability of the site could be threatened if groundwater declines due to drought or changes in water management.  

Key Issues Addressed

Riparian vegetation is dependent on shallow groundwater, especially in the arid Colorado River Delta, where summer temperatures are frequently above 115°F. Areas that currently have shallow groundwater are being targeted for restoration. Philanthropic foundations and government agencies are investing significant funding to re-plant native vegetation and deliver water to restoration sites and remaining native ecosystems.

Groundwater levels in the Delta are affected by agricultural practices, groundwater pumping, water use by plants, and environmental flows. An existing groundwater model was used to predict groundwater depth in the central Delta that would support different riparian ecosystem types under plausible future scenarios and explore if environmental flows could mitigate the effects of regional groundwater changes. 

Project Goals

• Establish groundwater depth thresholds for native vegetation and wetlands in the Delta
• Refine a groundwater model to predict average monthly groundwater depth under different scenarios
• Estimate the area of aquatic, cottonwood-willow, and mesquite cover types that could be supported under each scenario

Project Highlights

• Groundwater Threshold Analysis: Although depth to groundwater thresholds and sensitivity to groundwater fluctuations for cottonwood and willow species have been previously studied, these thresholds were estimated specifically for the Colorado River Delta by correlating local groundwater elevations with locations of remnant riparian vegetation.
• Groundwater Model Refinement: An existing regional MODFLOW groundwater model was refined to analyze potential groundwater levels at high spatial resolution. The regional (parent) groundwater model was integrated with the central Delta riparian corridor (child) model.
• Integrated Scenario Development:  The project team worked with experts from the region to validate scenarios. Upstream subsurface inputs were based on upstream piezometer groundwater data ranges. Agricultural return flow scenarios were based on changes in groundwater recharge due to fallowing, crop changes, and/or irrigation efficiency improvements. Plant water use and evaporation scenarios were developed from regional models, and took into account potential regional water use impacts of active restoration and the arrival of the tamarisk leaf beetle. Modeled environmental flows reflect current delivery plans being used by conservation organizations in the Delta.

Lessons Learned

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Critical Environmental Flows: Model results indicate that environmental flows have the potential to mitigate declining groundwater levels resulting from possible future changes in agricultural practices.

• Agricultural return flows are the most significant factor influencing groundwater levels in this part of the Delta. Model results suggested that if agricultural return flows were reduced by 25%, over 50% of the potential cottonwood-willow and mesquite forest area may no longer be supported by shallow groundwater.
• Modeled changes in evaporation and water use by plants did not significantly impact groundwater levels. Revegetation with native plants (higher water use) or arrival of the tamarisk leaf beetle (lower water use) is not expected to impact local groundwater levels.
• Model results suggest that environmental flow deliveries are the second most important factor for sustaining groundwater levels. Under baseline agricultural return flow levels, environmental flows would greatly increase the spatial extent of the region that would support cottonwood-willow habitat. Just as importantly, environmental flows would mitigate nearly all impacts of groundwater reductions resulting from modeled decreases in agricultural return flows.
• The sensitivity of groundwater to agricultural management practices and the mitigation potential of environmental flows could be incorporated into an adaptive management program for restoration sites. Regular monitoring of groundwater levels is needed to allow conservation organizations to respond with environmental flow deliveries.

Next Steps

• Incorporate potential impacts of groundwater decline and long-term water needs into restoration planning
• Use environmental flows to mitigate potential negative impacts of climate change climate change
  Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.
  
  Learn more about climate change and reduced agricultural water
• Advise binational water policy through data and modeling analyses
• Use an adaptive management framework to test future flow volumes and delivery points
• Create water management scenarios for different levels of groundwater decline to establish water delivery programs for quick implementation

Funding Partners

Resources

Contacts

• Karen Schlatter, University of Florida Water Institute (formerly Sonoran Institute): [email protected]
• Matt Grabau, US Fish and Wildlife Service (formerly Sonoran Institute): [email protected]

Case Study Lead Author

• Deanna Morrell, Program Analyst, US Bureau of Reclamation

Suggested Citation

Morrell, D., P. (2018). “Shallow Groundwater in the Colorado River Delta: Modeling Susceptibility and Management Options.â€� CART. Retrieved from /project/shallow-groundwater-colorado-river-delta.