Continued Field Measurement of Riparian ET, Lower Colorado River Basin

Project ID: 6224
Principal Investigator: Ian Ferguson
Research Topic: Water Resource Data Analysis
Funded Fiscal Years: 2014 and 2015
Keywords: evapotranspiration, riparian, invasive species, long-term monitoring

Research Question

Evapotranspiration (ET) from riparian and phreatophytic vegetation is an important component of reach, sub-basin and basin-scale water budgets in the Southwest, accounting for a significant fraction of non-agricultural ET in the region. Recent research has improved our understanding of riparian systems, including our ability to measure riparian ET via ground-based and remote sensing (RS) methods. However, the complex interaction between hydrologic, meteorological, and ecological dynamics of riparian systems remains a challenge to water, land, and wildlife managers in the Southwest.

Since 2005, Reclamation has conducted field measurements of ET and groundwater levels at three riparian sites adjacent to the Colorado River at Cibola NWR. Measurements have been used to quantify water use by Tamarisk, a riparian invasive species common throughout the West, and to evaluate RS-based ET algorithms, including the Lower Colorado River Accounting System (LCRAS).

All three ET measurement sites were destroyed by wildfire in August 2011. Two of the three sites were rebuilt prior to the 2012 growing season. This project will continue measurement of ET at the two rebuilt sites and groundwater levels at 13 remaining monitoring wells through the 2016 growing season in support of ongoing and future research efforts, including:

- continued improvement of RS-based riparian ET estimates

- evaluation of variability in riparian ET with respect to climate, hydrology, and vegetation conditions

Importantly, continued measurement of ET at the remaining sites provides a unique opportunity to evaluate the relationship between ET and vegetation density during the regrowth period following the 2011 wildfire. In addition, the tamarisk beetle is expected to reach the site within the next 3-5 years. Continued monitoring will provide a unique opportunity to directly measure the effects of this biological control agent on tamarisk ET.

Need and Benefit

Estimates of ET from riparian vegetation are an important input to environmental resources assessment and management, particularly in semi-arid and arid basins in the Southwest where riparian ET is a significant component of the water budget. Throughout the region, accurate estimates of "the disposition of water once it is released from [a reservoir]"--including the amount of water loss from the river and shallow aquifer via riparian ET--is a critical in order to effectively manage river systems. In the Lower Colorado River Basin, for example, the Lower Colorado River Accounting System (LCRAS) is used to track and manage releases from Hoover Dam. In other basins such as the Mojave, Reclamation has used ET estimates to quantify the potential water savings from removal of riparian invasive species such as Tamarisk and Russian Olive.

Field-based measurement of ET is critical for developing, calibrating, and verifying remote-sensing and numerical methods used to calculate riparian ET estimates needed by water and environmental resources managers. In addition to use in planning and operations support, maintaining a long-term field site for monitoring of riparian ET will provide the observational data necessary to support a wide range of research activities. Key research topics include quantifying the impact of riparian vegetation on reach, sub-basin, and basin-scale water budgets; evaluating the impact of riparian ET on surface water-groundwater interactions; evaluating correlations between climate, hydrology, and riparian ET, including evaluating climate change impacts on riparian ET; and evaluating relationships between riparian vegetation density, riparian ET, and riparian habitat quality for habitat and wildlife management.

Importantly, this project also provides a unique opportunity to quantify the effects of vegetation growth and density on ET and groundwater elevations. By monitoring ET during the vegetation regrowth following the 2011 wildfire, this project will provide new information regarding the relationship between riparian ET and riparian vegetation density and condition. In addition, continued monitoring through the arrival of the tamarisk beetle--a biological control agent initially released in the Grand Canyon area and expected to arrive in Cibola NWR within 3-5 years--will provide a unique opportunity to directly measure the effects of the beetle on tamarisk ET. To our knowledge, no other measurement site within the basin can provide similar measurements of the effects of fire, regrowth, and beetle arrival on tamarisk ET.

Contributing Partners

Contact the Principal Investigator for information about partners.

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Last Updated: 6/22/20