Impact of Invasive Species Water Use on Lower Colorado River Operations
The Bureau of Reclamation since 2006 has funded remote sensing and evapotranspiration (ET) studies at the Palo Verde Irrigation District (PVID) and Cibola National Wildlife Refuge (CNWR) in the Lower Colorado River system. The project made a considerable investment in flux towers to measure the ET of a tamarisk forest within the CNWR as well as in alfalfa and cotton fields within the PVID.
This project has come to an end, but some of the infrastructure remains functioning. Results from this effort have shown that, in 2008, tamarisk ET varied from an average of 1.8 millimeters per day (mm/day) in medium density tamarisk subject to ground water with low water quality to 4.4 mm/day in dense tamarisk with reasonably good water quality. Measured alfalfa ET was 6.1 mm/day during the same period, while reference ET0 grass) was 5.5 mm/day. The flux towers at the CNWR are uniquely positioned to provide long-term measurements of ET and could provide direct measurements of the impacts of future defoliation by the tamarisk leaf beetle (Diorhabda elongata), which were released in the Upper Colorado Basin, are migrating south, and will eventually adapt to the higher temperatures present in the Lower Colorado River system. There are several wells installed in the CNWR where depth to the water table and water quality are routinely measured.
Due to the presence of large, wide flood plains covered with tamarisk in the Lower Colorado system, a significant savings of water and improvement of water quality are anticipated from the control and elimination of tamarisk. Presently, not much data or information exist as to how much water could be saved by large scale control of tamarisk. We propose to estimate the changes of tamarisk ET and resulting changes in demand for river water as the tamarisk beetle progresses down the Colorado River system through a combination of airborne and satellite remote sensing, along with spatial modeling of river and flood plain ground water interactions.
Need and Benefit
Invasive vegetation species such as tamarisk (Tamarix sp.) and Russian olive (Eleagnus angustifolia) have spread throughout the Western U.S. water systems and rivers, outcompeting and replacing native species such as cottonwoods (Populus spp.) and willows (Salix spp.) in the Upper Colorado Basin and different varieties of mesquite (Prosopis spp.) and other desert trees and shrubs in the flood plains of the Lower Colorado Basin. Tamarisk, in particular, is a significant water user; in the Lower Colorado Basin, it grows in medium to dense stands covering large areas of the generally wider flood plains. This poses operational challenges for the management of the river, due to the connection between riverflows and water levels with fluxes into the flood plain ground water system resulting from riparian vegetation ET demands.
The ET of native and non-native species in the riparian zone and how it is impacted by water quality and salinity, depth to the water table, and vegetation density is still relatively unknown. There is a need to improve models of riparian ET, taking into account these aspects, as well as estimates of the ground water fluxes contributed from the river to the riparian ground water system for supplying the riparian ET demand. Improvements in these estimates through modeling will improve river management and optimize upstream releases, allowing for characterizing potential future increases or decreases in riparian ET resulting from climate change and beetle defoliation and other forms of tamarisk control and eradication. In addition, very little is known about Russian olive ET, which potentially could increase its presence as tamarisk is biologically controlled.
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