Impact of Invasive Species Water Use on Lower Colorado River Operations
The Bureau of Reclamation since 2006 has funded remote sensing and evapotranspiration studies at the Palo Verde Irrigation District (PVID) and the Cibola National Wildlife Refuge (CNWR) in the Lower Colorado River system. The project made 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 mm/day in medium density Tamarisk subject to groundwater 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 ETO (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 elongate), release in the Upper Colorado Basin and that are migrating south and will eventually adapt to 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 floodplains covered with Tamarisk in the Lower Colorado system, a significant savings of water and improvement of water quality is anticipated from the control and elimination of Tamarisk. Presently there are not much data or information 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 by 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/floodplain groundwater interactions.
Need and Benefit
Invasive vegetation species such as Tamarisk (Tamarix sp.) and Russian olive (Eleagnus angustifolia) have spread throughout the Western US water systems and rivers, out-competing 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 floodplains of the Lower Colorado Basin. Tamarisk in particular is a significant water user and in the Lower Colorado Basin it grows in medium to dense stands covering large areas of the generally wider floodplains. This poses operational challenges for the management of the river, due to the connection between river flows and water levels with fluxes into the floodplain groundwater system resulting from riparian vegetation evapotranspiration demands.
The evapotranspiration 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 groundwater fluxes contributed from the river to the riparian groundwater 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 evapotranspiration, which potentially could increase its presence as Tamarisk is biologically controlled. Also, the study of ET rates for native species such as Mesquite and other desert trees and shrubs will allow for a better undestanding of the water use of eventual replacement vegetation.
The Southwest Willow Flycatcher is an endangered species that inhabits the Lower Colorado River. The Tamarisk beetle is rapidly moving into that few remaining nesting sites. Defoliation of these nesting sites has let to nest failure. Monitoring temperature under the canopy at one, two and three meters heights will provide scientists with critical information to understand flycatcher habitat. In addition, we will monitor temperatures under native willows near the Palo Verde drain. Willow Flycatchers have traditionally nested in willows, but willow stands have been declining due to tamarisk i
1. Acquisition and quality control of Bowen ratio flux data and groundwater, river stage, and remote sensing data, analysis of this data, estimation of ET for different riparian vegetation species, preparation of journal quality publications and preparation of data for public release.
2. Report describing the ET estimates and potential water savings by reach of the river. It will include ET maps of the study area obtained from the remote sensing technique, description of present and previous ET data collected at the towers and comparisons with the remote sensing approaches, groundwater data and analysis and its relationship with riparian ET, description of modeled baseline results at CNWR, and the Lower Colorado River system
3. Report describing Willow Flycatcher habitat analysis at the CNWR.