BOARD01713

• Board Meeting Date: 7/28/2003
• Description: CF Section - Tamarisk Control and Eradication Update
• Board Meetings - Doc Type: Memo

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Impact of Tamarisk Infestation on the
Water Resources of Colorado
May 3D, 2003

3.0 Non-Beneficial Water Use

Consumption of water by tamarisk is the unifying issue that brings diverse groups ofpeople to
agree that there may be a need to control this invasive plant. Environmental organizations,
government agencies, conservation groups, ranchers, landowners, sportsmen, and others who
often times have been at odds on other issues seem to be willing to work together because of the
non-beneficial use of water by tamarisk. For this study, the term "non-beneficial water use" is
defined as the difference in water consumption (evapotranspiration) between tamarisk and the
native plants it has replaced.

Water Usage by Different Vegetative Types

Limited evidence indicates that water usage per leaf area of tamarisk and the native
cottonwood/willow riparian communities may not be that different. However, because tamarisk
grows into extreme thickets, the leaf area per acre may actually be much greater; thus, water
consumption would also be greater on an acre basis (Kolb 2001). Probably the most insidious
aspect of tamarisk and its consumption of water is that its much deeper root system (up to 100
feet compared to healthy cottonwoods and willows stands at 6 feet (Baum 1978, USDI-BOR
1995)) allows tamarisk to grow further back from the river and thus can occupy a larger area and
use more water across the floodplain than would be possible by the native phreatophytes. This is
especially significant, because the adjacent uplands and floodplain typically occupy a cross-
sectional area several times that of the riparian zone. In these areas, less dense areas of mesic
plants (such as bunch grasses, sagebrush, rabbit brush, skunk bush, and greasewood) can be
replaced by tamarisk resulting in overall water consumption several times that associated with
these other plants (DeLoach 2002).

From thirteen different studies conducted between 1972 and 2000 on tamarisk evapotranspiration
rates, the average water use reported is approximately 5.3 feet per year (Hart 2003). More recent
work performed on the Pecos River in Texas over the last three years indicates water use by
tamarisk of 7.7 feet per year (Hart 2003). Recent research by the U.S. Department ofInterior on
the middle Rio Grande estimates evapotranspiration rates on the order of 4.3 feet per year
(Interior 2003). These studies were performed using different methods of measurement, at
different locations, and Dr different densities of infestation. Native cottonwood/willow
communities have been estimated to use approximately one foot less per year than tamarisk
(Weeks, 1987) while the native shallow-rooted upland plant communities of grasses, sage, etc.
use only the moisture received by precipitation. Unpublished research on the Bosque del Apache
National Wildlife Re fuge on the middle Rio Grande River in New Mexico indicates that
Russian-olive has very similar evapotranspiration rates as tamarisk (Bawazir 2003).

Estimates of Non-Beneficial Water Use in Colorado

Estimates on water consumption by tamarisk vary a great deal depending on location, maturity,
density of infestation, and depth to groundwater. This will also be true for the

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