introduced) biological control. Although mathematically an estimated 1,000 acres of Arundo are
controlled by the natural insect infestation, the area of reduction occurs throughout the 58,000
acres and does not represent a separate and independent 1,000 acres of Arundo.
Water Use and Valuation
To decipher the total value that controlling giant reed has in association with the amount
of water saved, the percent control from the release of the insects is applied to the total acres
infested with Arundo donax. The number of acres controlled are then multiplied by the Arundo
water use per acre to obtain total potential reduction in water consumption by Arundo for the
region. After accounting for water uptake from natural vegetation regrowth and Mexico’s
allotment of the water, the amount of U.S. water saved in year one totals 912 acre-feet and
amounts to more than 57,000 acre-feet in year 2058.2 The net water savings amount to
approximately one acre foot per acre of expected infestation.
Composite Acre
The composite acre approach was developed to calculate the value of the water to the
region and is assumed to reflect a representative acre of the crops in the Lower Rio Grande
Valley. The composite acre with low-marginal crop values, including corn, cotton, and sorghum,
has an estimated return to water of $190 per acre-foot using market prices, and $139 per acre-
foot using normalized prices (Table 1). The high-marginal value crops composite acre include
corn, cotton, sorghum, citrus, vegetables, and sugarcane, and has an estimated composite acre
The study area is also located in a gaining reach of the River, where several “no name” tributaries add water to
the main stream. Any measured gains from these tributaries in this reach of the River can be considered as split
50/50 between the United States and Mexico (Rubinstein 2008).
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