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1. Introduction <br />This memorandum summarizes the procedures and results for quantifying phreatophyte <br />evapotranspiration (ET) along the South Platte River and its major tributaries in Colorado <br />east of the mountain front. The objective of this task was to estimate annual phreatophyte <br />ET, to create ageographically-referenced phreatophyte ET dataset, to develop a series of <br />methods for evaluating ET at monthly intervals, and for dynamically simulating how ET <br />will change if the depth to water changes in the South Platte riparian corridor. This work <br />constitutes the following task deliverables: <br />65.1 - A shape file enclosing the riparian zone along the South Platte River, Cache la <br />Poudre River, and significant tributaries. <br />65.2 - A technical memorandum documenting and summarizing the procedures used to <br />estimate ET plus electronic files that are grid- and pixel-based. <br />65.3 - A depth-to-water versus ETg curve for estimation of changing ETg within the <br />groundwater model to enable dynamic simulation. <br />2. Background <br />Phreatophyte ET estimates are an integral component of the Consumptive Use and Water <br />Budget for the South Platte Decision Support System (SPDSS) effort. As described in <br />the memorandum for Task 82, water budgets are created based on the conservation of <br />mass principle where: <br />Inflows -Outflows =Change in Storage <br />As an outflow to this system, the consumptive use of groundwater by phreatophyte <br />vegetation occurs along well-defined riparian corridors that follow the South Platte River <br />and its tributaries and is largely absent elsewhere. <br />By definition, "phreatophyte" is a plant of any species that derives at least part of the <br />water it uses from the water table. The occurrence of phreatophyte vegetation is, <br />therefore, governed by the presence of ground water within its maximal rooting depth. In <br />this system, groundwater levels are commonly linked directly to the flow and presence of <br />surface of water replenished by river flow and agriculture return water. <br />The largest proportion of phreatophyte ET occurs along the mainstem of the South Platte <br />River in a riparian corridor that was mapped in this study at up to two miles wide. Of <br />interest is the water used by phreatophytes that comes from groundwater (ETg). For these <br />calculations, ETg was simply represented by residual ET -Precipitation. This convention <br />enables manipulation of spatially-defined precipitation in any manner by the team <br />performing groundwater modeling. <br />Calendar year 2001 was chosen for documentation of ETg on the South Platte system. <br />2 <br />