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this application, peak season NDVI* was used to developed a single mid-summer <br />satellite scene. A single scene was used because the presence of a water table confers <br />reasonably steady state conditions of water availability and so, phreatophyte vegetation is <br />generally not water limited. The plants are generally not water limited and are <br />unconstrained by the highly variable and limiting supply afforded by precipitation as is <br />the "dryland" vegetation growing where the water table is significantly below the root <br />zone. As a simplification, the plants will remain leafed at close to a maximum through <br />the growing season. With water supplies tending to be non-limiting, plant water use for <br />phreatophytes becomes a factor of the driving force for the atmosphere acting upon the <br />leaf cover and vigor achieved by the vegetation. As both a measure of leaf cover and <br />vigor (chlorophyll) in the canopy, NDVI*, then can be used as an estimator of ET. <br />Single mid-summer images were used to differentiate the water use of the South Platte <br />vegetation for 2001. With the water table in a position shallow enough to supply the root <br />zone, groundwater-coupled vegetation will reach its maximal canopy cover and <br />greenness at about the time of mid-summer peak annual solar radiation (approximately <br />Summer Solstice; Or and Groeneveld, 1994) . To a much lesser extent, this relationship is <br />also dependent upon incident precipitation with higher greenness afforded by higher <br />precipitation, even with the water table present (Baugh and Groeneveld, 2006). The effect <br />of variable precipitation over the region of the basin was neglected in the calculations <br />here because the phreatophyte vegetation of interest is predominantly thick, riparian, and <br />supported by very shallow water tables-thus precipitation is only a fraction of the water <br />supplied by the groundwater. Also, the effect of precipitation on vegetation growth was <br />dealt with throughout the basin in a manner described in Equation 3, below. <br /> 35oa <br /> <br />c 3000 <br /> <br />v 2500 <br />v <br />x <br />2000 <br />a <br /> ~5ao <br />Y <br />~ao0 <br /> <br />~ <br /> 500 <br /> <br /> 0 <br /> a o.~ 0.2 a.3 a.4 <br /> NDVI <br /> 3500 <br /> <br />c 3000 <br /> <br />ti 2500 <br />d <br />x <br />200D <br />a <br /> ~5a0 <br />r <br />X000 <br /> <br />~ <br /> 500 <br />c) <br /> 0 <br /> 0 0.1 0.2 0.3 0.4 <br /> NDVI" <br />Figure 2. <br />Illustration of the effect of calculation <br />of NDVI* using data extracted from a <br />set region within San Luis Valley, <br />Colorado. The data portrayed in <br />Figure 1 is one of the CDFs shown. <br />a. NDVI CDFs (each for a separate <br />mid-summer date). A large degree <br />of scatter in NDVI values occurs <br />due to non-systematic variation. <br />b. NDVI* for the same data set starts <br />after application of Equation 2. <br />All curves initiate at a point of <br />assumed zero vegetation cover. <br />