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<br />001824 <br /> <br />to downstream nodes encompassing the element. Reservoir elements pro- <br />vide for storage and permit modification of the inflow hydrology to <br />achieve a desired outflow hydrology. The confluence of several river <br />branches is represented by the junction elements. <br /> <br />, <br /> <br />Although nodes are numbered sequentially starting with 1, there is <br />no specific order in a geographic sense. By convention, elements <br />take the number of the downstream node. In Figure 2, Reaches 5, 6, <br />7, and 8 were included to allow introduction of water developments <br />affecting each individual drainage area in the upper portions of <br />the basin. Node 9 would physically be located in Lake Powell, but <br />in abstraction in applying the model is considered as a point just <br />above the reservoir. Node 18 was introduced to allow ungaged inflows <br />to be included for the river system between Hoover and Parker Dams. <br />Program convention in dealing with water inputs in reservoirs required <br />the introduction of Node 19 to allow diversions for the Central Ari- <br />zona proj ect (CAP) and the Metropolitan Water District (l-IWD) to be met <br />by releases from both Lake Powell and Lake Head. <br /> <br />~ <br /> <br />Time- and Flow-weighted Concentrations <br /> <br />If the same weight is given to each monthly concentration and sim- <br />ple descriptive statistics computed (mean and standard deviation), <br />they are referred to as time-weighted values. [2, Section 5.7] In <br />general, it is found that the product of the time-weighted concen- <br />trations and the total flow for the period does not yield the total <br />mass. To avoid this problem, the mean flow-weighted concentration <br />is computed. It is simply the ratio of the total salt mass to the <br />total volume. It may be regarded as a fictitious concentration based <br />on uniform flow, with the variance due to both flow and salt mass <br />consolidated into a single value. For natural streams, the flow- <br />weighted concentration is less than the time-weighted value. <br /> <br />'~ <br />r <br /> <br />The method employed in the Biennial Report[l] is actually a mix <br />or hybrid of the time- and flow-weighted approaches. Monthly val- <br />ues are summed to obtain total annual salt and water masses. Con- <br />centrations are computed for the year and then given equal weight <br />in computing statistics for the total period. Thus annual values <br />are based on flow-weighted computations using each monthly value, <br />and the entire period statistics are computed using time-weighted <br />annual values. As noted in the section "Base Runs" and in Table VI, <br />this method will give values intermediate to the flow- and time- <br />weighted approaches. <br /> <br />Statistics contained in the computer listings are time weighted. <br />Flow-weighted mean concentrations are included in the results for <br />the various model runs. <br /> <br />4 <br />