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<br />1>/::0. <br />o <br />W <br />...:.! <br /> <br />c' <br /> <br />Cc <br />f.c, <br /> <br /><" <br /> <br />the principal inflows and outflow of each impoundment (Fig- <br />ure 1). Individual ion and TDS data were not available for <br />the Las Vegas Wash inflow to Lake Mead during the 1951-60 <br />period or for the Muddy River inflow during either period. <br />The annual data were used to calculate flow-weighted average <br />concentrations over the 10 yr periods on the combined in- <br />flows and outflow of each impoundment. <br />The effects of evaporation on ion and TDS concentra- <br />tions could not be evaluated directly. Gross'evaporation <br />rates have been measured annually in Lake Mead since 1952, <br />I but water inputs from precipitation and ungaged inflows, <br />which are necessary to compute net evaporation, have only <br />been measured during special studies [S,9J. Gross evapora- <br />tion rates and other variables of the water budget were only <br />measured in Lake Powell during 1973-74 [11 J. Water losses . <br />, <br />I from the impoundments, nonetheless, exceed water inputs. In <br />'ordl'lr to assess the effects of this on ion and TDS concen- <br />I trations, we computed average annual, net rates of ,/Ster <br />losD for each impoundment and time period using Equation 1. <br /> <br />R = Ig - Og -; Od - ~S <br /> <br />(1 ) <br /> <br />. where R = <br /> Ig <br /> Og = <br /> Od <br /> ~S <br /> <br />net water loss (precipitation + ungaged <br />inflows) - (evaporation + ~bank storage) <br />gaged inflows (as in Figure 1) <br />gaged outflows (as in Figure 1) <br />diversions <br />change in impoundment storage <br /> <br />, D ' <br />j,The ion and T S loads computed for the inflows to each im- <br />I 'poundment were then divided by the term (Ig-R) to estimate <br />ion and TDS concentrations expected in the outflow as a <br />result of the net water losses. The net water losses were <br />assumed to be due primarily'to evaporation, and we acknowl- <br />edge that factors like bank storage will have some influ- <br />ence on estimated changes in ion and TDS concentrations for <br />years when lake fluctuations ~re more severe. However, <br />I averages over long term periods should not be largely <br />affected. <br />Ion budgets, adjusted for net water losses, were used <br />to estimate chemical precipitation and dissolution rates in <br />the impoundments. Calcite precipitation was estimated from ' <br />molar changes in carbonate (= bicarbonate/2.03) [1SJ. Gypsum' <br />(CaSO.) and halite (NaCl) dissolution were estimated from <br />molar changes in sulfate and chlorid,e. <br /> <br />5 <br /> <br />.~ <br />