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<br /> <br />0021170 <br />. <br /> <br />3.2 Computation Procedure for Quality <br /> <br />With river flows and reservoir releases determined, the corre- <br />sponding salinities can be computed. The general approach is <br />nearly identical to that used in computing flows. Computations <br />commence with the first upstream boundary referenced on input and <br />proceed downstream until a junction is encountered. Computations <br />are then restarted at the next upstream boundary, proceeding as <br />before until a junction is encountered. If each upstream branch <br />into the junction has been entered, computations proceed down- <br />stream. Otherwise, another upstream boundary is used in continuing <br />the process. <br /> <br />I~en a river reach is encountered, the net salinity effect of the <br />import/exports, irrigation, and exports is computed. Only their <br />aggregate effect and not their sequence within the reach is <br />important. For the import/exports either the concentration or <br />mass of salts is specified. Irrigation effects are computed using <br />the per acre pickup rate per month and the specified number of <br />acres. Exports are diverted at the computed upstream inflow <br />concentration to the reach. <br /> <br />. <br /> <br />An internal sign convention is employed in which a negative aggre- <br />gate value denotes a removal of salt from the river while a <br />positive quantity denotes salt has been added. 'The aggregate <br />effect is added to the mass of salts entering the upstream end <br />of the reach to determine the salt outflow. Since the mass of <br />salts specified on input may exceed the mass in the river, a <br />negative sum may result. Clearly, this attempt to remove more <br />salt than exists is an impossible situation. Consequently, the <br />salt outflow is set to zero and the negative value is retained <br />as a variable called delta salt on output. <br /> <br />A second situation that can produce a value for delta salt arises <br />when the water outflow from the reach is zero. Since the salt <br />outflow may not be zero, the outflow concentration is undefined <br />and meaningless. For program purposes, it is assumed that with- <br />out water there is no vehicle to transport the salt. The actual <br />salt outflow is set to zero and the delta salt variable to the <br />computed value. By convention, the program assumes these salts <br />are lost from the river system rather than remaining in the river <br />bed to be picked up by subsequent river flows. The corresponding <br />outflow concentration is set to zero so that exports in the next <br />reach divert no salts. <br /> <br />. <br /> <br />In either of the above cases, the delta salt value represents the <br />computed salt outflow which was reset to zero because of an <br /> <br />3-7 <br /> <br />(rev. 07/75) <br /> <br />