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<br />lH118 1 'G <br /> <br />physical proceSses such as soil-water transformations due to an <br />irrigation regime must be specified by the user. It is this require- <br />ment which distinguishes the model from a true simulation model. <br /> <br />Total dissolved solids are used as the quality parameter and the <br />salinity system is treated as a conservative one. Because mass bal- <br />ance concepts are employed, chemical precipitation, dissolution, and <br />reactions of individual constituents are not explicitly considered. <br />These effects must be included by appropriate inputs. While flow is <br />assumed to be independent of quality, quality does depend on flows. <br /> <br />Both bank storage and evaporation are included for reservoirs. Com- <br />plete mixing of surface and ground water in bank storage is assumed, <br />resulting in a uniform reservoir water quality. Stratification, <br />incomplete mixing, varying detention times or short circuiting, and <br />variable withdrawal levels are ignored. For normal conditions, <br />reservoirs use a lag of 1 month for computing the quality of releases. <br /> <br />t~ <br />Fl <br />" <br />i <br />, <br />I <br />!. <br /> <br />Reservoirs may be operated in a variety of ways. Total releases <br />from individual reservoirs can be specified for each month, a min- <br />imum release may be specified and additional water demands met by <br />further releases, or the reservoir may be operated to only meet <br />demands. <br /> <br />Reservoirs operating conjunctively may do so using releases deter- <br />mined by one of three methods: <br /> <br />- Fixed percentages of downstream demands. <br />- Balanced operations in which releases are made so that the <br />level in each reservoir results in the same portion of <br />the zone between the top of the dead or inactive storage <br />pool and the spillway crest being full. <br />- Zoned approach in which as many as six zones are established <br />by input control levels. Operations are similar to those <br />for balanced operations, except the water level is also <br />desired to be within the same zone. <br /> <br />~ <br />" <br /> <br />The effects of activities within the basin are established through <br />four types of water use inputs: <br /> <br />" <br /> <br />- Import/exports in which the quantity of water and salt <br />are both specified. <br /> <br />- Exports in which the quantity of water exported is specified <br />but the corresponding salinity depends on the prevailing <br />concentration at the diversion point. <br /> <br />2 <br />