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.I '-.- power releases to meet generation requirements, or demands which draw from upstream reservoirs and move water through the reservoir. There also may be spills from the reservoirs. Evaporation, bank storage change, and power production are cakul'ated each time water is moved through a reservoir. When calculations have proceeded through the last sequence point of the 'last node, flo'W8 are in balance throughout the entire basin. Sa- linities are then computed throughout the .system 'by a mass balance accounting procedure. Computations are then complete for the monthly time frame. Before calculations are started lor the next month, hydrology in- 'puts {or the new month are updated Irom the tempor'ai'y hydrology file. 'll it is'the beginning of a new year. demand and/or reservoir operational input data are updated. Simulation of basin operation for the new monthly time frame is then repeated. This process is 'repeated through the final month of the last year specified in the input data. Special Colorado River Basin Features Several features have been incoporated into the general river basin model to reflect specific Colorado River operations. These in- clude use of snowmelt-runoff forecasts for January.July reservoir operations, distribution of water between the Metropolitan Water Db.trict of California (MWD) and the Central A;rizona Project (CAP). water splitting between the Upper and Lower Basins and storage re- quirements of the Upper Basin described in section 602.(a) of Public Law 90-537., and flood opt:rations. The model has a procedure to provide a forecast of ,spring runoff with the same error properties as actual forecasting during actual opera'- tion. ]n the model, the spring runo!! (from the month under considera- tion through July) is summed from the disk tile containing one year of hydrology data. An error- term is applied each month which reflect. the historical accu~acy of the forecast in that month. l'hu.', in any 15Z . ----Sll month the flow used as the forecasted value will be high or low to the same degree as actual forecasted flows are in field operations. The legal constraints which govern the flow of water ,between the Upper and Lower Basins are incorporated in the model through a spe- cial subroutine. This aspect of the basin operation uses the forecast flows to determine the monthly-release irom Lake Powell and Lake Mead. Before the Upper-Lower Basin analysis is run, the Upper Basin resel'... voirs are analyzed and their rule curves established. To determine whether Powell will release more or less than the normal 8.2.3 million acre.feet (10,151. 71 million cubic me,ters) (from a monthly schedule). 'an estimate is made of Powell and Mead contents ior the upcoming October. The forecasted inflow to Lake Ppwell (from the current month through July) plus the average August and September inflow forms the expected total Powell inflow. The total release Irom Powell expected through September is calculated as B.2.3 MA.F minus the amount delivered up to the current month. The October Powell contents are then the current contents plus the expected inflow. minus the planned releases. Similarly. the October contents oi Lake Mead can be estimated from the current contents plus expected inflow (same as Powell release's plus gain between Powell and the Grand Canyon gage) minus the planned releases. Once the October contents of Lake Mead are estimated. a Lower Basin shortage or surplus may be declared. If Mead's ending water surface elevation exceed 1190 feet above sea level (362..7 meters). water above that level is declared surplus and distributed to the Central Arizona Project (CAP) and the Metropolitan Water District 01 California (MWD). II Mead is below elevation 112.4 (342..6 meters), a shortage is declared and CAP is reduced below its normal demand. Between eleva_ tions 1124 and 1190. the normal release pattern associat.ed with an 8. Z3 MAF/year total release is followed. The declaration of a. shortage or surplus in the Lower Basin does not depend upon the status. of Lake Powell. However, water splitting between Powell and Mead may in... crease the supply to the Lower Basin and affect the declaration. 153