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69 6.0 BASIN MODELING 6 . 1 "FHAR" - COMPUTER PROGRAM Reclamation uses the Flood Hydrograph and Routing (FHAR) computer program to convert excess precipitation to runoff and generate the flood hydrograph for the particular storm event being studied. FHAR, which was developed by Reclamation, uses unitgraph theory. The program derives the flood hydrograph by applying the increments of excess precipitation to the unit hydrograph. The unit hydrograph is computed from the dimensionless graph, given the basin area, lag time, and unit time. The unit time is computed by dividing the lag time by 5.5. The subbasin lag times are shown in table 3.1. The unit time selected for this study is one hour, based on the smallest unit times computed for all of the subbasins. The Tatum method was used to route flood hydrographs from one subbasin to the next downstream subbasin, and to combine them with additional flood hydrographs as the floods move downstream. The Tatum method is a successive average lag procedure. It is commonly used to route hydrographs through channels which have no appreciable storage or large tributary inflow, or where costs of obtaining channel cross-section and other data needed for more sophisticated methods are prohibitive. The method uses the travel time of the flood hydrograph for each river reach, the hydrograph ordinates, and the unit time. Development of the travel time was discussed in a previous section of this report. FHAR uses the modified PuIs method to route floods through reservoirs or through short stream reaches in which the time of travel and wedge storage is negligible. When used to route floods through reservoirs, a fixed elevation-discharge-storage relationship is assumed. The change in storage is accounted for by a change in reservoir water surface elevation. The storage change is equal to the difference between average inflow and average outflow. 6.2 RESERVOIR OPERATING CRITERIA The reservoirs in the Colorado River basin above Hoover Dam are operated as an integrated system. The system has a total flood control space requirement of 5,350,000 acre-feet, which must be evacuated from storage by January 1. At least 1,500,000 acre-feet of that space must be in Lake Mead, which is the only facility in the system with exclusive flood control space. One of the primary goals of the flood control operations for the Colorado River system is to keep the exclusive flood control storage at Hoover vacant year-round to regulate potential rain floods. The 1982 field working agreement between the Bureau of Reclamation and Corps of Engineers for flood control operations of Hoover Dam and Lake Mead establishes the reservoir operating criteria. Two sets of operating rules are used to operate the system. During the