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3-9 ~ The holdout hydrograph at a flood control structure site is a discharge hydro graph consisting of the difference between the inflow and outflow hydrographs. The discharge values will be negative when the outflow exceeds the inflow. At present, the holdout hydrographs can be obtained only at the structure sites. This version of TR-20 will not allow the negative portion of the holdout hydrograph to be reach routed downstream. Furthermore, the routing coefficient in the Modified Att-Kin reach routing method varies with the magnitude of the inflow hydrograph peak and volume (See theory in Appendices G and H). Thus, the holdout portion of a storm hydrograph cannot be routed downstream separately with any degree of consistency to compare with the unmodified total storm hydrograph. The amount of attenuation and kinematic translation will vary with magnitude of the hydro graph being routed. Additional study will determine what shortcuts or improvements may be needed to continue use of this routing technique commonly called tabular routing. The holdout procedure must be completed by other means at this time. ~. The holdout procedureVis to route the holdout hydrographs from structure sites for a particular storm downstream without adding any uncontrolled area. This entails a separate set of Standard Control statements that develop the inflow hydrographs above the structures, reservoir routes through the structures, then reach routes through consecutive reaches generating output files for evaluation at selected points of interest. The holdout hydrographs from the structures must be combined algebraically with the proper timing at all stream junctions to form a composite holdout hydrograph. The modified storm hydrograph at any point of interest may be obtained by subtracting the composite holdout hydrograph from the total storm hydrograph obtained by the regular TR-20 procedure. The advantage of the procedure is that individual site or group of site holdout hydrographs can be developed and routed downstream and subtracted from the composite holdout hydrograph to analyze their separate effect. . To properly evaluate the downstream effect of the combined structure release rates and their timing, the main time increment should be small enough to adequately define the hydrograph peak yet large enough so that the entire hydrograph will fit within the 300 elements of the hydrograph..