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<br />Because the pan evaporation at Grapevine Dam was used as input to <br />the model during calibration, it was desirable to use the equivalent for <br />the entire period of peak-discharge simulation (1914-70). Ratios of <br />monthly evaporation as measured at Grapevine Dam and at Denton were de- <br />fined from concurrent periods of observation. The ratios varied from <br />1.25 in December and January to 1.85 for May through July. These ratios <br />were applied to the records for Denton from January 1917 to October 1953. <br /> <br />Distance-Area Curves <br /> <br />The shape of the runoff hydrograph is dependent upon storage char- <br />acteristics and travel time. The routing procedure in the hydrologic <br />model transforms rainfall excesS from drainage-area increments into dis- <br />charge by application of proper travel time and reservoir delay. <br /> <br />Travel time was assumed to be proportional to distance. Isochronal <br />lines representing varying travel times were drawn on maps as simple arcs <br />with varying radii, with some weight given to abrupt changes in the shape <br />of the basins and directions of the channel. The effects of storm sewer- <br />ing on travel time were not investigated. Isochronal subareas were meas- <br />ured, accumulated, and plotted against channel distances to derive the <br />distance-area curves. <br /> <br />COMPARISON OF OBSERVED AND COMPUTED PEAK DISCHARGES <br /> <br />Observed peak discharges and flood volumes were compared with those <br />computed by the hydrologic model to determine if the model produced rea- <br />sonable results. Both the observed and computed data are given in table <br />6. The results are shown graphically as scatter diagrams for the control <br />basins (those for which observed flood-volume data were available to derive <br />moisture-accounting parameters) and noncontrol basins (those in which <br />moisture-accounting parameters were estimated from the parameters derived <br />for the control basins) in figures 4 and 5, respectively. <br /> <br />For the control basins, the average standard error for simulated <br />flood peaks is 26 percent; for the noncontrol basins, .the average standard <br />error for the simulated flood peaks is 46 percent. <br /> <br />EXTENSION OF FLOOD RECORDS IN TIME AT GAGING STATIONS <br /> <br />~ <br /> <br />The long-term rainfall records collected by the National Weather <br />Service since 1914 at Dallas and nearby long-term records of measured or <br />estimated pan evaporation were used as input to the hydrologic model to <br /> <br />-16- <br />