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The climate of Dallas is generally temperate with hot summers and mild winters. Mean temperatures range from 450F (7.20C) in January to 850F (300C) in July. The most common storms are thunderstorms that occur frequently in the spring and summer. Long-duration low-intensity storms triggered by southward-moving continental polar fronts occur during the fall and winter. In late summer and early fall, hurricanes moving inland from the Gulf of Mexico cause some of the heaviest rainfall. Individual storms, although most frequent in the spring, may cause serious flooding during any season. Mean annual rainfall at Dallas for 1913-70 was 34.90 inches (88.6 centimeters). Lake evaporation usually exceeds rainfall from late March to early November. The major stream draining the area is the Trinity River, which di- vides the city into two parts. The principal tributaries are Joes Creek, Bachman Branch, Turtle Creek, White Rock Creek, Coombs Creek, Cedar Creek, and Fivemile Creek. Most streams and their tributaries have well incised channels with steep banks of limestone, particularly in the upper reaches. The average channel slopes commonly exceed 30 feet per mile (5.7 meters per kilometer). DESCRIPTION OF THE HYDROLOGIC MODEL A digital model developed by Dawdy, Lichty, and Bergmann (1972) and modified by Lichty (written commun., 1971) was used to simulate long rec- ords of peak discharges under existing conditions of urbanization. The structure of the model is shown by the diagram on figure 2. The input pa- rameters are identified in table 1. Figure 2 shows the general sequence of computations and shows that the output from one component is the input to the next. The antecedent-moisture accounting component (fig. 2), which is a more sophisticated version of the antecedent-precipitation index (API), measures the effects of antecedent conditions on the infiltration compo- nent. The infiltration component is based on an equation described by Philip (1954), in which infiltration rates are computed as a function of soil moisture and rainfall intensity. Infiltration does not occur in impervious areas, but some retention does occur. The model assumes 0.05 inch (0.12 centimeter) of water depth as the maximum retention in an impervious area. The surface-routing component (fig. 2) is based on unit hydrograph concepts (Sherman, 1932), and assumes a conceptual model composed of linear reservoirs and channels. Rainfall excess is converted into flood hydrographs by procedures representing the effects of varying travel times and reservoir delays, which are derived from distance-area curves. The derivation assumes that travel time and reservoir delay are propor- tional, but some flexibility is permissible. -5-