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by Ihe model with the corresponding measured hydro- graph from the prototype. From the results, relation. ships between model coefficienls and various urbaniza. tion characteristics, such as percent impervious cover, are established. These relalionships then can be ap. plied in predicting the effects of future urban develop- ment. For urban flood sludies, only surface runoff need be considered. The relevant processes for both rural and urban drainage areas are shown within the dotted line of Figure 3.2. Experimental and analytical results also are used in establishing and testing the mathematical relation. ships included with the model. Hydrologic parameters needed for operation of the hydrologic model are esti- mated: 1) from available data, 2) by slatistical corre. lation techniques, and/or 3) through calibration of the model ilself. The Hydrologic Balance A dynamic system consists of 1) a medium or media acted upon, 2) a set of constraints, and 3) an energy supply or driving force. In a hydrologic sys- tem, water is the medium of interest. The constraints are properties of the physical basin, and Ihe driving Eva oration I- I I I I I I L__ Precipitation, Pr Interce tion Water at Groundsurfac. Infiltration Root Zone Storage Evapotranspiration forces are supplied by solar energy, gravity, and cap- illary potential. The flow of water through the sys- tem is governed by principles of continuity of mass and momentum. Except where high velocilies are encountered, such as in channel flow, the effects of momenlum are negligible, and the continuity of mass predominates. Continuity of mass is expressed by the equation: Output = Inpul:t Change in slorage. . . (3.1) A hydrologic balance is the application of this equa. tion 10 physical or hydrologic measuremenls within a particular unit. It provides a basis for routing to pre- dict the rnovemenl of water through a system in space and lime. The inpuls to a hydrologic unit are pre. cipitalion and surface and groundwater inflow while the output is divided among surface outflow, ground- \Wter outflow, and evapotranspiration. As water passes through, storage changes occur on the land sur. face, in the soil moisture zone, in the groundwater zone, and in the stream channels. These changes oc- cur rapidly in surface locations and more slowly in the subsurface zones. I I I I I I I ---l Interflow Groundwater Outflow FIgUre 3.2. A schematic diagram of a typical hydrologic system. 24