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CHAPTER III MODELING THE HYDROlDGIC COMPONENT OF THE SYSTEM Introduction Urban land use planners must work with a limited available land area in supplying the needs of the com- munity for housing, commerce, industry, recreation, and transportation. Inlerspersed over the same land area, Ihe hydrologic system must carry away treated waste effluenl and often supplies water used by Ihose downstream. Simultaneously, it must carry away the slorm runoff from the city but convey Ihe flow al suf- ficiently low rales so as not to inflict excessive flood damages. Planners with urban flood water management responsibility need quanlitalive information on how storm runoff is affected by various urban land use pat- terns 1) so Ihat Ihey can effectively design structural measures for conveying the flood water through the urban area and/or 2) select nonslruclural measures 10 keep damageable property out of the way of the flood- ing that does occur. One approach 10 gelling lhis in- formation and using it in design is computer simulation using a mathematical model to investigate the behav- ior of the system. In the study reporled here, a com- puter model is used to simulate the hydrologic res- ponses of an urban watershed from measured variables representing urbanization. The model represenls the system by funclions which describe various physical phenomena on Ihe watershed and can be used to ap- praise proposed changes within the corresponding pro- totype. The Conceptual Models of the Hydrologic System The hydrologic models utilized in this sludy are outgrowihs of those developed earlier for both rural and urban watersheds (Narayana et aI., 1969, Evelyn el aI., 1970, Shih, 1971, and Chambers, 1973). The hydrologic models are developed Ihrough mathemali- cal representation of the various hydrologic processes and routine functions that are not specific to any par- ticular geography. The outflow hydrograph is modeled by chrono- logically deducting from precipitation input losses due to interception, inf1ltration, and depression storage and then routing the residual through surface and chan- nel slorages (Figure 3.1). Testing and verification of the basic mathematical model is done by using observed rainfall and runoff data from instrumented watersheds. The coefficienls representing interception, depression storage, and inftltralion are calibraled by trial-and- error matching of the outflow hydrograph predicted 1 ntercept ion Step 1 Infiltration Depression Ston'ile Step 2 Overland flow Routing Step 3 Tributary Channel Routing Step '+ Main Channel Routing Figure 3.1. Schematic representation of the steps used to obtain the rnnoff hydrograph 23