Small watershed response models: Hydrologic or hydraulic?

The response of a small, 32.3 hectare (80 acre), watershed in Harris County to rainfall is simulated entirely hydrologically using the geomorphic instantaneous unit hydrograph (GIUH) methods of Cleveland and others. The simulation is repeated using SWMM 5.0 and only hydraulic elements (entirely hydraulic simulation) for the same watershed, and the same rainfall input. The results are compared to observed rainfall and runoff for the watershed, and produce similar performance. The two simulation approaches are comparable to each other suggesting, among other things, that: 1 A hydrologic model, adequately parameterized captures more than enough hydraulic behavior to estimate watershed response. 2 A hydraulic model for a small watershed is a feasible tool to parameterize a hydrologic model. Such an exercise would be of value when constructing complex integrated hydrologic models, and when incorporating physical changes on the watershed from development, drainage improvements, etc. 3 The GIUH results in this study for this fully developed watershed indicates that the topographic relief and a characteristic velocity reflective of full development captures the essential behavior of the watershed. These findings indicate that hydraulic models can be used to develop a hydrologic responses that are fully traceable in terms of physical characteristics on the watershed, and that the decision to use a hydraulic model or hydrologic model for small watersheds is one of analyst choice, data availability, and project need.