The role of unit hydrograph theory in the flood prediction process is to provide an estimate of streamflow given the precipitation. A unit hydrograph shows the temporal change in flow, or discharge, per unit of runoff from excess precipitation. In other words, the unit hydrograph shows how the flow of a stream will be affected over time by the addition of one unit of runoff. This module offers a thorough introduction to the use of unit hydrographs and the application of unit hydrograph theory in flood prediction. Through use of rich illustrations, animations, and interactions, this module explains key terminology and assumptions, outlines the steps in creation of a unit hydrograph, examines the issues surrounding application of unit hydrograph theory, and discusses important considerations for forecasters.
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1. Define key features of unit hydrographs and of unit hydrograph theory.
• Explain why we need unit hydrographs.
• Describe how unit hydrograph theory is used as a tool in forecasting runoff.
• Define the basic components of a unit hydrograph.
2. Identify important terms and assumptions in unit hydrograph theory.
3. Explain key issues for application of unit hydrograph theory.
• Identify reasons that some precipitation events may not be accurately represented by a unit hydrograph.
• Explain the impact of using English or metric units of measure.
• Describe the process for application of unit hydrographs to storms covering multiple time durations.
4. Recognize the forecast implications of unit hydrograph theory for real precipitation events.
• Describe potential effects on actual hydrograph data based on storm coverage and basin changes.
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