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Western Dam Engineering Technical Note May 2017 13 classification and can be defined with return periods ranging from a 50-year event to extreme events with return periods less frequent than a 1,000-year event (like the probable maximum flood [PMF]). Step 2: Develop the IDF storm hyetograph  Estimate a depth-duration-frequency relationship (from local precipitation gages, gage-adjusted radar rainfall data, NOAA Atlases, Hydrometeorological Reports [HMRs], Site-Specific or Statewide Probable Maximum Precipitation [PMP] studies, etc.)  Estimate areal reductions (if applicable).  Estimate elevation reductions (if applicable).  Estimate spatial and temporal distributions. Step 3: Estimate watershed loss parameters and excess precipitation  The most pertinent watershed loss parameters (as they relate to dam safety) are surface retention (initial losses) and infiltration.  Some common methodologies to estimate these loss parameters include: Green and Ampt, Natural Resources Conservation Service (NRCS) Curve Number, and Initial and Constant Loss. Step 4: Transform watershed excess precipitation to discharge hydrographs  Excess precipitation (i.e., runoff) is translated to discharge hydrographs using a translation methodology—the unit hydrograph is generally the most preferred and is based on physical watershed properties and associated flood travel times.  Some common methodologies to estimate unit hydrographs include: o Watershed-Specific Unit Hydrographs – Use watershed precipitation and stream discharge data. o Synthetic Unit Hydrographs – Where insufficient watershed data is available, common methodologies include those of the U.S. Bureau of Reclamation, NRCS, Clark, and the U. S. Geological Survey (USGS). Step 5: Estimate watershed runoff volumes and hydrographs  Use a hydrologic model, like HEC-HMS, and the data estimated in the previous steps to estimate peak runoff rates and volumes.  If applicable, combine and route sub- watershed hydrographs through the watershed drainage network watercourses. Some common watercourse routing methodologies include: Kinematic Wave, Muskingum-Cunge, Lag, and Modified Puls. So that’s runoff in a nutshell—the reader is encouraged to reference the WDETN runoff article [3] for a more detailed discussion. But how do we know if the flood modeling runoff results are reasonable and reliable? This is where a variety of calibration and validation techniques and methodologies can be applied to justify and refine results and the input parameters that contribute to them. Watershed Model Calibrations General Discussion and Overview Model calibrations comprise adjustments to model input watershed parameters such that model results closely approximate observed or predicted data. Watershed parameters are initially estimated and adopted based on the methodologies and techniques discussed in the previous runoff review section (as well as in the WDETN runoff article [3]). Although these methodologies are reflective of industry standards and state of the practice, they typically require adjustments to produce reasonably accurate results. Figure 2 presents an example in which the modeled runoff volume, time to peak discharge, and the peak discharge do not reasonably replicate measured discharge data, which could indicate that the model results are inaccurate and/or questionable. To justify selection of adopted hydrologic model input parameters and verify associated model runoff results, model calibrations and validations are necessary.