August 2016 Hydrology Technical Note No. 4 Hydrologic Analyses of Post-Wildfire Conditions Natural Resources Conservation Service Title 210, Hydrology Technical Note 4, Aug 2016 Title 210 – Hydrologic Analyses of Post-Wildfire Conditions August 2016 Cover photo: Nabours Mountain directly east of Glenwood, NM. Photo courtesy of USDA Forest Service, Gila National Forest, Silver City, NM The USDA prohibits discrimination against its customers, employees, and applicants for employment on the bases of race, color, national origin, age, disability, sex, gender identity, religion, reprisal, and where applicable, political beliefs, marital status, familial or parental status, sexual orientation, or all or part of an individual's income is derived from any public assistance program, or protected genetic information in employment or in any program or activity conducted or funded by the Department. (Not all prohibited bases will apply to all programs and/or employment activities.) 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Title 210 Hydrology Technical Note 4, Aug 2016 Title 210 – Hydrologic Analyses of Post-Wildfire Conditions Acknowledgments This technical note was developed, as noted, by Dan Moore, Hydraulic Engineer, West National Technology Support Center, NRCS, Portland, OR, (Case Study 4: Whitewater Creek; Gila Wilderness, NM); Nathaniel Todea, Hydraulic Engineer, NRCS, Salt Lake City, UT, (Case Study 5: Saratoga Springs Wildfire, UT);Geoff Cerrelli, retired, Hydraulic Engineer, NRCS, (Case Study 2: Bitterroot Wildfires, MT);Steven Yochum, Hydrologist, U.S. Forest Service, Fort Collins, CO, (formerly NRCS, Lakewood, CO), (Case Study 1: High Park Fire, CO, and Case Study 3: West Fork Complex Fire, CO); John B. Norman, III, Soil Scientist, Major Land Resource Area, Soil Survey Office, NRCS, Fort Collins, CO (Case Study 3: West Fork Complex Fire, CO); and Claudia Hoeft, National Hydraulic Engineer, NRCS, Washington DC. Title 210 Hydrology Technical Note 4, Aug 2016 Title 210 – Hydrologic Analyses of Post-Wildfire Conditions Table of Contents Introduction .............................................................1 The Role of NRCS in Post-Fire Assessments and Modeling ....................1 Important Terms and Terminology . 2 Burned Area Reflectance Classification (BARC).......................................3 Burn Severity ..................................................................3 Fire Intensity...................................................................3 Fire Severity ...................................................................4 Hydrophobicity or Soil Water Repellency ........................................... 4 Landsat Differenced Normalized Burn Ratio (dNBR) ..................................4 Rapid Assessments of Vegetation Condition After Wildfire (RAVG).......................4 Soil Burn Severity Mapping .......................................................5 Low Soil Burn Severity ...............................................................5 Moderate Soil Burn Severity...........................................................5 High Soil Burn Severity...............................................................5 Wildfire Impacts on Watershed Hydrology . 5 Assessment of Post-Fire Soil and Vegetation Conditions................................6 Assessment of Post-Fire Soil Hydrophobic Conditions ..................................7 Hydrologic Modeling of Burned Watersheds .................................9 Methods and Models .............................................................9 Adjustment of Event-Based Runoff Modeling Components for Burned Areas...................10 Runoff CNs . ..10 Adjustment of CNs for Wildfire Effects .................................................12 Limitations of the CN Method.........................................................15 Time of Concentration . 16 Watershed Lag Method . 16 Velocity Method . 16 Adjustment of Runoff Flow Time for Wildfire Effects . 18 Infiltration Parameters..........................................................20 Adjustment of Infiltration Parameters for Wildfire Effects .................................22 Unit Hydrographs ..............................................................22 NRCS Dimensionless Unit Hydrograph .................................................24 Adjustment of Unit Hydrograph for Wildfire Effects.......................................25 Kinematic Wave Transformation..................................................27 Adj. of Kinematic Wave Transformation Parameters for Wildfire Effects ......................27 Time-Area Histogram Synthetic Unit Hydrograph Development ............................27 Title 210, Hydrology Technical Note 4, Aug 2016 Title 210 – Hydrologic Analyses of Post-Wildfire Conditions Sedimentation Estimation ................................................29 Models for Estimating Sediment Transport .........................................33 Adjustments for Modeling Sediment Transport From Burned Areas..........................33 Sediment Bulking ..............................................................34 Model Uncertainties, Calibration, Validation ...............................36 References ..............................................................37 Appendix A: Case Study 1, High Park Fire, Colorado Background............................................................A–1 Methods ...............................................................A–2 Runoff CN Estimation .........................................................A–3 Rainfall .....................................................................A–4 Lag Time ....................................................................A–5 Flow Routing .................................................................A–5 Sediment Bulking .............................................................A–5 StreamStats..................................................................A–5 Results and Discussion ..................................................A–6 Comparison With Regression Predictions ..........................................A–8 Accuracy and Limitations.......................................................A–8 Conclusions ............................................................A–9 Acknowledgements .....................................................A–9 References .............................................................A–9 Appendix B: Case Study 2, Bitterroot Wildfires, Montana Background............................................................B–1 Methods ...............................................................B–4 Runoff CN Estimation for Burn Areas.............................................B–7 Hydrophobic Soils ............................................................B–10 Time of Concentration (Tc) and Assumed Watershed Shape ..........................B–10 Results and Discussion .................................................B–11 Limitations .................................................................B–12 References ............................................................B–13 Title 210, Hydrology Technical Note 4, Aug 2016 Title 210 – Hydrologic Analyses of Post-Wildfire Conditions Appendix C: Case Study 3, West Force Complex Fire, Colorado Background............................................................C–1 Methods ...............................................................C–3 Runoff CN Estimation .........................................................C–4 Rainfall .....................................................................C–6 Lag Time ....................................................................C–6 Flow Routing .................................................................C–7 Sediment Bulking .............................................................C–7 Erosion Modeling .............................................................C–7 Results and Discussion .................................................C–13 Runoff Modeling Results.......................................................C–14 Sediment Modeling Results ....................................................C–14 Modeling Limitations . .C–16 Rainfall-Runoff Modeling Limitations............................................C–16 Sediment Modeling Limitations.................................................C–17 Conclusions ...........................................................C–17