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Modeling Wildfire Impact on Hydrologic Processes

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Modeling Wildfire Impact on Hydrologic Processes MODELING WILDFIRE IMPACT ON HYDROLOGIC PROCESSES USING A PRECIPITATION-RUNOFF MODEL by Ryan Logan A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Civil and Environmental Engineering). Golden, Colorado Date Signed: Ryan Logan Signed: Dr. Terri Hogue Thesis Advisor Golden, Colorado Date Signed: Dr. John McCray Professor and Department Head Department of Civil and Environmental Engineering ii ABSTRACT As large magnitude wildfires persist across the western United States, understanding their impact on hydrologic behavior and predicting regional streamflow response is increasingly important. Peak flows, sediment flows, and debris flows in burned watersheds are often addressed, but wildfires also alter the timing and overall volume of runoff, making the prediction of post-fire streamflow critical for water resources management. Six wildfire-impacted watersheds in the western United States are modeled using the Precipitation Runoff Modeling System (PRMS), a distributed-parameter, physical process based watershed model. Two change detection modeling approaches are applied in order to better understand post-fire changes and their related processes. First, the model is used to determine if each watershed shows significant changes in flow regimes following each wildfire. Second, post-fire parameterization is examined using a generalized likelihood uncertainty estimation (GLUE) approach and a national-scale sensitivity analysis. Three of the six watersheds showed significant increases in the difference between observed and modeled daily streamflow following the wildfire. For these watersheds, the parameterization analysis using PRMS revealed that changes in immediate surface runoff processes are best represented through preferential flow and imperviousness, and changes in evapotranspiration can be best represented through soil zone capacities. iii TABLE OF CONTENTS ABSTRACT................................................................................................................................................. iii LIST OF FIGURES ..................................................................................................................................... vi LIST OF TABLES ...................................................................................................................................... vii ACKNOWLEDGEMENTS ....................................................................................................................... viii CHAPTER 1 INTRODUCTION .............................................................................................................. 1 1.1 Watershed Impacts .............................................................................................................. 1 1.2 Post-wildfire Modeling ....................................................................................................... 3 1.3 The Precipitation Runoff Modeling System ....................................................................... 4 1.4 The National Hydrologic Model ......................................................................................... 6 1.5 Study Objectives ................................................................................................................. 7 CHAPTER 2 STUDY AREAS AND MODEL SETUP ........................................................................... 8 2.1 Burned Area Determination ................................................................................................ 8 2.2 Identification of Case Study Watersheds ............................................................................ 9 2.3 Description of Case Study Watersheds ............................................................................. 11 2.4 Identification of Burned National HRUs .......................................................................... 15 2.5 Model Calibration Methods .............................................................................................. 16 2.5.1 General Calibration Scheme ..................................................................... 16 2.5.2 Model Forcing ........................................................................................... 20 2.6 Calibration Results ............................................................................................................ 20 2.6.1 Station Distribution Methods .................................................................... 21 2.6.2 PRISM Method ......................................................................................... 22 CHAPTER 3 CHANGE DETECTION .................................................................................................. 24 3.1 The Change Detection Method ......................................................................................... 24 3.2 Study Methods .................................................................................................................. 25 iv 3.3 Results ............................................................................................................................... 26 CHAPTER 4 POST-FIRE PARAMETERIZATION ............................................................................. 32 4.1 Case Study Watershed Parameterization .......................................................................... 33 4.1.1 Methods ..................................................................................................... 33 4.1.2 Results ....................................................................................................... 35 4.2 Evapotranspiration ............................................................................................................ 40 4.2.1 National Model Vegetation and Evapotranspiration Response ................. 40 4.2.2 National Model Vegetation and Evapotranspiration Sensitivity ............... 41 CHAPTER 5 CONCLUSIONS .............................................................................................................. 45 REFERENCES ........................................................................................................................................... 47 v LIST OF FIGURES Figure 1.1 Hydrologic Process modeled in PRMS (from Markstrom et al. 2015). .............................. 5 Figure 2.1 Areas considered as study area watersheds. (a) All GAGES II watersheds in the NHM with over 25% total burn. (b) Those GAGES II with no significant dams. (c) Those watersheds selected as case study watershed based on performance with a calibration scheme .............................................................................................................................. 10 Figure 2.2 HRU delineation and burn severities for the case study watersh ...................................... 14 Figure 2.3 Burned National HRUs ..................................................................................................... 15 Figure 2.4 Nash-Sutcliff Efficiency for all NHM basins for which the general calibration scheme was applied ....................................................................................................................... 16 Figure 3.1 Annual volume comparisons. Watersheds in the top row showed a statistically significant increase in daily flow residuals ....................................................................... 30 Figure 3.2 Monthly volume comparisons. Watersheds in the top row showed a statistically significant increase in daily flow residuals. ...................................................................... 31 Figure 4.1 Surface runoff parameter ranges of the top 50 behavior models in the GLUE analysis for each watershed for each time period. .......................................................................... 36 Figure 4.2 Interception parameter ranges of the top 50 behavior models in the GLUE analysis for each watershed for each time period. ................................................................................ 38 Figure 4.3 Evapotranspiration parameter ranges of the top 50 behavior models in the GLUE analysis for each watershed for each time period ............................................................. 39 Figure 4.4 Solar radiation parameter ranges of the top 50 behavior models in the GLUE analysis for each watershed for each time period. .......................................................................... 39 Figure 4.5 Comparison of ensemble mean time series for remote sensing procudts, NDVI and PRMS actual evapotranspiration for all burned national HRUs ....................................... 42 Figure 4.6 Method used by PRMS to calculate evapotranspiration based on soil zone saturation for each soil type. .............................................................................................................. 43 Figure 4.7 Sensitivity of actual evapotranspiration to soil_moist_max. (Left) the first 100 HRUs are shown for simplicity. (Right) HRUs are categorized by aridity index and the ensemble mean for each range is shown. .......................................................................... 44 vi LIST OF TABLES Table 1.1 Datasets used in the parameterization of the National Hydrologic Model ......................... 7 Table 2.1 Characteristics of the Case Study Watersheds .................................................................. 14 Table 2.2 Case study watershed burn severities
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