ABSTRACT Title of Thesis: SENSITIVITY OF A SHENANDOAH WATERSHED MODEL TO DISTRIBUTED RAINFALL DATA Emily K. Hill, Master of Science, 2013 Directed By: Dr. Kaye L. Brubaker Department of Civil and Environmental Engineering Precipitation is a primary input for hydrologic modeling. In this study, the use of spatially varying gridded precipitation data obtained from the North American Land Data Assimilation System (NLDAS) is investigated for possible improvements to stream flow predictions within the Shenandoah River watershed over the use of gage based precipitation data. The sensitivity of hydrologic responses to gridded and gage precipitation data is analyzed using the Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS). Precipitation is the only input which varies between models. Model performance of each precipitation input is assessed by comparing predicted and measured stream flow during the 1995 to 1996 water year and calculating a number of goodness of fit measures. Results indicate that the use of spatially varying gridded precipitation data can improve stream flow predictions within the Shenandoah River watershed. Future research directions include the calibration and validation of the HEC- HMS model using the gridded precipitation data. SENSITIVITY OF A SHENANDOAH WATERSHED MODEL TO DISTRIBUTED RAINFALL DATA by Emily K. Hill Thesis submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirement for the degree of Master of Science 2013 Advisory Committee: Associate Professor Kaye L. Brubaker, Chair Professor Richard H. McCuen Research Professor Gerald E. Galloway, Jr. Table of Contents Chapter 1: Introduction ................................................................................................................ 1 1.1 Introduction ............................................................................................................................ 1 1.2 Project Objectives .................................................................................................................. 2 1.3 Introduction to Study Area .................................................................................................... 2 1.3.1 Potomac River Basin ....................................................................................................... 2 1.3.2 Shenandoah River Basin ................................................................................................. 4 Chapter 2: Literature Review ....................................................................................................... 7 2.1 Introduction of a Watershed ................................................................................................... 7 2.2 Overview of Hydrologic Modeling ........................................................................................ 8 2.2.1 Lumped Hydrologic Models ........................................................................................... 9 2.2.2 Distributed Hydrologic Models .................................................................................... 10 2.2.3 Semi-Distributed Hydrologic Models ........................................................................... 11 2.3 Current Semi-Distributed Hydrologic Modeling Environments .......................................... 12 2.3.1 VIC ................................................................................................................................ 12 2.3.2 TOPMODEL ................................................................................................................. 14 2.3.3 HEC-HMS ..................................................................................................................... 15 2.4 Introduction of Rainfall Data ............................................................................................... 28 2.4.1 NLDAS ......................................................................................................................... 28 2.5 Introduction of HEC-GeoHMS ............................................................................................ 32 2.6 Current Studies ..................................................................................................................... 35 Chapter 3: Methods and Model Setup ....................................................................................... 37 3.1 GIS Watershed Processing ................................................................................................... 37 3.1.1 GIS Watershed Processing ............................................................................................ 37 3.1.2 Terrain Processing and Watershed Delineations .......................................................... 39 3.1.3 HEC-HMS Project Setup .............................................................................................. 44 3.1.4 Basin Processing ........................................................................................................... 44 3.1.5 Stream and Watershed Characteristics .......................................................................... 45 3.1.6 Hydrologic Parameter Estimation ................................................................................. 47 3.1.7 HEC-HMS Model Setup ............................................................................................... 56 ii 3.2 Other Parameter Estimations ............................................................................................... 58 3.2.1 Storage Parameters ........................................................................................................ 58 3.2.2 Soil Moisture Accounting Parameters .......................................................................... 59 3.2.3 Clark UH/ModClark Parameters ................................................................................... 60 3.2.4 Initial Baseflow ............................................................................................................. 61 3.2.5 Evapotranspiration ........................................................................................................ 61 3.3 Precipitation Processing ....................................................................................................... 62 3.3.1 NLDAS ......................................................................................................................... 62 3.3.2 Inverse Distance ............................................................................................................ 64 3.4 General Methods for Model Analysis .................................................................................. 66 3.4.1 Precipitation Data .......................................................................................................... 66 3.4.2 Model Goodness of Fit Measures ................................................................................. 66 3.4.3 Other Criteria ................................................................................................................ 68 Chapter 4: Model Results ........................................................................................................... 70 4.1 SCS Loss Method ................................................................................................................ 70 4.2 Data and Model Analysis ..................................................................................................... 75 4.2.1 Precipitation Data .......................................................................................................... 75 4.2.2 Model Goodness of Fit Measures ................................................................................. 85 4.2.3 Other Criteria ................................................................................................................ 96 Chapter 5: Conclusion and Discussion ...................................................................................... 98 5.1 Conclusion ........................................................................................................................... 98 5.2 Data and Model Analysis ................................................................................................... 100 5.2.1 Calibration and Validation .......................................................................................... 100 5.2.2 Timing Differences ..................................................................................................... 101 5.2.3 Gridded SMA Method ................................................................................................ 101 Appendix A ................................................................................................................................. 102 Appendix B ................................................................................................................................. 109 References ................................................................................................................................... 113 iii Chapter 1 Introduction 1.1 Context and Motivation Currently, many hydrologic studies employ gage rainfall data as precipitation inputs to watershed models. However, distributed precipitation data, such as Next-Generation Radar (NexRad) and NASA’s North American Land Data Assimilation Systems (NLDAS), are becoming more accessible. Therefore it is important to understand the implications of using