CRWR Online Report 95-3

Transcription

1 CRWR Online Report 95-3 A GIS Procedure for Merging NEXRAD Precipitation Data and Digital Elevation Models to Determine Rainfall-Runoff Modeling Parameters by Seann M. Reed Research Assistant and David R. Maidment, Ph.D. Principal Investigator September, 1995 CENTER FOR RESEARCH IN WATER RESOURCES Bureau of Engineering Research.The University of Texas at Austin J.J.Pickle Research Campus. Austin, TX This document is available online via World Wide Web at

2 ACKNOWLEDGEMENT This research was supported by the United States Army Corps of Engineers Hydrologic Engineering Center in Davis, CA. Information provided by Norm Bingham and Dale Lillie at the Arkansas-Red Basin River Forecast Center (ABRFC) and Dong- Jun Seo and Dennis A. Miller at the NWS Hydrologic Research Laboratory at Silver Springs, Maryland, is also appreciated. 2

3 ABSTRACT The National Weather Service (NWS) Next Generation Weather Radar (NEXRAD) radar program generates a product called StageIII which offers gridded precipitation estimates spatially averaged over grid cells of approximately 16 km 2 and temporally averaged over 1 hour. Hydrologists need to consider how such distributed precipitation estimates may be translated into improved streamflow forecasts. Researchers at the U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC) have proposed using a modified version of the Clark unit hydrograph method to incorporate NEXRAD rainfall data into their streamflow forecasts. The proposed method requires information about the area of each rainfall cell falling within each modeled subbasin and the average flow length from each rainfall cell to the corresponding subbasin outlet. A set of programs was written to obtain this information using Arc/Info GIS and USGS digital elevation models. Properly positioning NEXRAD rainfall cells relative to digital elevation model cells is an important issue. A fundamental problem is that NEXRAD estimates are referenced to a spherical earth datum while data sets describing the land surface (i.e. digital elevation models) are most commonly referenced to an ellipsoidal earth datum. A study of the equations required to transform NEXRAD cells and digital elevation model cells into a common ellipsoid-based map projection is presented. 3

4 TABLE OF CONTENTS Acknowledgment 2 Table of Contents 3 List of Tables 4 List of Figures Introduction Executive Summary Description of Data NEXRAD Data Digital Elevation Models Gage Locations, HUCs, and RF1 Files Selecting a Study Area Scales of Analysis Choosing A Map Projection and Datum for the Study Methodology Obtain and Process Digital Elevation Model Create a Hydrologic DEM for the Region of Interest Locating Study Region Obtaining Digital Elevation Models Projecting the Digital Elevation Model Filling Sinks Process DEM for Stream and Watershed Delineation Creating a Point Coverage of Watershed Outlets Selecting Grid Cells for Watershed Outlets Delineating Watersheds Creating a Vector Coverage of Watersheds and Streams Process DEM for Travel Length or Travel Time Parameter 40 4

5 3.0 Positioning the NEXRAD StageIII Cells Relative to the Digital Elevation Model Cells Geodetic (Ellipsoidal) and Geocentric (Spherical) Coordinates Conversions Between Geodetic and Geocentric Latitudes The HRAP Coordinate System Forward Transformation from Geocentric to HRAP Coordinates From Geographic to Polar Coordinates From Polar to Cartesian Coordinates From Cartesian to HRAP Coordinates Reverse Transformation from HRAP to Geocentric Coordinates Using NEXRAD Data with an Ellipsoidal Datum Distortions Involved with Using the HRAP Coordinate System Scale Factor The Shape of HRAP Cells Shape Factor, C S Reconsidering the Mapping Problem Verifying Consistency with Arkansas-Red Basin River Forecast Center HRAP Cells Description of FORTRAN and AML Codes to Generate Cells and Transform to the Common Coordinate System Determine the Area and Mean Travel Length for Each HRAP Cell in a Subwatershed Intersect Processed Digital Elevation Model with the Coverage of HRAP Cells Results Conclusions 81 References 73 Appendix 86 5

6 LIST OF TABLES 1.1 Internet Addresses for Data Sources Parameters of Albers Map Projection Summary Script for Creating a Hydrologic DEM Flow Measurement Locations Summary Script for Stream and Watershed Delineation Differences Between Geodetic and Geocentric Latitudes for the GRS 80 Ellipsoid Approximate Scale Factor at Different Latitudes Coordinate Values for Figure 3.6a USGS Gaging Stations Identified from Arkansas-Red Basin River Forecast Center Snapshots Sample Session for Generating a NEXRAD Mesh 68 6

7 LIST OF FIGURES 1.1 StageIII Cells Overlaid on Watersheds at Tenkiller Procedure Overview Identifying the Tenkiller Hydrologic Cataloging Unit Buffered Tenkiller HUC with RF1 Features Creating a Drainage Network Delineated Tenkiller Streams and Watersheds with RF1 Streams and HUC Boundary Close-up of Dam Location A Few Misplaced Cells During Raster to Vector Conversion Computing Flowlength for Each Sub-basin Geocentric and Geodetic Coordinates Elevation View of a Polar Stereographic Map Projection Plan View of a Polar Stereographic Map Projection Conceptual Diagram of the Correct Steps in the HRAP to Albers Transformation Size and Shape of an HRAP Cell (701, 263) in Several Coordinate Systems a Four HRAP Cells Plotted in Geographic Coordinates b Four HRAP Cells Transformed into Albers Equal-Area Projection Arkansas-Red River Basin Control Points at the Corners of State Boundaries USGS Gaging Stations in the HRAP Plane Files Used to Create a Coverage of HRAP Cells HRAP Coverage Intersected with a Subwatershed Coverage Mean Flow Length from HRAP Cells to Subwatershed Outlets 78 7