GIS for Water Resources and Watershed Management - Chapter 6

CHAPTER 6 GIS Modeling and Visualization of the Water Balance during the 1993 Midwest Flood Pawel J. Mizgalewicz, W. Scott White, David R. Maidment, and Merrill K. Ridd INTRODUCTION The characterization of the water balance during a major flooding event is useful to scien-tists, land-use planners, and government agencies who need such information to develop strategies for coping with future floods. Recent technological developments in the form of com-puter mapping and analysis databases have provided the means to accurately analyze flood-re-lated data sets, such as streamflow and precipitation measurements, over extended periods of time. This study investigates the use of geographic information systems (GIS) to model spatially distributed and time-varying hydrologic and meteorologic data sets, and the use of scientific vi-sualization techniques in the interpretation of the results. The data sets referred to deal specifi-cally with the 1993 Midwest flood which affected a large part of the Upper Mississippi River Basin. The study area encompasses the main stem of the Mississippi River above Cairo, Illinois, and the main stem of the Missouri River below Gavins Point Dam near Yankton, South Dakota. Also included are the many tributaries of these rivers such as the James, Des Moines, Illinois, and many other rivers and streams. This region of nearly 700,000 km2 was previously defined by the Scientific Assessment and Strategy Team (SAST) as the area most affected by the spring and summer floods of 1993 (SAST, 1994). States included in this study are Illinois, Indiana, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota, and Wis-consin (Figure 6.1). Exactly 180 Hydrologic Unit Code (HUC) subbasins (8 digit) cover the study area. The goal of this project is twofold: 1. Develop, demonstrate, and document procedures used to model the water balance of a large-scale flooding event, such as the 1993 Midwest floods, utilizing hydrologic and meteoro-logic data sets in a GIS database; and 2. Develop a comprehensive understanding, in terms meaningful to scientists as well as policy and decision makers, of the climatic and hydrologic conditions related to the 1993 floods and the spatiotemporal dynamics of the events. This understanding will result from the use of visualization software in the depiction of water storage change over the land surface dur-ing the flooding period. 61 © 2003 Taylor & Francis Chapters 1, 3, 5 & 6 © American Water Resources Association; Chapter 13 © Elsevier Science; Chapter 14 © American Society for Photogrammetry and Remote Sensing 62 GIS FOR WATER RESOURCES AND WATERSHED MANAGEMENT Figure 6.1. Study area—Upper Mississippi and Lower Missouri River Basins. SURFACE HYDROLOGY CONSIDERATIONS The water balance of a particular area represents a measure of the inputs to the hydrologic sys-tem and the outputs from that system over a specified period of time. In the case of this study, daily water balance maps were desired, therefore a decision was made to partition the study area into incremental drainage units based on the location of stream gauge stations. These drainage units or gauging station zones provided the basis on which areal interpolation of precipitation measurements were made. The outlets of the 180 HUC boundaries rarely corresponded with a stream gauge site, thus necessitating the subdivision of the study area into customized zones. The creation of the gauging station zones is shown in Figure 6.2. There are three possible zones: (1) no inflow; (2) one inflow; or (3) two or more inflows. The equation for calculating the water balance of each gauging station zone is: dS/dt = I – Q (1) where S is the volume of water stored in each gauging station zone, t is the time index, and I and Q are inflow to the gauging station zone and outflow from the gauging station zone, respectively. The term on the right (I – Q) can be rewritten to account for the area of the gauging station zone: © 2003 Taylor & Francis Chapters 1, 3, 5 & 6 © American Water Resources Association; Chapter 13 © Elsevier Science; Chapter 14 © American Society for Photogrammetry and Remote Sensing GIS MODELING, VISUALIZATION OF THE WATER BALANCE DURING THE 1993 MIDWEST FLOOD 63 Figure 6.2. Gauging station zone configurations based on (a) no inflow, (b) one inflow, and (c) two or more inflows. I – Q = (P – E)A + SQin – Qout (2) where P is the rate of precipitation, E is the rate of evaporation, A is the area of the gauging station zone, SQin is the sum of the inflows recorded at gauges whose flows enter the zone and Qout is the outflow at the downstream end of the zone. The change in storage ( S) can be found by combining Equations 1 and 2 and integrating each term over a time interval of one day ( t): S = P tA – E tA + SQin t – Qout t (3) In order to express S in terms of average depth over the gauging station zone area, Equation 3 can be rewritten as the following ( y = S/A): y = P t – E t + (1/A)[SQin t – Qout t] (4) where y is the change in storage depth per unit area of a gauging station zone. The values of y were computed on a daily basis (01/01/93 through 09/30/93) for each zone. GIS AND WATERSHED DELINEATION The raster GIS program GRID, a module available in the ARC/INFO® software package, al-lows for several hydrologic modeling procedures including the determination of flow direction, flow downstream accumulation, and watershed delineation. One of the strengths of this cell-based modeling package is the availability of map algebra functions. With map algebra, the variables in a logical expression actually are map (raster grid) layers. Algebraic manipulation of these grids can be performed at the local or individual cell level, neighborhood level (cells surrounding the cell of interest), zonal level (entire cell groups change in value), or at a global level (entire grid changes in value) (Tomlin, 1990). Prior to delineating the gauging station zones, several data sets were acquired from the GCIP Reference Data Set (GREDS) CD-ROM, produced by the U.S. Geological Survey. This CD-ROM is a collection of several geographic reference data sets of interest to the global change research community (Rea and Cederstrand, 1994). ARC/INFO export files of 8-digit HUC boundaries and current streamflow gauge sites were obtained from this CD-ROM, as was a 15² digital elevation model (DEM) of the region (500 meter resolution). RF1 river reach files in ARC/INFO export for-mat for the Upper Mississippi and Missouri Rivers were downloaded via the Internet from the USGS node of the National Geospatial Data Clearinghouse (http://h2O.er.usgs.gov/nsdi/ wais/water/rf1.HTML). The first step in creating the gauging station zones was to edit the RF1 vector coverage to re- © 2003 Taylor & Francis Chapters 1, 3, 5 & 6 © American Water Resources Association; Chapter 13 © Elsevier Science; Chapter 14 © American Society for Photogrammetry and Remote Sensing 64 GIS FOR WATER RESOURCES AND WATERSHED MANAGEMENT move circular arcs and isolated lakes. Once a definite stream network was available, the vector coverage was converted to a raster grid, and then embedded into the 15² DEM, which had been corrected for spurious sinks or pits. This embedding procedure “raised” the elevation of the off-stream grid cells surrounding the network in the DEM relative to the stream grid cells. By embed-ding these stream cells, a more precise network was created on the DEM that exactly matched the original paths of the RF1 vector coverage. This stream “burn in” technique has been shown to be particularly effective in areas of low relief (Maidment, 1996). The resulting grid was then clipped with a polygon coverage of the study boundaries. A 50 km buffer had been applied to this bound-ary coverage to account for drainage outside of the study area. The next steps involved the actual delineation of the stream network within their embedded channels on the DEM. In ARC/INFO’s version of this process, the flow direction is first determined by examining the neighborhood of eight grid cells that surround the cell of interest. The flow direc-tion function identifies the lowest cell value in the neighborhood, and assigns a flow direction value to the corresponding cell in an output grid, thus creating an implicit stream network between cell centroids. Once this process was complete over the entire study area, a flow accumulation grid was made. The flow accumulation function in ARC/INFO uses the flow direction grid to determine the number of “upstream” cells. Anew value is assigned to the cells in an output grid showing the num-ber of cells that contribute or flow to downstream cells. High values indicate confluences of streams, whereas values of zero indicate watershed boundaries (Maidment, 1995). A conditional statement was then set up in ARC/INFO which isolated those cells that met a certain threshold of flow accumulation. Stream links were created, and the stream network was then in place. After a suitable terrain model had been made, the next step was to precisely locate the USGS gauge stations on the stream grid. The point coverage of station locations was converted to a grid, and then viewed as a background grid against the stream grid. Unfortunately, most of the gauge cells did not lie directly on top of a stream cell, so the stream cell closest to a gauge cell was given a unique value (the USGS-assigned station number) to differentiate it from adjacent nongauge stream cells. Initially, 460 gauge cells were located on the stream grid; however, it was later deter-mined that a number of these stations contained incomplete streamflow records. A total of 50 gauges were removed which did not have complete records. This left 410 gauges, which were un-evenly distributed over the stream network. Some streams had many gauges, whereas other stream segments contained one or less. The gauge locations were viewed along with the HUC coverage to provide a better spatial representation of the gauges with respect to watersheds. It was decided that those gauges whose contributing drainage areas (an attribute in the ARC/INFO point coverage of the gauges) were less than 1,000 km2 would be removed from the collection. Also removed were gauges that were concentrated in a particular watershed. Most HUCs contained between one and three gauge stations, but several contained more than three. Unless the HUC was large in area, those extra gauge stations exceeding three were also removed from the collection. Approximately 260 gauge stations on the stream grid were retained, and these gauge locations were for the most part uniformly distributed throughout the stream network. The gauging station zones were then determined using the flow direction grid and the edited stream gauge grid (Figure 6.3). The resulting grid contained approximately 260 subbasins defined on the basis of the stream gauge locations. Each gauging station zone was checked against its cor-responding HUC to ensure that the two sets of boundaries were mutually compatible. GIS DATABASE AND MAP CREATION Streamflow daily values for the 1993 water year were provided by the Water Resources Divi-sion of the USGS. After removing the 1992 values and reformatting the data into comma-delim- © 2003 Taylor & Francis Chapters 1, 3, 5 & 6 © American Water Resources Association; Chapter 13 © Elsevier Science; Chapter 14 © American Society for Photogrammetry and Remote Sensing GIS MODELING, VISUALIZATION OF THE WATER BALANCE DURING THE 1993 MIDWEST FLOOD 65 Figure 6.3. Map of gauging station zones defined on the basis of the location of USGS streamflow gauge stations. ited ASCII text, the values were incorporated into the attribute table of the gauge station point cov-erage. This point coverage reflected the edits made to the stream gauge grid. Complete daily records for the gauge stations were assigned to the point coverage for a total of nearly 71,000 daily values from January 1 to September 30, 1993 (273 days). Precipitation values obtained from National Climatic Data Center Summary of the Day files were treated in a manner similar to the streamflow data. A total of 1,078 climate stations were mapped. This number includes stations within the study area, and those within a 50 km buffer of © 2003 Taylor & Francis Chapters 1, 3, 5 & 6 © American Water Resources Association; Chapter 13 © Elsevier Science; Chapter 14 © American Society for Photogrammetry and Remote Sensing ... - tailieumienphi.vn