Exercise 6: Non-Point Bacterial Loadings to Copano Bay
CE 394K GIS in Water Resources – Fall 2005
University of Texas at Austin
Prepared by: Carrie Gibson and David R. Maidment
Table of Contents
· Goals of Exercise
· Computer and Data Requirements
· Procedure
o Downloading Data and Registering DLLs
o Calculating Runoff in Copano Bay Watershed
o Creating Fecal Coliform Concentration Grid
o Creating Non-Point Bacterial Load Grid
o Calculating Non-Point Bacterial Loading per Watershed
o Modeling Non-Point Bacterial Transport
o Summary of Items to be Turned in
Goals of Exercise
The goals of this exercise are to become familiar with the use of Model Builder and Spatial Analyst tools in ArcGIS 9.1, in order to model bacterial load and transport in the Copano Bay watershed.
The Copano Bay watershed contains three impaired water segments in which sampled data are exceeding Texas Surface Water Quality Standards for fecal coliform bacteria. These three water segments are Copano/Port/Mission Bay, Aransas River, and Mission River. Using tools in Arc Toolbox, an Arc GIS 9.1 model has been created to determine and accumulate the watershed bacterial loads for each of the impaired water body segments. Processing engines have also been developed (using dynamic linked libraries or DLLs) to simulate bacterial load transport. This exercise is going to replicate a portion of the modeling procedure.
Computer and Data Requirements
This exercise requires use of Arc GIS 9.1 using the Spatial Analyst toolbar. The data for this exercise can be downloaded from http://www.ce.utexas.edu/prof/maidment/giswr2005/ex6/bacterialmodel.zip.
This is a very large file (77MB) because of a large land use coverage it contains. I have put this zipped file and its unzipped contents (~220MB) on the class server in the LRC in class/Maidment/giswr2005/BacterialModel When you obtain this file, unzip it, and view the contents in MS Explorer. You will see:
And if you go to this folder and open it in ArcCatalog, you will see:
We’ll explore these various components and datasets as the exercise proceeds.
Procedure
- Downloading Data and Registering DLLs
You need to copy the entire BacterialModel.zip folder to your computer, unzip it, and runBacterialModel/_INSTALL.bat to register the necessary dynamic linked libraries (DLLs) and Toolboxes for ArcGIS. This will enable the Schematic Processor (what this is will be described later) to run properly. If you double-click on Install.bat, you will see it high-lighted and then some messages will appear asking whether you want to have dlls registered. Say ok and let the processes run.
- Calculating Runoff in the Copano Bay Watershed
Open ArcMap and add to the display the Watershed, SchemaLink and SchemaNode feature classes. Symbolize the watersheds by means of JunctionID to get something like the display shown below. What we are going to do in this exercise is to calculate the runoff and bacterial pollutant loading from each watershed and then transport them downstream through the stream system to Copano Bay along the “Schematic Network” formed by the SchemaLinks and SchemaNodes that are shown in the display. We are going to use a “Schematic Processor” function to do the mass balancing and decay of bacteria as they pass through the watersheds, into the rivers, and into the bay.
Runoff calculations are made by using the empirical equations derived from Ann Quenzer’s thesis, A GIS Assessment of the Total Loads and Water Quality in the Corpus Christi Bay System(http://www.crwr.utexas.edu/reports/1998/rpt98-1.shtml). The annual precipitation data is obtained from the National Resources Conservation Service (NRCS) and the Spatial Climate Analysis Service (SCAS) at Oregon State University (OSU). The land use land cover dataset was obtained from the United States Geologic Survey (USGS).(The web addresses for these data sources are given in the datasources.htm document that you received earlier in the semester.) These equations are used to predict runoff based on land use type:
Agriculture:
Q = 0.008312 * exp ( 0.011415 * P )
Forest:
Q = 0.0053 * exp ( 0.010993 * P )
Urban:
Q = 0.24 * P
Open Water:
Q = 0
Where: Q = Runoff (mm/year)
P = Precipitation (mm/year) – from PRISM
The precipitation data was obtained from PRISM in polygon feature class format. Open up ArcMap, and add AnnualPrecipitation_PRISM (feature class located in "Data" feature dataset of BacteriaModel.mdb) to the layout view as shown below.
If you symbolize the precipitation based on Range and use graduated colors, you can see the range of annual precipitation as shown below, from 29 to 39 inches per year. A lot of variation for not a very large watershed!
The precipitation feature class needs to be converted into a raster. Using the Feature to Raster tool in Arc Toolbox, convert the polygon feature class into a raster based on the field, “RANGE”, which is the annual precipitation in inches, and set the output cell size to 100 (as shown below). Do not use the Feature to Raster conversion tool on the Spatial Analyst toolbar because it does not give you the option of saving the resulting raster inside the personal geodatabase. To find the Feature to Raster tool in Arc Toolbox, use the Index, and type Feature to Raster in the search window. Save the resulting raster in the BacteriaModel.mdb as Precip_Grid. In order to make all the rasters consistent, the same grid size (100m) is used throughout this exercise.
The annual precipitation is converted to millimeters by using Spatial Analyst's Raster Calculator: [Precipitation in in/year] * (25.4 mm/inch) = [Precipitation in mm/year] = P, where [] represents a raster. However, the extent and cell size first needs to be set. Go to Spatial Analyst | Options, and set the Extent and Cell Size as the same as the Precip_Grid (as shown below.)
Then go to Spatial Analyst | Raster Calculator... to convert in/year into mm/year, and press "Evaluate".
A new raster will be automatically added to ArcMap called "Calculation". This raster is only temporary and is not stored in your personal geodatabase. In order to make the raster permanent, right-click on the "Calculation" raster and click "Make Permanent…," which is shown below. Save the raster in BacteriaModel.mdb as a Personal Geodatabase Raster as PrecipGrid_mm/yr.
If you get an error message that says “The Object named PrecipGrid_mm/yr could not be found”, then Right Click on Calculation, use Properties to change the name of the Calculation to PrecipGrid_mm/yr, and just keep going. Save the map file as Ex6.mxd so you can recover the link to this temporary grid if you have to do so. This error may occur because you have a lock file open on the geodatabase. If so, you can quit out of Arc Map and delete the lock file ( a 1 kb file with the same name as the geodatabase) and then continue.
To be Turned in:
· Display of precipitation raster in mm/year for the Copano Bay watershed with a legend and north arrow.
· What is the range of precipitation in mm/year across the Copano Bay watershed?
In order to calculate the runoff for each land use classification in Quenzer's rainfall-runoff equations, the precipitation grid needs to be divided into four different rasters based on land use classifications. In order to save time on tedious work, precipitation rasters for each land use have already been created for you. Precip_Agriculture, Precip_OpenWater, Precip_Rangeland_Forest_Other, Precip_Urban can all be found in BacteriaModel.mdb. A description for how these were created is described on this website (http://www.crwr.utexas.edu/gis/gishydro05/Modeling/WaterQualityModeling/BacteriaModel.htm) in Appendix C (if you happen to be curious for how this was done!)
Now, runoff grids for each land use can be created. Add the precipitation rasters for the four different land use classifications to ArcMap. Set the Extent and Cell Size to Precip_Grid in the Spatial Analyst options. Then go to Spatial Analyst | Raster Calculator..., and evaluate runoff for each of the four land uses using the equations that were derived by Quenzer (given above). For example, this is how the runoff grid for Agriculture is calculated. Use the button at the bottom of the Raster Calculator window to extend the raster calculator to the right and obtain the exponentiation function Exp(). Note that you are using the Precip_Agriculture grid in this compuatation (a grid that has cells only where there is agriculture). (Note: while trying out this exercise, I would obtain an error message if I plugged in the numbers to the equations using the keyboard, so in order to avoid receiving any error messages, use the mouse to click on the number buttons in the raster calculator to plug in all the functions and numbers into the runoff equations.)
Rename the resulting calculation Runoff_Agr This grid has cells only where the land use is agriculture.
Now, lets do the same thing for Forest and Rangeland (which have the same runoff equation). Add the Precip_Rangeland_Forest_Other grid to the map display, and perform the raster calculation.
Rename the Calculation Grid to Runoff_Forest and save the map file Ex6.mxd again. Notice how the range of runoff from forest and rangeland is less than for runoff from agriculture.
Next, lets compute the runoff from urban areas. This is a simpler equation (there are no streamgages in urban land use in Corpus Christi, so a simple rainfall-runoff relationship valid in Austin was used instead). Add the Grid Precip_Urban to the map display, do the computation in the raster calculator as shown below, and rename the result Calculation as Runoff_Urb. Notice the sparse layout of cells for “urban” that appears also to be accounting for roads in this largely rural area.
The runoff grid for Open Water does not need to be evaluated since the Runoff = 0.
In ArcToolbox, Use the "Mosaic to New Raster" tool (do not use the tool entitled just Mosaic) located under Data Management Tools | Raster | Mosaic to New Raster. The Mosaic to New Raster tool is used to combine multiple rasters into one single raster.
You can drag the runoff rasters Runoff_Urb, Runoff_Forest, and Runoff_Agr from the ArcMap window into the tool window. Set the output location to BacteriaModel.mdb and name the raster Runoff_mmyr. Leave all the other fields as the default selections as shown below, and set the cell size to 100m.
Go to the Spatial Analyst toolbar and Options, and set the Extent and Cell Size to "Same as Layer Runoff_mmyr". Then go to Spatial Analyst | Raster Calculator... to convert the runoff into m3/year. Since each cell is 100m x 100m, or 10,000 m2 in area, 1 mm of runoff over a cell o f this area generates 10,000 * 1/1000 = 10 m3 of runoff. Hence, we can calculate the runoff in m3/year as:
Use Make Permanent to save the resulting Calculation as a Grid Runoffm3year in the BacteriaModel.mdb geodatabase.
The runoff per watershed can now be calculated by using Zonal Statistics and the delineated watersheds. In order to save time, the watersheds for Copano Bay have already been delineated for you. Delineated watersheds were produced by conducting Terrain Preprocessing, which is found in the Arc Hydro toolbar, on the Digital Elevation Model (DEM) to determine the flow patterns in the basin (flow direction), and subwatersheds were delineated for the Copano Bay watershed based on the Critical Points (USGS gauge stations, water segment endpoints, and bacteria monitoring stations). After Terrain Preprocessing is complete (procedure found at http://www.crwr.utexas.edu/gis/gishydro05/Modeling/WaterQualityModeling/BacteriaModel.htm in Appendix A), Water Rights Analysis Package (WRAP) Hydro is used to delineate watersheds for the basin (procedure described in Appendix B on same webpage).
Add the Watersheds feature class (found in BacteriaModel.mdb and Data feature dataset) to ArcMap.
Zonal Statistics is a tool that is used to determine statistics within specified 'zones' or boundaries. In this instance, the boundaries or 'zones' are the delineated watersheds and will be defined by JunctionID (each watershed's unique identifier), and the runoff values of all the grid cells (from the runoff raster) within each watershed will be summed.
In the Spatial Analyst toolbar, select the Zonal Statistics function
Fill in the resulting table as shown below. Be sure to click the Join output table to zone layer. Save the resulting table as Runoff_Stats.dbf.
When you open the Watershed Feature Attribute Table, you’ll see that the Runoff_Stats are joined to the Watershed Features:
Ok, that was a lot of work, but we’ve figured out how much runoff comes from each watershed in the Copano Bay drainage basin. Pretty neat! Save the Ex6.mxd file.
To be Turned in:
· Make a map of runoff per watershed (m3/year) for the Copano Bay watershed with a legend and north arrow.
· Determine the total annual runoff (m3/yr) for the whole drainage basin of Copano Bay
- Creating a Fecal Coliform Concentration Grid
There are several GIS data layers that are used to calculate the bacteria concentration in the Copano Bay watershed. The event mean concentration (EMC) values can be approximated for each type of land use. For this project, fecal coliform EMCs for each land use code were determined by Reem Jihan Zoun in her thesis, Estimation of Fecal Coliform Loadings to Galveston Bay (2003), which are listed in Table 1 below.