OHD/HL - Threshold Runoff: Workshop1

INTRODUCTION

Objectives:

• explore ThreshR input data and gain an understanding of how ThreshR parameters are computed

• improve the user's ArcView and Spatial Analyst skills

SETUP

On UNIX systems,  copy the extension file threshr.avx to either your Home directory or to the directory /ext under the ArcView3 installation directory.  [On a PC, copy threshr.avx to the ArcViewext32 directory.]

You can obtain the most recent version of threshr.avx via anonymous ftp to ftp.nws.noaa.gov/oh/gis/threshr.
 

WORKSHOP INSTRUCTIONS

Problem statement:  Compute the 1 hour threshold runoff at USGS Gage 1546500 along Spring Creek Near Axemann, PA (77.790 W, 40.890 N) and at a nearby gage USGS 1558500 along Shaver C NR Petersburg PA (78.007 W, 40.611 N).

Characteristics of a unit graph for Spring Creek are known.  The 1 hour unit graph has a peak flow of 2910.7 cfs and a basin lag of 8.67 hours.

These basins are located in USGS Region 5 of Pennsylvania.  For most states and regions, equations for several different return periods are reported in USGS WRIR 94-4002.  Most states report the 2 year flooding flow Q2 which we are using as an approximation for the "flooding flow."  In Pennsylvania Region 5, the 2.33 year equation is provided rather than the 2 year equation.  For this exercise we will assume that the Q2.33 is a good approximation for Q2.  Thus, the Q2 equation for Pennsylvania Region 5 is

Equation 1. 

ARM = drainage area (mi2)
PRE_PA = an index equal to mean annual precip minus mean annual potential evapotranspiration

Creating/Loading Data Sets

1.  Start ArcView and load the AV-ThreshR Extension.  [This can be done by accessing the menu item File --> Extensions when either a Project window or a View window is active.  In the Extensions dialog, scroll down the list of available Extensions and place a check mark next to "ThreshR."  To see information about the version number of ThreshR in the "About" frame, click the mouse on the "ThreshR" text.  Threshr.avx is dependent on the Spatial Analyst so the Spatial Analyst Extension is automatically loaded when the ThreshR extension box is selected.  An error will occur if the Spatial Analyst Extension is not available.]

2.  At the UNIX prompt, create a new directory for output files in your workspace using the UNIX "mkdir" command.   E.G. "mkdir output1"  In ArcView, specify this directory as the project Working Directory by clicking on Project --> Properties with the Project window active.  Type the name of your output directory in the space next to "Work Directory" and click OK.

3.  Create a point shape file to display the location of the Spring Creek and Shaver Creek stations.

•  Create a comma delimited ASCII file containing the ID, coordinates, and field name in a header line.  An example ASCII file is shown below.  The file you create MUST be created with a ".txt" extension.

 

 
 
 
 
 
 
 

Example ASCII file:
------------
 id,usgsid,lon,lat
 1,1546500,-77.790,40.890
2,1558500,-78.007,40.611
 ------------

•  Load this comma delimited ASCII file into ArcView as a Table (Hint:  Click  once and click the Add button.  You have to select "List files of Type" Delimited Text in the Add Table window).
•  With a View active (create a new View if you have not already), click View --> Add Event Theme to create a new point Shapefile.  An "Add Event Theme" window will appear and you should select the Table you just added and "Lon" as the X field and "Lat" as the Y Field.

• The point Theme you have created is displayed in Geographic Coordinates (decimal degrees).  An easy way to tell if you are viewing points in this coordinate system is by positioning your mouse cursor in the View window and looking at coordinate values in the upper right corner of the View window.

•   Use the   to project your point Theme.
•   Delete the unprojected point Theme from your View since it is no longer needed.  (Edit --> Delete Themes.)

 Note that a more general tool to project data Themes from one projection to another is the ArcView "Projector!" extension available for free from ESRI.  Similar Theme projection functionality comes standard with ArcView Version 3.2.  Theme projection capabilities only work with vector Themes NOT Grid Themes.

4.  For reference, load the following Themes into the same View as your projected station locations (Use the "Load Themes" button )

./marfc/rf1.shp --  EPA's River Reach File 1 (modified by NOHRSC)
./marfc/rfcbndp.shp -- RFC boundary file
./marfc/regions.shp -- USGS Regions where different regression equations apply.
./marfc/workshop1/padem -- DEM clipped for a portion of PA (units are meters)
./marfc/workshop1/pafd -- flow direction grid clipped for a portion of PA
./marfc/workshop1/pafa -- flow accumulation grid clipped for a portion of PA
./marfc/workshop1/pafld -- flow length grid grid clipped for a portion of PA
 

• Zoom to the extent of these Themes once they are all loaded .
• If you wish, you can hide the default legends that are displayed with the Grid Themes and take up a lot of space by making the Grid themes Active (use the Shift key for multiple Themes) and clicking Theme --> Hide/Show Legend.

5.  Make the DEM Active and click Theme --> Properties to get information about this Theme.

What is the cell size for this grid?
How many rows and columns are there?
What type of grid is this (Integer or floating point?)
 
• You can view statistics for a grid Theme by double clicking on the Theme name or Legend to get the "Legend Editor" window and then clicking on the "Statistics" button.

           What are the maximum, minimum, and mean elevations for cells in this grid?

6.  Just for fun, let's create a topographic relief map that appears to be 3-D.
 

•    Make padem active.
•    Surface --> Compute Hillshade (use the default Azimuth and Altitude).  A Hillshade Theme is added to your View.
•     Double-click on padem to edit its legend.
•     In the legend editor, click "Advanced" and choose "Hillshade of padem" as the Brightness Theme. Click OK
•     Select "Elevation # 1" as the Color Ramp to see a relief map.  You will need to scroll down in the Color Ramps combo-box to see this option.

7.  Move your stations Theme to the top of the Table of Contents for display and Zoom into the vicinity of the gaging stations of interest (Zoom Tool )

8.  Flow Directions.  If you hid the Legend for the Flow Direction Theme, unhide it by making pafd Active and clicking Theme --> Hide/Show Legend.  Doing this, you will see that there are eight possible values for each cell in the Flow Direction grid.  Make the flow direction grid ("pafd") the active Theme and use the tracing tool  to visualize flow paths predicted by the flow direction grid.  This is a fun tool to play around with.  The tracing tool creates graphics (green lines) in the View window, not Themes.  To remove these graphics, select Edit --> Select All Graphics and then Edit --> Delete Graphics.

9.  Flow Accumulation.  Another way to visualize stream flow paths is to create a synthetic stream network using the flow accumulation grid.  In Workshop 2,  a synthetic stream network is generated automatically by Av-ThreshR and used internally when multiple basins are delineated.  However, it's useful to see how these synthetic streams can be defined manually using standard Spatial Analyst features.
 

• Click  Analysis --> Map Query and enter the following string

    ([pafa] < 500).setnull(1.asgrid)

Click Evaluate.  A Theme named "Map Query 1" is added to your View.  Make this Theme visible to see the synthetic streams.  (Note: The "Map Query" window can be closed from the pull-down menu in its upper-left hand corner.)

The   tool is provided with AV-ThreshR to make it easy to compute the cumulative drainage area of individual grid cells in mi2 or km2 rather than in units of cells.  THE FLOW ACCUMULATION THEME MUST BE THE ACTIVE THEME in order for this tool to give the correct result.  Using this tool is nearly identical to using the identify tool   to query the flow accumulation grid except that it automatically converts the number of cells upstream to mi2 and km2 rather than just reporting the number of cells upstream.
 

• Make the flow accumulation Theme active and use the  tool to query the source points of some of the synthetic streams that resulted from your Map Query.  This will verify that the synthetic streams begin when drainage area jumps above 30.9 mi2 or 500 grid cells.

10.  Delineate Spring Creek and Back-calculate Snyder Cp and Ct values.

For station 1546500 on Spring Creek we have a unit hydrogaph but we do not have a unit hydrograph for station 1558500 on Shaver Creek.  If you have lost track of which station is which, just make the station point Theme active and use the  .

To derive a Snyder unit hydrograph for Shaver Creek, we can using Cp and Ct estimates computed from the data available at Spring Creek.  Cp and Ct values can be calculated for Spring Creek given the unit hydrograph peak flow (cfs), the time to peak, the drainage area in square miles (ARM), the length of the longest flowpath (CHLN), and the length from the basin outlet to a point on the longest flow path opposite the centroid (CHCN).
 

•  Delineate the basin boundary for Spring Creek by making the flow direction Theme "pafd" active and using the  tool.  To do this, it is helpful to zoom in close to the USGS gaging station of interest so that you can select an individual stream cell with the mouse.  Displaying your "Map Query 1" on top of "padem" will help you to see which cells are "stream" cells.  Using ,  select the stream cell nearest  the USGS gage with the mouse.  When the delineation is complete, you will be asked to enter the name of the output Shapefile.  Suggested name:  "Sprcreek.shp"  This Shapefile file is then added to your View along with a grid file of the watershed boundary (by default the grid is named "Grid1" or "Grid2" if "Grid1" exists, etc).  Remove the check mark next to Grid1 since this Theme is not needed until later.

Of the geometric parameters need for the Snyder calculation, ARM and CHLN are easy to compute manually while CHCN is slightly more complicated (as discussed in Lecture).  Thus, a simple Avenue script is provided to calculate ARM, CHLN, and CHCN for you.   This program also calculates CHSL (also a more complex calculation) which is not needed for the USGS Flood Frequency regression equation in this region, but is often needed in other regions and is also needed if the "second" form of the Snyder lag equation is used.
 

• Make your Spring Creek polygon Theme active, and select your Spring Creek basin polygon  and then click ThreshR-Utility -->  Single Basin Geometry.  Type in the appropriate Theme names when the dialog comes up.  This program calculates the following parameters and adds them as attributes to the subbasin Shapefile (sprcreek.shp).

    ARM    drainage area (sq.mi)
    CHLN    channel length (mi)
    CHCN    length to point on the main channel opposite the centroid (mi)
    CHSL    USGS channel slope (ft/mi)

The program also adds three additional Themes to your View:
    flow length grid (meters): "fld1"
    grid representing the longest flow path: "lfpg1"
    center point Shapefile: "cntp1.shp"

• Open the attribute table for your basin Shapefile to see the results of your calculations (use ).
• To confirm that CHLN was calculated correctly, Display the Longest Flowpath grid ("lfpg1"), make the Flow Length ("fld") active and query the flowlength value on the most upstream cell in the longest flow path using .   The longest flowlength value in Spring Creek should be about 26300 m  or 16.35 miles.

You now have all of the information you need to estimate Cp and Ct for this basin.  The method for doing this is described by in Chow, Maidment, and Mays, 1988 (p. 224-226).  The sequence of calculations required is summarized here:

' GIVEN VALUES
t_pR  = basin lag for the observed flow = 8.67 hrs
t_R = duration = 1 hr
Q_pR = observed unit hydrograph peak (cfs) = 2910.7 cfs

'EQUATIONS
q_pR = Q_pR / ARM = 2910.7 / ARM
'--- determine time to peak for the synthetic unit graph
t_p=1.04762*(t_pR-(t_R/4))
'--- solve Applied Hydrology Eq. 7.7.2 for C_t
c_t=t_p/((CHLN*CHCN)^0.3)
c_p=q_pR*t_pR/640

You can make these calculations manually or run a utility script is available in AV-ThreshrR to make these calculations for you:
 

• Make sprcreek.shp the active Theme and run Threshr-Utility --> Cp and Ct.  Enter the known information and Cp and Ct will be calculated based on the information in the fields "ARM", "CHLN", and "CHSL" of sprcreek.shp.  A report giving these values is displayed.  In this example, Ct should be about 2.0 and Cp should be about 0.5.

11.  Calculate ThreshR for Spring Creek.

You need one more piece of information to calculate ThreshR for Spring Creek.   That information is the net Precipitation Index (PRE_PA) used in the flood frequency regression equation for region 5 of Pennsylvania.  A grid of this precipitation index is provided for you in the "/marfc/pa" directory.
 

• Load the grid called "pre_pa" into your View using .

• Calculate the mean value of PRE_PA for Spring Creek.  To do this, make the Spring Creek boundary Shapefile active and   Select Analysis --> Summarize Zones.  Select "Gridcode" as the field that defines the zones and select "Pre_pa" as the Theme containing the variable to summarize.  Just hit Cancel when you are asked to "Select a statistic to chart:" because it is silly to make a Chart out of only one data point.

A table titled "Stats of Pre_pa within the Zones of Sprcreek.shp" is opened.  The "Mean" field in this table is the mean value of PRE_PA (inches) for Spring creek.

You can now calculate 1 hr threshold runoff (R_1hr) using Equation 1 to estimate flooding flow:

    R_1hr = Q₂/Q_pR

Your result should be around 0.88.

12.  Calculate ThreshR for Shaver Creek
 

• Use  to delineate the basin boundary for Shaver Creek.  (Remember the "pafd" grid must be Active).  The boundary for Shaver Creek looks quite blocky and maybe a little unusual in shape.  The boundary does make sense, however, when looking at the shaded relief map created in Step 4.  Also, remember that a basin of this size is at the lower end of the size ranges where we can expect reasanable accuracy in defining boundaries from the 400-m DEM.  (Remove the check mark next to "Grid2")

• Maker your Shaver Cr. shapfile active and select the polygon boundary of the basin ().  Click ThreshR --> Single Basin Geometry

• Compute the unit graph peak using the Snyder method and assuming the Ct and Cp values are the same as those for Shaver Creek.  Do this calculation manually in this workshop.  This calculation will be done automatically in Workshop 2.

t_p = C_t*(CHLN*CHCN)^0.3
t_r = t_p/5.5
t_pR = t_p - (t_r - t_R)/4
Q_pR = 640*ARM*C_p/t_pR
• Compute the flooding flow using Equation 1 (Use Analysis --> Summarize Zones again to compute mean PRE_PA for Shaver)

• Estimate ThreshR as Q₂/Q_p.   Answer should be around 0.85 inches.

References

Chow, V.T., D.M. Maidment, and L.W. Mays, Applied Hydrology, McGraw-Hill, INc., 1988.

Jennings, M.E., Thomas, W.O., and Riggs, H.C., "Nationwide Sumary of U.S. Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods for Ungaged Sites, 1993"  USGS Water-Resources Investigations Report 94-4002, Reston, VA, 1994.

Notes:
USGS says the drainage area for Shaver Creek is 46.4 mi2 indicating a DEM-based error of ~5.5%
USGS says the drainage area for Spring Creek is 87.2 mi2 indicating a DEM-based error of ~9.6%
Assessment of the errors in drainage area delineation using the Av-ThreshR flow directions will be discussed in a lecture.