MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_NextPart_01CA3D39.AA689010" This document is a Single File Web Page, also known as a Web Archive file. If you are seeing this message, your browser or editor doesn't support Web Archive files. Please download a browser that supports Web Archive, such as Windows® Internet Explorer®. ------=_NextPart_01CA3D39.AA689010 Content-Location: file:///C:/6A09C639/Ex2_2009.htm Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset="us-ascii"
Pre= pared by David R. Maidment
This exercise is intended for you to build a base map of geographic and
streamflow data for a watershed using the San Marcos Basin in
To complete this exercise, you'll need to run ArcGI= S 9.3.1 from a PC.
The HUC boundaries are a subdivision of the
The NHDPlus data for the United States can be downloaded over the intern= et:
NHDPlus http://www.horizon-systems.com/NHDPlus/
Get the NHDPlus data for Region 12:
http://www.horizon= -systems.com/NHDPlus/data.php
For = those ambitious students that would like the experience of downloading NHDPlus da= ta for themselves, follow the instructions in this section. Otherwise, skip ah= ead to the Procedure for the Assignment Section wher= e you will find a zipped file with all the necessary data.
Foll= ow the link to get NHDPlus data, and click on the Regi= on 12 location in the map (or another region if you want a different area of the country).
Ther= e you will download the following files and save them in a directory of your choosing:
- Region 12, Version 01_01, Catchment Flowline Attributes
- Region 12, Version 01_01, National Hydrography Dataset
Don’t download the = grid files because they are not needed for this exercise and they are huge in si= ze.
After extracting the zipp= ed files, you should have something similar to the following:
=
Watershed Boundary DataSet These data can be obtained from http://=
www.ncgc.nrcs.usda.gov/products/datasets/watershed/ Click on “Obtain Data” at this addres=
s, and
then in colorful display that follows, go to the top left and say “Get Data”. To get the data for Texas, select
“Quick State” in the box on the lower left, and then select TX
Texas in the drop-down menu that follows.
At
Step 2, select 12 Digit Watershed
Boundary Dataset 1:24,000
At
Step 3, just leave the options as the standard ones: Geographic coordinates=
in
NAD83 datum in one ESRI Shape File
At
Step 4, fill in the delivery information:
Then
go to Step 5 and the estimated download time is given. When the file is ready, you get an=
email
message, and then you download the resulting file via a web link. In this case, the compressed file =
was
74MB in size. I saved t=
his
file into a folder called WBD
And
when unzipped, this creates a folder called hydrologic_units, whose contents
look like:
These
are shape files for the 12 Digit Watershed Boundary Dataset for Texas.
Logon to the computer of your choice and make a directory in your worksp= ace for this exercise. I've saved the needed files in the LRC class directory <= b>civil.ce.utexas.edu\class\class_files\MAIDMENT\giswr2009\Ex2 They may also be downloaded as h= ttp://www.ce.utexas.edu/prof/maidment/giswr2009/Ex2/Ex2Data.zip This file is 156MB and the total fi= le space used for this exercise is a little more than 1GB. If you don’t have this amoun= t of file space available to you, get the smaller file http://www.ce.utexas.edu/prof/maidment/giswr2009/Ex2/SanmarcosGDB.zip= a> and proceed to the section on= Adding Attributes to the = Flowlines.
Unzip the file to get the following:
In the HydrologicUnits directory that n=
ormally
comes with the NHD file download, I have replaced
Creating a Geodatabase
Open ArcCa= talog and create a new file geodatabase by right clicking the directory where= the NHDPlus data is saved and selecting New/File Geodatabase and name it SanMarcos.gdb. = p>
Right click on the new geodatabase and selec= t New/Feature Dataset.
Name the new feature dataset Basemap,= and hit Next to set the projection= and map extent.
Select Import from the choices in the menu displayed.
We will import the coordinate system, so select Import and then navigate to the NHDPlus data that was just downloaded. Select the nhdflowlines= shapefile.
Hit Add to select this hori= zontal coordinate system. Hit Next and = leave the Vertical Coordinate system set at None.
Hit Next and leave the default XY Tolerance= s as they are, then hit Finish to complete the specification of the spatial reference of the feature dataset.= If you right click on the resulting Basemap feature dataset and open Properties, and tab to XY Coordinate System, you’ll see t= he coordinate system is GCS_North_Ame= rica_1983. This means that the coordinate sys= tem is in geographic coordinates using the North American Datum of 1983.
Displaying Streams and Watersheds
Open ArcMap and use the button to add data. Navigate to the
folder where the downloaded data is saved.
We will first add the subba=
sin
and flowlines layers. The NHDflowl=
ine.shp
shapefile is located in the Hydrography folder and the=
wbdhu12_a_tx.shp shapefile
is located in HydrologicUnits folder. Please note that in this HydrologicUnits folder I have substituted the Watersh=
ed
Boundary Dataset HUC12 watersheds for the normal HUC8 watershed files that =
come
with the NHDPlus dataset. NHDPlus is
being updated to include the HUC12 watersheds but that work is not complete
yet.
Recolor the themes if you wish. The res=
ult
is a map similar to the following:
=
You
can see the watersheds that are roughly in the outline of Texas and the NHD
stream network that covers Water Resource region 12.
Use File/Save As to save the ArcMap document as Ex2.mxd (to save your own customized colors).
The HUC12 Watershed, and NHDflowlines feature classes cover a large region and we only want to work in the San Marcos Bas= in. We'll use ArcMap to identify the San Marcos SubBasin= span> and to create new feature classes using pertinent portions of the feature classes for Region 12.
(1) Turn off the nhdflowline theme in the display, and open the attribute table for the HUC12 watersheds= by right clicking on its feature class name:
You’ll see a displa= y that looks like this. We want all = the HUC12 subwatersheds that lie within the San Mar= cos subbasin, which has a HUC8 value of [HUC_8 =3D 12100203].
At the bottom of the Attr= ibutes table, click on Options and us= e Select by Attributes
Click on “HUC8̶= 1;, “=3D”, Get Unique Values and “12100203” to form the expression
"HUC_8" =3D '12= 100203'
In the
selection window. Be careful about how you do this since the form of=
the
expression is important. Once=
you’ve
executed this query, you’ll see that 32 HUC12 subwatersheds are selected, and if you hi=
t the
“Selected” button instead of “All” at the bottom of=
the
Attribute table, you’ll see the selected records, and also their
highlighted images in the map.
(2) M= ake sure that Arc Catalog is closed or the next steps won’t work. In ArcMap, Right Click on Subbasin and select Data/Expo= rt Data to produce a new theme. If you get a message saying you can’t do this, it means that you haven’t shut down Arc Catalog = before trying the data export. Close= Arc Catalog and repeat the export steps if this happens
Browse inside your geodatabase to the Basemap Feature dataset (you̵= 7;ll have to change the Save as Type to File and Personal Geodatabase feature classes first), name this new feature class as Watershed and save it in the geodatabase as a File and Personal Geodatabase feature class. Don’t save it as a Shapefile, which is the default option you = are first presented with.
You will be prompted to whether add this theme to the Map, click Yes. In ArcMa= p, Use Selection/Clear Selected Featu= res to clear the selection you just made.
And then Zoom to Layer to focus in on your selected Watersheds.
It is useful to have a single polygon that is the outline of the San Mar= cos Basin. To do this, we’l= l use the Dissolve Tool in ArcToolbox. = span>In Arc Toolbox, select the Index = tab
and
type dissolve in the window at the top of the panel, and select Dissolve (management) Input Watershed
as the Input Features, and name the resulting output feature class Basin. Click on HUC8 as the Dissolve Field. You’ll see a new feature class ca=
lled
Basin is created that covers our study area. Its attribute table has only one r=
ecord
in it, for the HUC basin 12100203: Use the Symbology properties of the fea=
ture
classes to make the Watershed and Basin feature classes hollow so that you =
can
see the streams within them: To Be Turned In: How many HUC12 <=
span
class=3DSpellE>subwatersheds2? What is the total area of HUC12 subwatersheds in this basin in km2? What is the ratio of the len=
gth of
the streamlines to the area of the HUC12 subwatersheds=
(called the drainage density) in km-1?
Now we can create a layer wit=
h just
the flowlines in the San Marcos Basin. In ArcMap, use
Select/Select by Location to s=
elect
the features from nhdflowline
with their centroid in Basin.
This selects all the flowline=
s in
the San Marcos basin.
Follow the same process to ex=
port
this data. Save it as Flowline
in the BaseMap feature dataset and add it as a =
layer
to the map. Remove the old nhdflowline and
wbdhuc12_a_tx themes from your map display, and save the Ex2.mxd file again.
Now lets look at some summary statistics of the
From this display, you can se=
e the
statistics of the LengthKm of the Flowlines. There are 557 flowlines
whose average length is 3.39 km and the total length is 1890 km. You can do the same query on the A=
cres
attribute of the Watershed feature class to get watershed areas.
To be turned in: How many HUC12 subwatersheds are there in the San Ma=
rcos
Basin? What is their average =
area
in acres and in km2?
What is the total area of this basin in km2? What is the ratio of the len=
gth of
the streamlines to the area of the HUC12 subwatersheds=
(called the drainage density) in km-1?
Now we will use the flowline
attributes table to symbolize the flowlines based on their mean annual flow=
. Add
the table flowlineattributesflow.d=
bf
to your ArcMap display.
Lets zoom into our Fl=
owlines
and use the Inquiry button
If you open the Attributes ta=
ble of
FlowLineAttributesFlow.dbf, you’ll see that it also has a COMID field=
and
lots of tabular attributes that tell you more about the properties of the <=
span
class=3DSpellE>flowline.
We’ll use COMID as a key field to link the two attribute tables
and transfer mean annual flow attributes to the Flowli=
ne
feature class.
Open the attribute table for =
the
feature class Flowline
and select Options/Add Field. =
You
should have Arc Catalog closed while you are doing this or it may not work.=
Name the field Mean _Annual_Flow and make it of the type Double.
Now we will join the Flowline=
layer with the flowlineattributesflow tab=
le
based on COMID. Right click on the Flowline layer and select Joins and Relates/Join.
Say no to creating
an index.
Now when you open the Flowline
attribute table you will find the information contained in the flowlineattributesflow table has been joined to the
existing features. Scroll over to the column labeled f=
lowlineattributesflow.MAFLOWU.
This field contains the Mean Annual Flow for each reach. It is estimated by
averaging the mean annual runoff over the drainage area above this reach.
We can set the value of our n=
ew
field Mean_Annual_Flow by using the field calcu=
lator.
Scroll back to the column we created, now called Mean_Annual_Flow, and right
click on the column label to select the field calculator.
Set this field equal to [flowlineattribu=
tesflow.MAFLOWU].
This populates the Mean Annual Flow field with the appropriate value.
Now we can remove the join by=
right
clicking on the Flowline
feature class and selecting Joins =
and
Relates/Remove All Joins.
Now our attribute table for <=
span
class=3DSpellE>SanMarcos_flowlines has a field called Mean_Annual_Flow
with the values populated. We can use this field to symbolize the flowlines=
. Right
click on Flow=
lines
and select properties. In the
properties menu, select the Symbol=
ogy
tab. Change the Symbology to display graduated
symbols for the Mean_Annual_Flow
field and hit OK.
The result is a map displayin=
g the
relative flow of the streams and rivers in the San Marcos basin. Use the Inquiry tool to find out the names of the various rivers in the=
map
display. Use View/Toolbox/Draw to open the Drawing toolbox and select a labe=
l:
And add a label to show Plum =
Creek:
Now you are going to build a new Feature Class yourself of stream gage locations in the San Marcos basin. I have extracted information from the US= GS stream gage data books information about 8 gages in this basin:
T= he location coordinates and flow data can be obtained from the USGS NWIS serve= r, as described subsequently in this exercise.
<= b>(a) Define a table containing an ID and the long, lat coordinates of the gages<= /b>
T= he coordinate data is in geographic degrees, minutes, & seconds. These val= ues need to be converted to digital degrees, so go ahead and perform that computation for the 8 pairs of longitude and latitude values. This is something that ha= s to be done carefully because any errors in conversions will result in the stat= ions lying well away from the San Marcos basin. I suggest that you prepare an Ex= cel table showing the gage longitude and latitude in degrees, minutes and secon= ds, convert it to long, lat in decimal degrees using the formula
D= ecimal Degrees (DD) =3D Degrees + Min/60 + Seconds/3600
R=
emember
that West Longitude is negative in decimal degrees. Shown below is a table =
that
I created. Be sure to format the columns containing the Longitude and
Latitude data in decimal degrees (LongDD and LatDD) so that they explicitly have Number format wit=
h 4
decimal places using Excel format procedures. Note the name of the worksheet tha=
t you
have stored the data in. I ha=
ve
called mine L=
atlong.
Save the file as an Excel 97-2003
Workbook or it will not appear in ArcMap. Close Excel before you procee=
d to
ArcMap.
<=
!--[if gte vml 1]>
<= b>(b) Creating and Projecting a Feature Class of the Gages
(=
1)
Open ArcMap and the Ex2.mxd file you created in the f=
irst
part of this exercise. Select=
the
add data button =
and navigate to your Excel spreadsh=
eet
<= span style=3D'mso-no-proof:yes'>
Double click on the spreadsheet to identify the individual worksheet within the spreadsheet that you want to a= dd to ArcMap (it’s a coincidence that they have the same name in this example and that is not necessary in general).
<= span style=3D'mso-no-proof:yes'>
H= it Add and your spreadsheet will = be added to ArcMap. Pretty cool!= ! This is the first time we’ve been able to add worksheets directly in ArcMap. Before ArcGIS 9.2 we = had always to save the worksheet as a .dbf file and add that instead. ArcGIS= 9.3 recognizes Excel 2007 files but ArcGIS 9.2 does= not do that, so if you are using ArcGIS 9.2, you wi= ll need to save your Excel file in Excel 2003 format. If you have trouble finding this f= ile in a folder where you know it exists, it can be because your file directory has been labeled Read Only. In Wi= ndows Explorer use File/Properties on your Ex2 folder and turn off the Read Only = property and you’ll be able to see the file just fine.
<=
!--[if gte vml 1]>
<=
!--[if gte vml 1]>
N= ow we are going to convert the tabular data in the spreadsheet to points in the ArcMap display.
(= 2) Right click on the new table, LatLong, and select= Display XY Data
<= span style=3D'mso-no-proof:yes'>
<= span style=3D'mso-spacerun:yes'> (3) Set the XY Table to latlong, the X Field to LongDD (or Longitude), the Y Field to LatDD (or Latitude), Hit Ed= it to change the spatial coordinate system, and then Import, and get the coordinate system from the feature dataset = Basemap, and you should end up wi= th a display that looks like the one below.&nbs= p;
<=
!--[if gte vml 1]>
H=
it
OK, to complete it and youR=
17;ll
get a warning message about your table not having an O=
bjectID. Just hit Ok and move on. Hit Ok to add the points and voila=
! Your
gage points show up on the map right along the San Marcos If you don’t see any points,
don’t be dismayed. Chec=
k back
at your spreadsheet to make sure that the correct X field and Y field have =
been
selected as the ones that have your data in decimal degrees.
C= lick on the point symbol under the legend label latlong event and recolor and resize the points so that they show up m= ore clearly.
W= hat you have created is called an “event” which means that it is a graphical display in the ArcMap window of latitude and longitude points that are stored in a table. It is = not a real feature class yet.
<=
!--[if gte vml 1]>
<= span style=3D'mso-spacerun:yes'> (4) Now, we’ll make a feature cl= ass out of the points. Right click on the latlong Events layer and select Data/Export Data
<= span style=3D'mso-no-proof:yes'>
A= nd export the data into the Basemap= b> feature dataset as the feature class MonitoringPoint= . Say Yes when you are ask= ed if you want to add the points to your map, and now you’ve got a new feat= ure class in the Basemap feature dataset with your points in the same projectio= n as the other features in Basemap (ArcGIS does the map projection automatically= as part of the data export process). Open the Attribute table of the Monitorin= gPoint feature class so that you can see that all the attributes have been correct= ly translated from the Excel file.
<=
!--[if gte vml 1]>
<=
!--[if gte vml 1]>
O= pen ArcCatalog and check out your = Basemap feature dataset. The MonitoringPoint feature cl= ass now resides there.
<=
!--[if gte vml 1]>
(= 5) Save your Ex2.mxd ArcMap document.
N= ow we are going to add the USGS gage ID, gage name, and annual discharge for e= ach gage as new attributes in ArcCatalog.
(= 1) Open ArcCatalog (make sure Arc= Map is closed) and navigate to the San Marcos geodatabase that contains your data.=
(= 2) In the Basemap feature dataset, right click on the MonitoringPoint featu= re class, and click on Properties= to open the Feature Class Properties window. Click on the Fields tab and scroll down to the end of the Fiel= d Name list. Type the following three entries:
Field
Name |
Data
Type |
USGSID |
Text |
Name |
Text |
Flow |
Double |
<=
!--[if gte vml 1]>
<= span style=3D'mso-spacerun:yes'> (3) Click OK and open your A= rcMap document ex2.mxd.
<= b>Adding Data to the Attribute Table
(= 1) In ArcMap right click on the MonitoringPoint feature class and open the attribute table. Scroll to the right until y= ou see the new fields you have created. Start editing by selecting Editor/S= tart Editing.
<= span style=3D'mso-no-proof:yes'>
If the Editing Toolbar is not visible it maybe add= ed by selecting View/Toolbars/Editor.
<= span style=3D'mso-no-proof:yes'>
S= elect Editor/Start Editing and select the San Marcos geodatabase as the ta= rget to be edited:
<=
!--[if gte vml 1]>
(= 2) You may now just add the appropriate data values to the table and for = the three attribute fields you added. Continue adding the data values until they are all done.
The following data are needed in the table:
|
Station Name |
Mean
Annual Flow (cfs) |
|
1 |
08170500 |
San Marcos Rv at San Marcos, TX |
176 |
2 |
08173000 |
Plum Ck nr Luling, TX |
114 |
3 |
08172000 |
San Marcos Rv at Luling, TX |
408 |
4 |
08172400 |
Plum Ck at Lockhart, TX |
49 |
5 |
08171300 |
Blanco Rv
nr Kyle, TX |
165 |
6 |
08173500 |
San Marcos Rv at Ottine, TX |
456 |
7 |
08172500 |
Plum Ck nr Lockhart, TX |
56 |
8 |
08171000 |
Blanco Rv
at Wimberley, TX |
142 |
<= span style=3D'mso-spacerun:yes'> (3) Select Editor/Stop Editing= b> to finish editing and say Yes when it asks if you would like to save your edit= s. Note that you can only edit tables for which you have write permission (i.e. your own tables not tables in my work area).
Labeling the Gages in View
R=
ight
click on the =
MonitoringPoint feature class and select Properties. Click on the Labels tab and from the drop down=
menu
select the label field name to be =
Name.
R= ight click on the MonitoringPoint feature cla= ss again and select “Label Features”. You can now create a view li= ke this:
<=
span
style=3D'mso-no-proof:yes'>
Pretty awesome! If you want t=
o move
the labels around to customize your map, you can convert the labels to
annotation and then they can be individually moved where you want them to b=
e:
<= span style=3D'mso-no-proof:yes'>
N= ote that in order to move the labels, you have to open the View/Toolbars/Editor and selected the SanMarcos.gdb geodatabase to edit, and the MonitoringPointAnno as the feature class to be edited:
<= span style=3D'mso-no-proof:yes'>
T= hen, you can move and change the size of the annotation as you wish. When you are done, say Stop Editing and Save your annotation in its new location. Save the ArcMap doc= ument Ex2.mxd also.
(1)&=
nbsp;
Open ArcMap to create a chart of the mean annual fl=
ow of
the San Marcos gages. The Mean Annual Flow at the gages is recorded in the
column labeled Flow in the att=
ribute
table. Open the MonitoringPoint attributes table and make a chart using the tools available in
ArcMap. Alternatively, you can export the attribute table to a .dbf file and
map the chart in Excel. You can also open the geodatabase tables directly in
Excel by using Data/Get External D=
ata
and doing a query on the Microsoft Access file that contains the geodatabase. The chart may look something like this
<= br> = span>
(=
2)
In ArcMap prepare a layout showing a map of the drainage area, the <=
b>graph
of its annual flows at each gage and a table of numerical values
describing the gages. You can import your Excel chart and worksheet from
the Insert/Object... option in ArcMap. If necessary resize the
original chart or table smaller so that it can be displayed in the layout.
You'll see in the chart that the flow in the San Marcos River at Luling and Ottine is much=
higher
than in the upstream stations. That is because of the cumulative effect
upstream at Luling and because Plum Creek joins the San Marcos River just
upstream of Ottine.
<= i>To be turned in: a layout showing the base map, chart and data table for the S= an Marcos River flows
T=
he
Edwards aquifer is one of the most critical water resources of
T= he Edwards aquifer coverage from TNRIS is in Decimal Degree coordinates. This = is the Edwards shapefile that you copied fr= om the zip file at the beginning of the exercise.
(1)&=
nbsp;
Import your Edwards.shp file into the SanMarcos geodatabase. From
the Toolbox window in ArcMap, select Conversion
Tools à
To Geodatabase à Feature Class to Feature Class. Although the name of the tool is f=
eature
class to feature class, it will also convert a shapefi=
le
to feature class.
(2)&= nbsp; Choose Edw= ards.shp as the Input Features, the Basemap= feature dataset of the SanMarcos geodatabase as the output location, Aquifer as the output feature class name, and leave the rest of= the inputs as their default values= . Press OK.
This will not only do the conversion from shapefile = to feature class, but also add the new feature class to your map.
R= ight click on the Aquifer feature class and select Properties. Click on i= ts Symbology tab and Label the theme using the attribute Aquifer. This attribute has three values: 1 for outcrop, 2 for downdip and 0 for holes within the outer bound= ary of the aquifer. Classify the values with Unique Value and color them appropriately.
<= br>
<= br> You'll see that as the San Marcos River flows So= uth East towards the Gulf Coast and it crosses first the outcrop and then the <= span class=3DSpellE>downdip portions of the Edwards aquifer. The downdip region is where the aquifer dips below the la= nd surface and is shielded from the surface rivers by overlying hydrogeological units of low permeability. The Edward= s is a fissured limestone aquifer whose fissures lie along its Southwest to Northe= ast orientation, so its flow moves in that direction, transverse to the directi= on of flow in the San Marcos basin. It is thus quite possible for water to drain = from the San Marcos river into the Edwards aquifer and then reappear as a spring further North in another river. Zoom in to the region where the aquifer cro= sses the San Marcos basin for a closer look.
<= span style=3D'mso-no-proof:yes'>
<=
br>
To be turned in: Between which two gaging stations does the Edwards aqui=
fer
outcrop area occur? What is the difference in mean annual flow at these two
gages? Comment on these data. Do they seem correct to you?
T= here are also other sources of flow data. One of them is real time data via Inte= rnet from the U.S. Geological Su= rvey (http://water.usgs.gov/real= time.html). This data is taken at the gage sit= es every 15-60 minutes and is transmitted to the USGS office every 1 to 4 hour= s. This data relayed using satellite, telephone, and/or radio and can be viewed within minutes on arrival to the office. Notice the difference between the = flow conditions in September 2002 to 2009. In 2002, the Northeast was experiencing a severe drought while in 20= 06 it was wetter than normal, in 2008, we had Hurricane Ike in Texas, and today there is a severe drought conditions in Texas. Hydrology is a wonderful phenomenon – always changing!
<= span style=3D'mso-no-proof:yes'>
<= span style=3D'mso-no-proof:yes'>
<= o:p>
<=
span
style=3D'mso-no-proof:yes'>
<=
!--[if gte vml 1]>
Y= ou can see the effect on streamflow of Hurricane I= ke which passed through this region Sept 12-15, 2008!
<=
!--[if gte vml 1]>
(= 1) Click on the site: http:= //waterdata.usgs.gov/nwis/rt
(=
2)
Once you have looked around a bit click on
<=
!--[if gte vml 1]>
To be turned in: The graph of flow of the San Marcos = River at Luling printed from the NWIS website. What are the 20%, 50%, and 8= 0% cumulative probability flows for the calendar day on which you do the download? Approximately what % cumulative probability is the flow currently? <= /span>
Summary
of Items to be Turned in:
2. How many HUC12 subwatersheds are there in the San Marcos Basin? What is their average area in km2? What is the total area of HUC12 subwatersheds in this basin in km2? What is the ratio of the len=
gth of
the streamlines to the area of the HUC12 subwatersheds=
(called the drainage density) in km-1?
3. A layout of the San Marcos Basin and streams. Add labels to show the San Marcos = River, the Blanco River and Plum Creek.
4.
A layout showing the base map, chart and data ta=
ble
for the San Marcos River flows.
5.
Between which two gaging stations does the Edwards aquifer outcrop area occ=
ur?
What is the difference in mean annual flow at these two gages? Comment on t=
hese
data. Do they seem correct to you?
6. The gr=
aph of
flow of the San Marcos