Updated April 2007
BASINS Technical Note 1
Creating Hydraulic Function Tables
(FTABLES) for Reservoirs in BASINS
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Technical Note 1
Creating Hydraulic Function Tables (FTABLES) for Reservoirs in BASINS
April 2007
INTRODUCTION
The HSPF model (Bicknell, et al, 2005) uses a hydraulic function table, called an
FTABLE, to represent the geometric and hydraulic properties of both stream reaches and
fully mixed reservoirs. When the WinHSPF module of the BASINS program constructs
the HSPF User Control Input (UCI) file, it automatically derives the FTABLEs from
stream attributes computed during watershed delineation. These stream attributes contain
only rudimentary channel characteristics and does not adequately describe the hydraulic
behavior of reservoirs. This technical memo describes a procedure that BASINS users
can follow to manually create FTABLEs appropriate for reservoirs.
The FTABLE in HSPF
The FTABLE describes the hydrology of a river reach or reservoir (RCHRES) segment
by defining the functional relationship between water depth, surface area, water volume,
and outflow in the segment (see Figure 1). The relationship described in the FTABLE is
independent of the shape of the reach or waterbody in that waterbodies with different
shapes could have the same FTABLE. That is, HSPF makes no assumptions regarding
the shape of a stream channel (e.g. that the cross-section be trapezoidal or even that the
shape be prismoidal). The assumption of a fixed depth, area, volume, outflow relationship
rules out cases where the flow reverses direction or where one RCHRES influences
another upstream of it in a time-dependent way. The routing technique falls in the class
known as "storage routing" or "kinematic wave" methods. In these methods, momentum
is not considered.
The user specifies the properties of a RCHRES in the FTABLE, which is a piecewise
linear function table (see Example 1). It has columns for the depth, surface area, and
volume, plus up to five columns for volume-dependent outflows. Each row contains
values corresponding to a specified water surface elevation. The system obtains
intermediate values by interpolation. Thus, the number of rows in the FTABLE depends
on the size of the cross section and the desired resolution. The stage-discharge
relationship, specified in FTABLE columns, defines volume-dependent outflows. If only
time-dependent releases are to be used, then no discharge column is necessary. The HSPF
manual (Bicknell et al, 2005) describes several ways to specify more complex outflow
situations, such as time-dependent releases, seasonal rating curve variability, and
combinations of volume-dependent rule curves with time-dependent release demands.
Page 1 of 8
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Lined
Channel
Outflow
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RCHTAB
B) Function table used to specify geometry
and hydraulic properties of a RCHRES
A) Typical reach and mixed reservoir
rngation
release i/ Power release
Figure 1. Typical RCHRES configurations and the method used to represent geometric
and hydraulic properties
The FTABLE in WinHSPF
WinHSPF uses a series of four simple text files in creating a new project. These files are
intended to be produced using the Model Setup plug-in of BASINS 4.0, accessed through
the Models:HSPF menu in the BASINS 4.0 GIS interface. Since these four files are text
files, they may be built manually. Once a new project (UCI) has been created, these files
will no longer be needed by the project. One of these files, called the Channel Geometry
File, is used by WinHSPF for computing FTABLEs. The Channel Geometry File has a
.ptf extension.
The Model Setup plug-in for HSPF writes the Channel Geometry File using the GIS data
layers specified through its interface. In that interface the user must specify fields of a
GIS layer containing stream attributes including stream length, slope, mean depth, and
mean width. In most cases the BASINS user will have computed those values through
the process of watershed delineation. Mean width and mean depth are computed during
watershed delineation as a function of upstream area. See the BASINS User's Manual
for details on the Watershed Delineation tools and the use of the Model Setup plug-in.
Page 2 of 8
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WinHSPF automatically creates FTABLEs at the point the UCI is built using data from
the Channel Geometry File. WinHSPF assumes a pre-set channel and flood plain
geometry and calculates outflow using Manning's equation. The following assumptions
are used in computing the channel cross-section geometry (see Figure 2):
• the channel cross-section is trapezoidal
• the channel sides have slopes of 1:1
• the channel depth is 1.25 times the mean channel depth
• the flood plain width, on each side of the reach, is equal to the mean channel
width
• the depth at which the flood plain slope changes is 1.5 times the channel depth
• the default slopes of the upper and lower flood plain are 0.5:1
• the maximum depth in the FTABLE is set to 50 times the channel depth
Figure 2. Assumed channel cross-section geometry used in WinHSPF
WinHSPF then uses the above cross-sectional geometry, the Manning's n and slope, and
Manning's equation, to calculate outflow for different depths. The user can specify
additional volume dependent outflows by adjusting the FTABLE size and increasing the
number of outflows. To change the number of rows or outflow columns in the FTABLE,
change the values in the NRows and NCols fields in the WinHSPF FTABLE interface
(Figure 3).
Page 3 of 8
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v EditFtable
FTable: J5-R5 Western Branch _^J
Depth (ft)| Area(acres)| Volume (acre-ft) Outflowl (ft3/s)
00 0! 0
1 0.7 1.41 0.47 15.48
1 1.4 2.82 1.88 98.27
1 2A 3.29 4.23 310.3
1 2.8 3.29 6.11 634.15
1 3.5 3.29 8.46 1047.62
1 4.2 3.76 11.28 1544.44
1 4.9 3.76 13.63 2121
1 5i 3.76 16.45 2775.13
1 6.3 4.23 19.27 3505.53
1 7 4.23 22.09 4311.5
1 7.7 4.23 24.91 5192.77
OK Cancel Apply Help
A|
NRows: IB
NCols: |~~
Import From
Cross Section
Compute New
F-Curve
^
4
Figure 3. The WinHSPF Interface for Editing an FTABLE
The above FTABLE editing interface can be found within the Reach Editor interface in
WinHSPF. The number of reach exits can be changed using the 'NExits' field in the
WinHSPF Reach Editor. The 'Import From Cross Section' feature can be used to
compute an FTABLE by specifying the full range of geometry measurements shown in
Figure 2, enabling a user to override the WinHSPF assumptions of cross-sectional
geometry. The 'Compute New' feature provides an alternative mechanism for computing
the stream mean width, mean depth, and Manning's roughness coefficient based on
physiographic province and drainage area (see technical note #2). See the WinHSPF
User's Manual for full details on the use of the WinHSPF Reach Editor.
POPULATING RESERVOIR FTABLES IN WINHSPF
The combination of default data and assumptions used in B ASINS/WinHSPF provide an
inadequate representation of reservoir characteristics; the user instead should manipulate
the FTABLE to describe the reservoir. WinHSPF allows the user to edit FTABLEs
through the user interface. To edit an FTABLE in WinHSPF, select the FTABLE button
from the Reach Editor window, select the reach of interest, and type values in the table.
Reservoir Geometry
As described above, FTABLEs describe how outflow, water volume and surface area
change as a function of depth in the reach or reservoir (RCHRES) segments. For
reservoir segments, the user must obtain data describing the reservoir. Data tables or
graphs describing the depth-area and depth-volume relationships are generally available
from the agency managing the reservoir. When depth-area and/or depth-volume data not
available, a bathymetric map of the lake must be created. Bathymetric methods can be
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found in texts such as Welsch, 1948. The surface area can then be calculated from
planimetry; the volume of the lake at a given depth is then:
A-.*dz
where Vmis the volume at depth zm of interest, Az is the surface area of the lake at depth
z, and dz is the incremental depth (Hutchinson, 1975).
Tables of depth-area and depth-volume data can be used directly to create the first three
columns of the FTABLE. The depth values may be given in terms of stage or elevation
rather than depth, in which case the depth is the stage minus the elevation of the deepest
point of the reservoir (parameter STCOR in HSPF section HYDR). Additionally, where
depths on the depth-area and depth-volume tables do not correspond exactly, the user
must interpolate on one table such that the depth, surface area, and volume data are
complete for each row in the FTABLE. Graphical data must be linearized by selecting
points which define linear sections of the curve, again using the same depth values on
both plots. Since HSPF linearly interpolates between rows in the FTABLE, regions of the
curve where the slope changes rapidly should be sampled more closely than relatively flat
portions. There is an upper limit of 100 on the number of entries (rows times columns) in
the FTABLE. Important elevations such as the gate invert and the spillway crest should
be among those chosen. Finally, the depth (and therefore storage) for the last row should
exceed the highest level ever expected, to prevent HSPF from terminating due to
FTABLE exceedance.
Reservoir Releases
The final step is to add any necessary discharge columns. Historical gage station data is
available from the United States National Water Information System (NWIS) data
retrieval web page at: http://waterdata.usgs.gov/nwis/sw. Methods for measuring stage
and discharge is discussed in USGS Circular 1123: Stream-Gaging Program of the U.S.
Geological Survey (Wahl, et al, 1995) at: http://pubs.usgs.gov/circ/circll23/index.html.
Up to five outflows may be specified, each of which may be routed to a downstream
reach (in HSPF), or may be assumed to leave the system (i.e. a diversion out of the
watershed). While HSPF can model two or more reservoir outflows separately (e.g. gate
and spillway) and still route them to the same downstream reach, WinHSPF currently is
capable of displaying a watershed schematic showing only one exit from each reach.
Three examples of UCI modifications for reservoirs follow. Example 1 shows a sample
FTABLE for a reservior. Example 2, below, describes modifications to the UCI file
required to route multiple outflows (as defined in the FTABLE) to the downstream reach.
Example 3 describes additional UCI file modifications required to route a specific
outflow to the downstream reach while allowing other outflows to be removed from the
system. The UCI file can be modified in any text editor or through the WinHSPF
interface. Page 5 of 8
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EXAMPLES
Example 1: The sample FTABLE shown in Table 1 is for a moderate-sized reservoir
with two exits. The first three columns were taken from a tabular summary of the stage-
area and stage-volume curves. The first discharge column is the rule curve for the gate
based on the storage in the reservoir. The second is the spillway rating curve. Both exits
will be routed to the same downstream reach (in the NETWORK or SCHEMATIC block
in HSPF). The gate invert lies 38 ft above the lowest point in the reservoir, with a dead
storage capacity of 1,148 acre-feet. There is no outflow below this elevation. The
spillway crest is at 123 feet, giving a total storage capacity of 28,949 acre feet.
Remember that the actual elevations of the gate invert and spillway crest are the
respective depths plus the stage correction parameter STCOR.
Table 1 - Example Reservoir FTABLE
FTABLE 1
ROWS COLS ***
14 5
DEPTH
AREA
(FT) (ACRES)
0.
8.
18.
28.
38.
48.
58.
78.
98.
113.
123.
126.
129.
132.
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
0.
4 .
24.
47.
86.
132.
182.
307.
504.
640.
734.
810.
891.
977 .
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
VOLUME
(AC-FT)
0.
17 .
143.
491.
1148.
2230.
3791.
8636.
16643.
24020.
28949.
33040.
37120.
41221.
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
GATE
(CFS)
0.
0.
0.
0.
0.
100.
750.
945.
1245.
1450.
1650.
1750.
1850.
2000.
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
SPILLWAY ***
(CFS) ***
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
800.
1750.
2740 .
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
.00
END FTABLE 1
*** Note that all lines containing three consecutive asterisks are comment lines, which
are ignored by the program. They may be included anywhere in the UCI file, often to
assist the user in aligning data fields. Numeric entries are generally right-justified within
the field.
Page 6 of 8
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Example 2: In Table 2, the reservoir is at the base of the containing subbasin between
upper and lower reaches of the river. Notice the number of exits (NEXITS) for the
reservoir is set to 2. This value, which corresponds to the number of outflows from the
reservoir, is specified in the WinHSPF Reach Editor. In the HYDR-PARM1 table of the
UCI file, the ODFVFG(2) value for reach 3 (the reservoir) was changed from 0 to 5 to
indicate that the fifth column of the reservoir FTABLE should be used to determine its
second outflow. Up to three additional outflows could be identified here as necessary.
With the above two minor modifications, the model will now remove flow from the
reservoir as per the second outflow column in the FTABLE (FTABLE column 5) as well
as the primary outflow; the total flow will be routed to the downstream reach.
Table 2 - Modifications to UCI file for Multiple Outflows
(Original)
GEN-INFO
RCHRES
Name
1 Lower River
2 Upper River
3 Reservoir
END GEN-INFO
Nexits Unit Systems Printer
-> User T-series
in out
1111
1111
2111
Engl Metr LKFG ***
34
34
34
HYDR-PARM1
RCHRES Flags for each HYDR Section ***
# - # VC Al A2 A3 ODFVFG for each *** ODGTFG for each
FG FG FG FG
0111
0111
0111
possible exit
40000
40000
40000
possible exit
00000
00000
00000
FUNCT for each
possible exit
10000
10000
11000
(Modified)
GEN-INFO
RCHRES
Name
1 Lower River
2 Upper River
3 Reservoir
END GEN-INFO
Nexits Unit Systems Printer
-X > User T-series Engl Metr LKFG
in out
1 1 1 1 34 0 0
1 1 1 1 34 0 0
2 1 1 1 34 0 1
HYDR-PARM1
RCHRES Flags for each HYDR Section ***
# - # VC Al A2 A3 ODFVFG for each ***
FG FG FG FG possible exit ***
1 0111 40000
2 0111 40000
3 0111 45000
ODGTFG for each
possible exit
00000
00000
00000
FUNCT for each
possible exit
10000
10000
11000
Page 7 of 8
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Example 3: To modify the link to the downstream reach, modify the MASS-LINK block
in the UCI file. Table 3 shows the original block for RCHRES to RCHRES connections
as well as a modified version that will connect only the first outflow to the downstream
reach. The first line in the modified version is the original code commented out with three
asterisks at the end of the line. The second line in the modified block indicates that the
first outflow (OFLOW, member # 1) from the reservoir (RCHRES 3) should be routed to
the inflow (INFLOW) time series for the lower river segment (RCHRES 1). The second
flow will then exit the model upon removal from the reservoir. Additional routing options
can be found in the HSPF user manual.
Table 3 - Modifications to UCI file to Route Only One Outflow Downstream
(Original)
(Modified)
MASS-LINK 3
<-Volume-> <-Grp> <-Member-X--Mult-->
x x<-factor->
RCHRES ROFLOW
RCHRES OFLOW 1
END MASS-LINK 3
Note: The above example shows the changes for the MASS-LINK block. Similar
changes would be required for the NETWORK block for a UCI that uses that block to
connect operations.
Page 8 of 8
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References:
Bicknell, B.R., J.C. Imhoff, J.L. Kittle Jr., T.H. Jobes, and A.S. Donigian, Jr.
Hydrological Simulation Program - Fortran (HSPF). User's Manual for Release 12.2.
U.S. EPA National Exposure Research Laboratory, Athens, GA, in cooperation with U.S.
Geological Survey, Water Resources Division, Reston, VA, 2005.
Duda, P.B., J.L. Kittle, Jr., M.H. Gray, P.R. Hummel and R.A. Dusenbury. 2001.
WinHSPF - An Interactive Windows Interface to HSPF: User's Manual. U.S. EPA Office
of Water, Washington DC.
Hutchinson, G. Evelyn. A Treatise on Limnology. Volume I, Parti - Geography and
Physics of Lakes. New York, John Wiley & Sons, 1975.
Wahl Kenneth L., Wilbert O. Thomas, Jr., and Robert M. Hirsch., et al, 1995, Stream-
Gaging Program of the U.S. Geological Survey: U.S. Geological Survey Circular 123,
Reston, Virginia, 1995 (available on the internet at
http://pubs.usgs.gov/circ/circll23/index.html).
Welch, P.S. Limnological Methods. Philadelphia, Blakiston, 381 pp., 1948.
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