Updated April 2007 

BASINS Technical Note 1 

Creating Hydraulic Function Tables 

(FTABLES) for Reservoirs in BASINS 
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 
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 
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 
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 BASINS/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 
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: 

where Vm is 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/circ1123/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 
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 stagearea 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 VOLUME GATE SPILLWAY *** 

 (FT) (ACRES) (AC-FT) (CFS) (CFS) *** 

 0.0 0.0 0.0 0.00 0.00 

 8.0 4.0 17.0 0.00 0.00 

 18.0 24.0 143.0 0.00 0.00 

 28.0 47.0 491.0 0.00 0.00 

 38.0 86.0 1148.0 0.00 0.00 

 48.0 132.0 2230.0 100.00 0.00 

 58.0 182.0 3791.0 750.00 0.00 

 78.0 307.0 8636.0 945.00 0.00 

 98.0 504.0 16643.0 1245.00 0.00 

 113.0 640.0 24020.0 1450.00 0.00 

 123.0 734.0 28949.0 1650.00 0.00 

 126.0 810.0 33040.0 1750.00 800.00 

 129.0 891.0 37120.0 1850.00 1750.00 

 132.0 977.0 41221.0 2000.00 2740.00 

 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
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 Nexits Unit Systems Printer *** 

 # - #<------------------><---> User T-series Engl Metr LKFG *** 

 in out ***

 1 Lower River 1 1 1 1 34 0 0 

 2 Upper River 1 1 1 1 34 0 0 

 3 Reservoir 2 1 1 1 34 0 1 

 END GEN-INFO 

 HYDR-PARM1 

 RCHRES Flags for each HYDR Section *** 

 # - # VC A1 A2 A3 ODFVFG for each *** ODGTFG for each FUNCT for each 

 FG FG FG FG possible exit *** possible exit possible exit

 1 0 1 1 1 4 0 0 0 0 0 0 0 0 0 1 0 0 0 0 

 2 0 1 1 1 4 0 0 0 0 0 0 0 0 0 1 0 0 0 0 

 3 0 1 1 1 4 0 0 0 0 0 0 0 0 0 1 1 0 0 0

(Modified) 

GEN-INFO 

 RCHRES Name Nexits Unit Systems Printer *** 

 # - #<------------------><---> User T-series Engl Metr LKFG *** 

 in out ***

 1 Lower River 1 1 1 1 34 0 0 

 2 Upper River 1 1 1 1 34 0 0 

 3 Reservoir 2 1 1 1 34 0 1 

 END GEN-INFO 

 HYDR-PARM1 

 RCHRES Flags for each HYDR Section *** 

 # - # VC A1 A2 A3 ODFVFG for each *** ODGTFG for each FUNCT for each 

 FG FG FG FG possible exit *** possible exit possible exit

 1 0 1 1 1 4 0 0 0 0 0 0 0 0 0 1 0 0 0 0 

 2 0 1 1 1 4 0 0 0 0 0 0 0 0 0 1 0 0 0 0 

 3 0 1 1 1 4 5 0 0 0 0 0 0 0 0 1 1 0 0 0

Page 7 of 8
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) 

 MASS-LINK 3 

<-Volume-> <-Grp> <-Member-><--Mult--> <-Target vols> <-Grp> <-Member-> *** 

<Name> <Name> x x<-factor-> <Name> <Name> x x *** 

RCHRES ROFLOW RCHRES INFLOW 

 END MASS-LINK 3

(Modified) 

 MASS-LINK 3 

<-Volume-> <-Grp> <-Member-><--Mult--> <-Target vols> <-Grp> <-Member-> *** 

<Name> <Name> x x<-factor-> <Name> <Name> x x *** 

RCHRES ROFLOW RCHRES INFLOW *** 

RCHRES OFLOW 1 RCHRES INFLOW 

 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
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, Part1 - 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, StreamGaging 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/circ1123/index.html). 

Welch, P.S. Limnological Methods. Philadelphia, Blakiston, 381 pp., 1948.