Updated April 2007 

# BASINS Technical Note 1 Creating Hydraulic Function Tables (FTABLES) for Reservoirs in BASINSTechnical Note 1 Creating Hydraulic Function Tables (F TABLES) for Reservoirs in BASINS April 2007

INTRODUCTION

The HSPF model (Bicknell, et al, 2005) us es a hydraulic function table, called an FTABLE, to represent the geometric and hydrau lic 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 auto matically derives the FTABLEs from stream attributes computed during watershed delineation. These stream attributes contain only rudimentary channel characteristics and do es 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 ri ver reach or reservoi r (RCHRES) segment by defining the functional relationship between water depth, surface area, water volume, and outflow in the segment (see Figure 1). Th e 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. Th at 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 fi xed 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 wa y. 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 Exam ple 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 interp olation. 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 FT ABLE 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 severa l ways to specify more complex outflow situations, such as time-dependent rel eases, seasonal rating curve variability, and combinations of volume-dependent rule curv es with time-dependent release demands.

Page 1 of 8Figure 1. Typical RCHRES configurations an d the method used to represent geometric and hydraulic properties

The FTABLE in WinHSPF

WinHSPF uses a series of four simple text f iles in creating a new pr oject. These files are intended to be produced using the Model Se tup 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 fo r computing FTABLEs. The Channel Geometry File has a .ptf extension.

The Model Setup plug-in for HSPF writes the Ch annel 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 incl uding stream length, slope, mean depth, and
mean width. In most cases the BASINS user will have computed those values through the process of watershed delin eation. Mean width and mean depth are computed during watershed delineation as a function of upstr eam area. See the BASINS User’s Manual
for details on the Watershed Delineation tools and the use of the Mode l Setup plug-in.

Page 2 of 8WinHSPF automatically creates FTABLEs at the point the UCI is built using data from the Channel Geometry File. WinHSPF a ssumes a pre-set channel and flood plain geometry and calculates outflow using Manni ng’s equation. The following assumptions are used in computing the channel cr oss-section geometry (see Figure 2):

• the channel cross-sect ion 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-se ction geometry used in WinHSPF

WinHSPF then uses the above cross-sectiona l geometry, the Manning’s n and slope, and Manning’s equation, to calculat e outflow for different depths. The user can specify additional volume dependent outflows by ad justing 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 NCol s fields in the WinHSPF FTABLE interface (Figure 3).

Page 3 of 8Figure 3. The WinHSPF Interfac e 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 Cr oss Section’ feature can be used to compute an FTABLE by specifying the full ra nge of geometry meas urements 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, a nd Manning’s roughness coefficient based on physiographic province and dr ainage 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 assump tions used in BASINS/WinHSPF provide an inadequate representation of reservoir charact eristics; the user instead should manipulate
the FTABLE to describe the reservoir. WinHSPF allows the user to edit FTABLEs through the user interf ace. 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-vo lume relationships are generally available from the agency managing the reservoir. Wh en depth-area and/or depth-volume data not available, a bathymetric map of the lake mu st be created. Bathymetric methods can befound in texts such as Welsch, 1948. The su rface area can then be calculated from planimetry; the volume of the la ke at a given depth is then:

where V m is the volume at depth z m 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 th e depth, surface area, and volume data are complete for each row in the FTABLE. Graphi cal data must be linearized by selecting points which define linear sections of the cu rve, 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 th e gate invert and the spillway crest should be among those chosen. Finally, the depth (and therefore storag e) 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 discharg e 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 leav e 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 rese rvoirs 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 allowi ng 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 8EXAMPLES

Example 1: The sample FTABLE shown in Table 1 is for a moderate-sized reservoirwith two exits. The first three columns were taken from a tabular summary of the stagearea and stage-volume curves. The first discharg e column is the rule curve for the gate based on the storage in the reservoir. The sec ond is the spillway rating curve. Both exits
will be routed to the same downstream r each (in the NETWORK or SCHEMATIC block in HSPF). The gate invert lies 38 ft above the lowest point in the re servoir, 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 th e gate invert and spillway crest are the respective depths plus the stag e 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 cons ecutive asterisks are comment lines, which are ignored by the program. They may be incl uded anywhere in th e UCI file, often to assist the user in aligning da ta fields. Numeric entries are ge nerally right-justified within
the field.

Page 6 of 8Example 2: In Table 2, the reservoir is at the ba se of the containing subbasin between upper and lower reaches of the river. Noti ce the number of exits (NEXITS) for the reservoir is set to 2. This value, which co rresponds to the number of outflows from the reservoir, is specified in the WinHSPF R each Editor. In the HYDR-PARM1 table of the UCI file, the ODFVFG(2) value for reach 3 (t he reservoir) was changed from 0 to 5 to
indicate that the fifth column of the rese rvoir FTABLE should be used to determine its second outflow. Up to three additional outflow s 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 out flow column in the FTABLE (FTABLE column 5) as well as the primary outflow; the total flow w ill 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 8Example 3: To modify the link to the downstrea m reach, modify the MASS-LINK block in the UCI file. Table 3 shows the origin al block for RCHRES to RCHRES connections
as well as a modified version that will conne ct 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 lin e 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 8References:

Bicknell, B.R., J.C. Imhoff, J.L. Kittle Jr., T.H. Jobes, and A.S. Donigian, Jr. Hydrological Simulation Program - Fortra n (HSPF). User's Manual for Release 12.2. U.S. EPA National Exposure Research Laborat ory, Athens, GA, in cooperation with U.S. Geological Survey, Water Res ources 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 Window s Interface to HSPF: User's Manual. U.S. EPA Office of Water, Washington DC.

Hutchinson, G. Evelyn. A Treatise on Lim nology. Volume I, Part1 - Geography and Physics of Lakes. New York, John Wiley & Sons, 1975.

Wahl Kenneth L., Wilbert O. Thomas, Jr., a nd Robert M. Hirsch., et al, 1995, StreamGaging Program of the U.S. Geological Surv ey: U.S. Geological Survey Circular 123, Reston, Virginia, 1995 (availa ble on the internet at http://pubs.usgs.gov/ci rc/circ1123/index.html ).

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