STREAM-AQUIFER PACKAGE

1. STREAM-AQUIFER PACKAGE • The Stream-Aquifer Package is an accounting program that tracks the flow in one or more streams which interact with groundwater. • The program limits the amount of groundwater recharge to available stream flow, allow the streams to go dry, and allows the streams to rewet. • The Stream-Aquifer Package is a calculated-stage package as opposed to a specified stage package—the river stages can be calculated by the Package instead of input as data. • The program permits two or more streams to merge into one, with the flow in the merged stream equal to the sum of the tributary flows. • The program permits diversions from the stream. STR6

2. STREAM-AQUIFER PACKAGE • Streams are divided into segments and reaches. • A reach corresponds to the individual cells in the finite-difference grid. • A segment consists of a group of reaches connected in a down stream order. • Leakage is calculated for each reach on the basis of the head difference between the stream and the aquifer and a conductance term, and then is added or subtracted to the amount of stream flow into the reach. STR6

3. STREAM-AQUIFER PACKAGE 1 Numbering and Ordering of Streams • A segment (e.g. ) is a stream or diversion in which streamflow from surface sources are added at the beginning or subtracted (as with a diversion) at the end of a segment, • A reach (e.g. 1) is the part of the segment that corresponds to an individual cell in the finite difference grid. • A segment consists of one or more reaches. • Segments are numbered sequentially from the farthest upstream segment to the last downstream segment as are the reaches within each segment. cell Note that more than one reach in different segments can be assigned to the same mode cell. STR6

4. STREAM-AQUIFER PACKAGE • Streamflow is specified for the first reach in each segment that enters the modeled area (e.g. , , ) • The streamflow that is computed at as entering an adjacent downstream reach in a segment (Sd) is equal to the inflow from the upstream reach (Su) plus or minus the leakage from or to the aquifer in the upstream reach (Lu) Sd = Su – Lu • The streamflowentering the model layer is assumed to be instantly available to downstream reaches. cell STR6

5. STREAM-AQUIFER PACKAGE • Streamflow into a segment that is formed from tributary streams is computed by adding the outflows from the last reach in each of the specified tributary segments. • If the segment is a diversion, then the specified flow into the first reach of the segment is subtracted from the flow in the main stream • If the specified flow for the diversion is greater than the flow in the stream it was diverted from, then the diversion is set to zero. (Note: this has been changed in later modifications of the Stream-Aquifer package) cell STR6

6. STREAM-AQUIFER PACKAGE • Leakage to or from the stream bed is computed by Dary’s Law, QL = CSTR(hS – ha) where, QL is the leakage to or from the streambed [L3/t], HS is the stream stage elevation [L] Ha is the head in the aquifer side of the streambed [L], CSTR is the streambed conductance [L2/t] • The stream stage elevation is for the center of the reach. • The head in the aquifer cell beneath the stream depends on the saturation. STR6

7. STREAM-AQUIFER PACKAGE • If the materials are saturated, the head of the aquifer side of the streambed is equal to the head in the model cell beneath the stream reach. • If the materials are unsaturated, the head in the aquifer beneath the streambed is equal to the elevation of the bottom of the streambed. • If leakage through the streambed to the aquifer in a stream reach is greater than the amount of streamflow that enters the reach, then the leakage is set equal to the stream flow that enters the reach, and flow out of the reach is set to zero. STR6

8. STREAM-AQUIFER PACKAGE Computing Stream Stage in Reaches • The Stream-Aquifer Package has an option to calculate the stream stage in every reach. • The stream stage is computed assuming incompressible steady state flow in the stream at constant depth and using Manning’s formula, where, Q is the stream discharge [L3/t], n is Manning’s roughness coefficient, A is the cross-sectional area of the stream [L2], R is the hydraulic radius [L], S is the slope of the stream channel, and C is a constant [L1/3/t], which is 1.486 for units of cubic feet per second and 1.0 for cubic meters per second. STR6

9. STREAM-AQUIFER PACKAGE • The cross-sectional area and the hydraulic radius for a rectangular channel are, and, where d is the depth of water in the stream [L], and w is the width of the channel [L], gives, which computes the stream depth. STR6

10. STREAM-AQUIFER PACKAGE • The streamflow estimates from Manning’s formula are accurate for the intermediate range of roughness coefficients shown in the table on the right. • Stream stage is calculated prior to calculating leakage to or from the aquifer. • An iterative procedure is adopted that for a reach, flow into the reach→calculate stream stage→calulate leakage→subtract (add) leakage from (to) flow→re-calculate stream stage→… Note that this iterative scheme can sometimes lead to instability. STR6

11. STREAM-AQUIFER PACKAGE Assumptions and Limitations • The stream-Aquifer Package is not a true routing model because the streamflow that enters into the first reach of each segment is instantly available in downstream reaches. • The stream stage calculations assume a rectangular channel where, if Manning’s equation is to hold, the width is much greater than the depth. • The width of a channel will likely vary with flow and stage in a real stream, requiring an external change of conductance values over stress periods. • The leakage into the stream is assumed to be instantaneous, which is a valid assumption if there is a hydraulic connection between the stream and aquifer, or if the break in connection is not to great. STR6

12. STREAM-AQUIFER PACKAGE NPSTR—isthe number of stream parameters. MXL—isthe maximum number of stream reaches that will be defined using parameters. MXACTS—isthe maximum number of stream reaches in use during any stress period, including those that are defined with parameters. STR6

13. STREAM-AQUIFER PACKAGE NSS—isthe number of stream segments. NTRIB—isthe number of stream tributaries that can connect to one segment. Must be less than or equal to ten. NDIV—isa flag, which when positive, specifies that diversions from segments are simulated STR6

14. STREAM-AQUIFER PACKAGE ICALC—is a flag, which when positive, specifies that stream stages in reaches are calculated CONST—isconstant value used in calculating stream stages in reaches, and is specified whenever ICALC≥0. The value is dependent on units. Cubic-feet/sec→CONST = 1.486 Cubic-meters/sec→CONST = 1.0 STR6

15. STREAM-AQUIFER PACKAGE ISTCB1—isa flag and unit number for the option to write seepage between the stream reaches and model cells into the list file or binary file. ISTCB1 > 0, it is the unit number to which seepage between each stream reach and the corresponding model cell will be saved when a non-zero value for ICBCFL is specified in output control. ISTCB1 = 0, seepage between each stream reach and the corresponding model cell will not be written to any file. ISTCB1< 0, stream flow for each reach and seepage between each stream reach and the corresponding model cell will be written to theList Filewhen a non-zero value forICBCFLis specified in output control. STR6

16. STREAM-AQUIFER PACKAGE ISTCB2—isa flag and unit number for the option to store stream flow out of reach into the list file or binary file. ISTCB2 > 0, it is the unit number to which streamflow in each reach will be saved when a non-zero value for ICBCFL is specified in output control. ISTCB2 ≤ 0, streamflow in each reach will not be written to any file. PARNAM—isthe name of a parameter. This name can consist of 1 to 10 characters, and is not case specific. STR6

17. STREAM-AQUIFER PACKAGE PARTYP—is parameter type. There is only one parameter type for the STRPackage—thestreambed conductance, STR. Parval—isthe parameter value. NLST—isthe number of stream reaches that are included in the parameter. . STR6

18. STREAM-AQUIFER PACKAGE ITMP—isa flag and a counter whose meaning depends on whether or not stream parameters are being used If NPSTR>0, ITMP is the number of stream parameters being used in the current stress period (similar to NP of other packages). If NPSTR=0, ITMP<0, non-parameter stream data from last stress periodwill be reused. ITMP≥0, ITMPwill be the number of non-parameter stream reaches read for current stress period. . STR6

19. STREAM-AQUIFER PACKAGE IRDFLG—is a flag, which when positive, suppresses printing of the stream input data for a stress period. If IRDFLG=0 and ICBCFL in output control is set, the input data are printed. IPTFLG—is a flag, which when positive, suppresses printing of the stream results for a stress period. If IPTFLG=0, ICBCFL in output control is set, and ISTCB1<0, the results are printed. STR6

20. STREAM-AQUIFER PACKAGE Layer—is the layer number of the stream reach. Row—is the row number of the stream reach. Column—is the column number of the stream reach. Seg—is a number assigned to a group of reaches. Segments must be numbered in downstream order and are read into the program in sequential order. . STR6

21. STREAM-AQUIFER PACKAGE Reach—is a sequential number in a segment that begins with 1 for the furthest upstream reach and continues in downstream order to the last reach in the segment. Reaches must be read in sequentially because the order in which reaches are read determines the order of connection. . STR6

22. STREAM-AQUIFER PACKAGE Flow—is the streamflow entering the segment. This value is used for the first reach of each segment. The value should be specified as either a 0 or a blank when the reach number is not 1. When the inflow to the first reach of a segment is the sum of the outflows from upstream tributary segments, Flow should be specified as -1. When the segment is a diversion, the Flow for each reach is the amount to divert, however, there will be no diversion if the segment from which the diversion is obtained contains less than the value of Flow. . STR6

23. STREAM-AQUIFER PACKAGE Stage—is the stream stage. The value of stage is not used if ICALC>0 Condfact—is the factor used to calculate the streambed hydraulic conductance from the parameter value. The conductance is the product of Condfact and the parameter value. Cond—is the streambed hydraulic conductance. Sbot—is the elevation of the bottom of the streambed. STR6

24. STREAM-AQUIFER PACKAGE Stop—is the elevation of the top of the streambed. The value of Stop is used if the option to calculate stream stage is active (ICALC>0) or when the streambed has zero flow. Pname—is the name of the parameter that is being used in the current stress period. ITMP parameter names will be read. They must be specified in an order that meets downstream ordering requirements for Seg and Reach. STR6

25. STREAM-AQUIFER PACKAGE Width—is the width of the stream channel. It is only read when stream stage is calculated (ICALC>0) Slope—is the slope of the stream channel. It is only read when stream stage is calculated (ICALC>0) Rough—is Manning’s roughness coefficient. It is only read when stream stage is calculated (ICALC>0) STR6

26. STREAM-AQUIFER PACKAGE Itrib—contains the segment number for each tributary that flows into a segment. NTRIB values are read for each segment. Unused values of Itrib should be set to zero. Itrib records are read only if NTRIB>0 Example: There are 4 segment and NTRIB=3, the Format is 10I5 0 0 0 0 0 0 0 0 0 1 2 3 Tributaries STR6

27. STREAM-AQUIFER PACKAGE Iupseg—is the number of upstream segments from which water is diverted. If the segment is not a diversion, Iupseg should be set to zero. Iupseg records are read only if NDIV>0 Example: There are 5 segment and NTRIB=2, the Format is I10 0 1 0 3 0 Diversions STR6

28. STREAM-AQUIFER PACKAGE No Parameters STR6

29. STREAM-AQUIFER PACKAGE No Parameters No Parameters, cont. STR6