RIV61 RIVER PACKAGE The purpose of the River Package is to simulate the effects of flow between surface-water features and groundwater systems. For each cell affected by surface water features, a terms representing seepage must be add to the groundwater flow equation. The River Package is a specified stage package as opposed to a calculated stage package—the river stages will be read into the Package as input data.

RIV62 RIVER PACKAGE The stream is divided into reaches so that each reach is completely contained in a single cell. Stream/aquifer seepage is simulated between each reach and the model cell that contains the reach. For the River Package, the numbering scheme in the figure on the right is arbitrary. Stream Reaches Discretization ignore

RIV63 RIVER PACKAGE The upper figure cross-section show a geomorphologic conceptualization of the stream being separated from the groundwater system by a streambed. The lower figure show the same cross- section as it is mathematically conceptualized in the River Package. In many cases, no discrete low- permeability streambed layer is present, but the concept can still be applied, provided proper interpretation is placed on the various parameters that are used.

RIV64 RIVER PACKAGE It is assumed that measurable head losses between the stream and the aquifer are limited to those across the streambed. If the underlying cell remains fully saturated, that is, it water level does not drop below the bottom of the stream bed, then where Q RIV is the flow between the stream and the aquifer, taken as positive if directed into the aquifer (losing stream). H RIV is the stage elevation of the stream. C RIV is the hydraulic conductance of the stream/aquifer interconnection ( KLW/M ).

RIV65 RIVER PACKAGE The formulation of a river conductance term to account for a three dimensional flow process is inherently an empirical exercise. Reliable field measurement of stream seepages (seepage run) and associated head difference can be used to estimate an effective river conductance. Adjustment of river conductance during calibration is almost always required. Limitations to the calibration adjustments can be based on factors such as –Actual river width and reach length, –Equivalent conductance for distinct recognized stream bed layers, and –Cell hydraulic conductivity as an upper level for vertical stream bed hydraulic conductivity.

RIV66 RIVER PACKAGE If the water level in the aquifer is above the above the stream stage elevation, the stream is a gaining stream ( I ). If the water level in the in the aquifer is below the stream stage elevation ( D w <3W ), the stream is a losing stream ( II ). For these two systems, Q RIV =C RIV (H RIV −h ijk ) That is, the flow is proportional to the head difference between the stream and the aquifer. DW is an indicator of the difference between aquifer water level and stream stage elevation.

RIV67 RIVER PACKAGE As the water level in in the aquifer drops, the seepage becomes less dependent upon the head in the aquifer ( III ). Ultimately the hydraulic connection between the bottom of the stream bed and the water table will break ( IV ). The interval below the stream bed is unsaturated, but the stream bed is assumed to remain saturated. The head at the bottom of the stream bed is its elevation, designated R BOT, and Q RIV =C RIV (H RIV −R BOT ) DW is an indicator of the difference between aquifer water level and stream stage elevation.

RIV68 RIVER PACKAGE The River Package simulates flow between a stream and a node i,j,k as, C RIV (H RIV – h ijk ), h ijk >R BOT Q RIV = C RIV (H RIV – R BOT ), h ijk ≤R BOT There is no flow to or from the stream to the aquifer if h ijk = H RIV. Flow is from the aquifer to the stream (a gaining stream) if h ijk > H RIV Flow is from the stream to aquifer (a losing stream) if h ijk < H RIV The flow is constant from the stream to the aquifer if h ijk < R BOT

RIV69 RIVER PACKAGE The independence between the head in the aquifer and seepage through the steam bed is established before a break in saturation occurs. The head gradient in the saturated connection in the figure to the right is approximately unity, and further lowering of the water table will not increase this gradient. In all situations, seepage from the stream must at some point become independent of head in the aquifer. Limiting Seepage from a stream at Unit Hydraulic gradient

RIV610 RIVER PACKAGE If the seepage reaches its limiting condition when the water table reaches an elevation h L, then R BOT should be taken as h L. Because the vertical gradient beneath the stream above is approximately unity, the seepage into the cell i,j,k is approximately KLW A value of the stream bed conductance can be estimated as Limiting Seepage from a stream at Unit Hydraulic gradient hLhL

RIV611 RIVER PACKAGE NPRIV —is the number of river parameters. MXL —is the maximum number of river reaches that will be defined using parameters. MXACTR —is the maximum number of river reaches in use during any stress period, including those that are defined with parameters.

RIV612 RIVER PACKAGE IRIVCB —is a flag and unit number. IRIVCB > 0, it is the unit number to which cell-by-cell flow terms will be written when SAVE BUDGET or a non-zero value for ICBCFL is specified in output control. IRIVCB = 0, cell-by-cell terms will not be written. IRIVCB < 0, river leakage for each reach will be written to the List File when SAVE BUDGET or a non-zero value for ICBCFL is specified in output control.

RIV613 RIVER PACKAGE [Option] —is an optional list of character values. AUXILIARY abc or AUX abc—defines an auxiliary variable named abc, which is read for each well as part of Items 4 and 6. Up to 5 variable can be specified, each preceded by AUXLIARY or AUX. CBCALLOCATE or CBC—indicates that memory should be allocated to store cell-by-cell flows for each well in order to make these flows available for use in other packages.

RIV614 RIVER PACKAGE PARNAM —is the name of a parameter. This name can consist of 1 to 10 characters, and is not case specific. PARTYP —is parameter type. There is only one parameter type for the RIV Package—the river bed conductance, RIV. Parval —is the parameter value. NLST —is the number of river reaches that are included in the parameter.

RIV615 RIVER PACKAGE ITMP —is a flag and a counter. ITMP<0, non-parameter river data from last stress period will be reused. ITMP≥0, ITMP will be the number of non-parameter reaches read for current stress period. NP —is the number of parameters in use in current stress period. Layer —is the layer number of the cell that contains the river reach. Row —is the row number of the cell that contains the river reach. Column —is the column number of the cell that the contains the river reach.

RIV616 RIVER PACKAGE Stage —is the stage elevation of the stream. Condfact —is the factor used to calculate the riverbed hydraulic conductance from the parameter value. The conductance is the product of Condfact and the parameter value. Cond —is the riverbed hydraulic conductance. Rbot —is the elevation of the bottom of the riverbed.

RIV617 RIVER PACKAGE [xyz] —represent the value of any auxiliary variable for a river reach that has been defined in Item 2. The auxiliary variable must be present in each repetition of of Item 4 and 6 if they are defined in Item 2. Pname —is the name of the parameter being used in the current stress period. NP parameter names are read.

RIV618 RIVER PACKAGE No Parameters

RIV619 RIVER PACKAGE Parameters