Reservoirs are impoundments located on the channel network of the watershed. Reservoirs receive loadings from upstream channels and potentially from surrounding routing units. They usually discharge into one downstream channel. Ponds may or may not receive flow and loadings from upstream channels. Some ponds have an outflow that connects them to a downstream channel, while others are not connected to the stream network.

Both reservoirs and ponds are simulated in SWAT+ using the reservoir input files:

• Main reservoir file: reservoir.res

• Reservoir and wetland initialization: initial.res

• Reservoir hydrology: hydrology.res

• Reservoir and wetland sediment: sediment.res

• Reservoir and wetland nutrients: nutrients.res

• Reservoir weir: weir.res

• Reservoir and wetland release: res_rel.dtl

The pointer to the reservoir and wetland initialization file is a foreign key referencing name in initial.res.

The SWAT+ Editor assigns a name to every object, which is typically the abbreviation of the object type followed by the object number in QSWAT+. However, the name of the reservoir is not used by SWAT+, so the user may change it.

The pointer to the reservoir and wetland nutrient file is a foreign key referencing name in nutrients.res.

The name of the reservoir and wetland initialization record is a primary key referenced by the foreign keys init in reservoir.res and init in wetland.wet. All names in the initial.res file must be unique.

The pointer to the organic-mineral initialization file is a foreign key referencing name in om_water.ini.

The pointer to the reservoir hydrology file is a foreign key referencing in .

The ID of the reservoir is a primary key referenced by the foreign key in . All IDs in the reservoir.res file must be unique.

The pointer to the pesticide initialization file is a foreign key referencing name in pest_water.ini.

The name of the reservoir hydrology record is a primary key referenced by the foreign key hyd in reservoir.res. All names in the hydrology.res file must be unique.

If 0 is input for yr_op and mon_op, the model assumes the reservoir is in operation at the beginning of the simulation.

If shp_co2 is set to 0, the model will estimate this value.

If shp_co1 is set to 0, the model will estimate this value.

The year specified here must be within the simulation period. If 0 is input for yr_op and , the model assumes the reservoir is in operation at the beginning of the simulation.

The pointer to the reservoir and wetland sediment file is a foreign key referencing in .

The name of the reservoir and wetland sediment record is a primary key referenced by the foreign keys in and in . All names in the sediment.res file must be unique.

The pointer to the reservoir and wetland release decision table file is a foreign key referencing name in res_rel.dtl.

For SWAT+ to calculate the reservoir surface area each day, the surface area at two different water volumes must be defined. Variables referring to the principal spillway can be thought of as variables referring to the normal reservoir storage volume, while variables referring to the emergency spillway can be thought of as variables referring to the maximum reservoir storage volume.

For SWAT+ to calculate the reservoir surface area each day, the surface area at two different water volumes must be defined. Variables referring to the principal spillway can be thought of as variables referring to the normal reservoir storage volume, while variables referring to the emergency spillway can be thought of as variables referring to the maximum reservoir storage volume.

The name of the reservoir and wetland nutrient record is a primary key referenced by the foreign keys in and in . All names in the nutrient.res file must be unique.

The amount of suspended solid settling that occurs in the water body on a given day is calculated as a function of concentration. Settling occurs only when the sediment concentration in the water body exceeds the equilibrium sediment concentration specified by the user.

A negative settling rate indicates that the reservoir sediments are a source of N. A positive settling rate indicates that the reservoir sediments are a sink for N.

A negative settling rate indicates that the reservoir sediments are a source of N. A positive settling rate indicates that the reservoir sediments are a sink for N.

SWAT+ calculates the median sediment particle diameter for impoundments located within a landscape unit using the equation

where $d_{50}$ is the median particle size of the sediment (µm), $m_c$ is percent clay in the surface soil layer, $m_{silt}$ is the percent silt in the surface soil layer, and $m_s$ is the percent sand in the surface soil layer.

Because reservoirs are located on the channel network and receive sediment from the entire area upstream, defaulting the sand, silt, and clay fractions to those of a single LSU or HRU in the upstream area is not appropriate. Instead, the user may set the median particle size diameter to a representative value.

The table below lists the D50 values of different sediment classes:

The chlorophyll a concentration in the reservoir is calculated from the total phosphorus concentration. The equation assumes the system is phosphorus-limited. The chlorophyll a coefficient was added to the equation to allow the user to adjust results to account for other factors not included in the basic equation, e.g. nitrogen limitations.

The model allows the user to define two settling rates for each nutrient and the time of the year during which each settling rate is used. A variation in settling rates is allowed so that impact of temperature and other seasonal factors may be accounted for in the modeling of nutrient settling. To use only one settling rate for the entire year, both variables for the nutrient may be set to the same value. Setting all variables to zero will cause the model to ignore settling of nutrients in the water body.

The model allows the user to define two settling rates for each nutrient and the time of the year during which each settling rate is used. A variation in settling rates is allowed so that impact of temperature and other seasonal factors may be accounted for in the modeling of nutrient settling. To use only one settling rate for the entire year, both variables for the nutrient may be set to the same value. Setting all variables to zero will cause the model to ignore settling of nutrients in the water body.

A negative settling rate indicates that the reservoir sediments are a source of P. A positive settling rate indicates that the reservoir sediments are a sink for P.

For natural lakes, measured phosphorus settling velocities most frequently fall in the range of 5 to 20 m/year although values less than 1 m/year to over 200 m/year have been reported (Chapra, 1997). Panuska and Robertson (1999) noted that the range in apparent settling velocity values for man-made reservoirs tends to be significantly greater than for natural lakes. Higgins and Kim (1981) reported phosphorus apparent settling velocity values from –90 to 269 m/year for 18 reservoirs in Tennessee with a median value of 42.2 m/year. For 27 Midwestern reservoirs, Walker and Kiihner (1978) reported phosphorus apparent settling velocities ranging from –1 to 125 m/year with an average value of 12.7 m/year.

The table below lists recommended apparent settling velocity values for phosphorus (Panuska and Robertson, 1999):

If seepage occurs in the reservoir, the hydraulic conductivity must be set to a value other than 0.

The clarity of the reservoir is expressed by the secchi-disk depth, which is calculated as a function of chlorophyll a. Because suspended sediment also can affect water clarity, the water clarity coefficient has been added to the equation to allow users to adjust for the impact of factors other than chlorophyll a on water clarity.

A negative settling rate indicates that the reservoir sediments are a source of P. A positive settling rate indicates that the reservoir sediments are a sink for P.

For natural lakes, measured phosphorus settling velocities most frequently fall in the range of 5 to 20 m/year although values less than 1 m/year to over 200 m/year have been reported (Chapra, 1997). Panuska and Robertson (1999) noted that the range in apparent settling velocity values for man-made reservoirs tends to be significantly greater than for natural lakes. Higgins and Kim (1981) reported phosphorus apparent settling velocity values from –90 to 269 m/year for 18 reservoirs in Tennessee with a median value of 42.2 m/year. For 27 Midwestern reservoirs, Walker and Kiihner (1978) reported phosphorus apparent settling velocities ranging from –1 to 125 m/year with an average value of 12.7 m/year.

The table below lists recommended apparent settling velocity values for phosphorus (Panuska and Robertson, 1999):

The name of the reservoir weir record is a primary key referenced by the foreign key xxx in res_rel.dtl. All names in the weir.res file must be unique.