The primary difference between ponds and wetlands is the method in which the outflow is calculated.

Pond outflow is calculated as a function of target storage. The target storage varies based on flood season and soil water content. The target pond volume is calculated:

$V_{targ}=V_{em}$ if $mon_{fld,beg}<mon<mon_{fld,end}$ 8:1.2.11

$V_{targ}=V_{pr}+\frac{(1-min[\frac{SW}{FC},1])}{2}*(V_{em}-V_{pr})$

if ​ $mon \le mon_{fld,beg}$ or $mon \ge mon_{fld,end}$ 8:1.2.12

where $V_{targ}$ is the target pond volume for a given day (m$^3$ H$_2$O), $V_{em}$ is the volume of water held in the pond when filled to the emergency spillway (m$^3$ H$_2$O), $V_{pr}$ is the volume of water held in the pond when filled to the principal spillway (m$^3$ H$_2$O), $SW$ is the average soil water content in the subbasin (mm H$_2$O), $FC$ is the water content of the subbasin soil at field capacity (mm H$_2$O), $mon$ is the month of the year, $mon_{fld,beg}$ is the beginning month of the flood season, and $mon_{fld,end}$ is the ending month of the flood season.

Once the target storage is defined, the outflow is calculated:

$V_{flowout}=\frac{V-V_{targ}}{ND_{targ}}$ 8:1.2.13

where $V_{flowout}$ is the volume of water flowing out of the water body during the day (m$^3$ H$_2$O), $V$ is the volume of water stored in the pond (m$^3$ H$_2$O), $V_{targ}$ is the target pond volume for a given day (m$^3$ H$_2$O), and $ND_{targ}$ is the number of days required for the pond to reach target storage.

The wetland releases water whenever the water volume exceeds the normal storage volume, $V_{nor}$. Wetland outflow is calculated:

$V_{flowout}=0$ if $V<V_{nor}$ 8:1.2.14

$V_{flowout}=\frac{V-V_{nor}}{10}$ if $V_{nor} \le V \le V_{mx}$ 8:1.2.15

$V_{flowout}=V-V_{mx}$ if $V>V_{mx}$ 8:1.2.16

where $V_{flowout}$ is the volume of water flowing out of the water body during the day (m$^3$ H$_2$O), $V$ is the volume of water stored in the wetland (m$^3$ H$_2$O), $V_{mx}$ is the volume of water held in the wetland when filled to the maximum water level (m$^3$ H$_2$O), and $V_{nor}$ is the volume of water held in the wetland when filled to the normal water level (m$^3$ H$_2$O).

Table 8:1-2: SWAT+ input variables that pertain to ponds and wetlands.