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), is the volume of water held in the pond when filled to the principal spillway (m HO), is the average soil water content in the subbasin (mm HO), is the water content of the subbasin soil at field capacity (mm HO), is the month of the year, is the beginning month of the flood season, and is the ending month of the flood season.

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

8:1.2.13

where is the volume of water flowing out of the water body during the day (m HO), is the volume of water stored in the pond (m HO), is the target pond volume for a given day (m HO), and 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 HO), is the volume of water held in the wetland when filled to the maximum water level (m HO), and is the volume of water held in the wetland when filled to the normal water level (m HO).

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