The volume of precipitation falling on the pond or wetland during a given day is calculated:

$V_{pcp}=10*R_{day}*SA$ 8:1.2.7

where $V_{pcp}$ is the volume of water added to the water body by precipitation during the day (m$^3$ H$_2$O), $R_{day}$ is the amount of precipitation falling on a given day (mm H$_2$O), and $SA$ is the surface area of the water body (ha).

The volume of water entering the pond or wetland on a given day is calculated:

$V_{flowin}=fr_{imp}*10*(Q_{surf}+Q_{gw}+Q_{lat})*(Area-SA)$ 8:1.2.8

where $V_{flowin}$ is the volume of water flowing into the water body on a given day (m$^3$ H$_2$O), $fr_{imp}$ is the fraction of the subbasin area draining into the impoundment, $Q_{surf}$ is the surface runoff from the subbasin on a given day (mm H$_2$O), $Q_{gw}$ is the groundwater flow generated in a subbasin on a given day (mm H$_2$O), $Q_{lat}$ is the lateral flow generated in a subbasin on a given day (mm H$_2$O), $Area$ is the subbasin area (ha), and $SA$ is the surface area of the water body (ha). The volume of water entering the pond or wetland is subtracted from the surface runoff, lateral flow and groundwater loadings to the main channel.

Ponds and wetlands are water bodies located within subbasins that received inflow from a fraction of the subbasin area. The algorithms used to model these two types of water bodies differ only in the options allowed for outflow calculation.

The water balance for a pond or wetland is:

$V=V_{stored}+V_{flowin}-V_{flowout}+V_{pcp}-V_{evap}-V_{seep}$ 8:1.2.1

where $V$ is the volume of water in the impoundment at the end of the day (m$^3$ H$_2$O),$V_{stored}$ is the volume of water stored in the water body at the beginning of the day (m$^3$ H$_2$O), $V_{flowin}$ is the volume of water entering the water body during the day (m$^3$ H$_2$O), $V_{flowout}$ is the volume of water flowing out of the water body during the day (m$^3$ H$_2$O), $V_{pcp}$ is the volume of precipitation falling on the water body during the day (m$^3$ H$_2$O), $V_{evap}$ is the volume of water removed from the water body by evaporation during the day (m$^3$ H$_2$O), and $V_{seep}$ is the volume of water lost from the water body by seepage (m$^3$ H$_2$O). $V_{seep}$ is added to shallow aquifer storage.

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

The volume of water lost to evaporation on a given day is calculated:

$V_{evap}=10*\eta*E_o*SA$ 8:1.2.9

where $V_{evap}$ is the volume of water removed from the water body by evaporation during the day (m$^3$ H$_2$O), $\eta$ is an evaporation coefficient (0.6), $E_o$ is the potential evapotranspiration for a given day (mm H$_2$O), and $SA$ is the surface area of the water body (ha).

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 surface area of the pond or wetland is needed to calculate the amount of precipitation falling on the water body as well as the amount of evaporation and seepage. Surface area varies with change in the volume of water stored in the impoundment. The surface area is updated daily using the equation:

8:1.2.2

where is the surface area of the water body (ha), is a coefficient, is the volume of water in the impoundment (m HO), and is an exponent.

The coefficient, , and exponent, , are calculated by solving equation 8:1.1.2 using two known points. For ponds, the two known points are surface area and volume information provided for the principal and emergency spillways.

8:1.2.3

8:1.2.4

where is the surface area of the pond when filled to the emergency spillway (ha), is the surface area of the pond when filled to the principal spillway (ha), is the volume of water held in the pond when filled to the emergency spillway (m HO), and is the volume of water held in the pond when filled to the principal spillway(m HO). For wetlands, the two known points are surface area and volume information provided for the maximum and normal water levels.

8:1.2.5

8:1.2.6

where is the surface area of the wetland when filled to the maximum water level (ha), is the surface area of the wetland when filled to the normal water level (ha), 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).​

The volume of water lost by seepage through the bottom of the pond or wetland on a given day is calculated:

$V_{seep}=240*K_{sat}*SA$ 8:1.2.10

where $V_{seep}$ is the volume of water lost from the water body by seepage (m$^3$ H$_2$O), $K_{sat}$ is the effective saturated hydraulic conductivity of the pond or wetland bottom (mm/hr), and $SA$ is the surface area of the water body (ha).

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.