Adjustment for Clear/Overcast Conditions

Maximum temperature and solar radiation will be lower on overcast days than on clear days. To incorporate the influence of wet/dry days on generated values of maximum temperature and solar radiation, the average daily maximum temperature, $\mu mx_{mon}$, and average daily solar radiation, $\mu rad_{mon}$, in equations 1:3.4.10 and 1:3.4.12 are adjusted for wet or dry conditions.

The continuity equation relates average daily maximum temperature adjusted for wet or dry conditions to the average daily maximum temperature for the month:

$\mu mx_{mon}*days_{tot}=\mu Wmx_{mon}*days_{wet}+\mu Dmx_{mon}*days_{dry}$

1:3.4.14

where $\mu mx_{mon}$ is the average daily maximum temperature for the month (°C), $days_{tot}$ are the total number of days in the month, $\mu Wmx_{mon}$ is the average daily maximum temperature of the month on wet days (°C), $days_{wet}$ are the number of wet days in the month, $\mu Dmx_{mon}$ is the average daily maximum temperature of the month on dry days (°C), and $days_{dry}$ are the number of dry days in the month.

The wet day average maximum temperature is assumed to be less than the dry day average maximum temperature by some fraction of ($\mu mx_{mon}-\mu mn_{mon}$):

$\mu Wmx_{mon}=\mu Dmx_{mon}-b_T*(\mu mx_{mon}-\mu mn_{mon})$ 1:3.4.15

where $\mu Wmx{mon}$ is the average daily maximum temperature of the month on wet days (°C), $\mu Dmx_{mon}$ is the average daily maximum temperature of the month on dry days (°C), $b_T$ is a scaling factor that controls the degree of deviation in temperature caused by the presence or absence of precipitation, $\mu mx_{mon}$ is the average daily maximum temperature for the month (°C), and $\mu mn_{mon}$ is the average daily minimum temperature for the month (°C). The scaling factor, $b_T$, is set to 0.5 in SWAT+.

To calculate the dry day average maximum temperature, equations 1:3.4.14 and 1:3.4.15 are combined and solved for $\mu Dmx_{mon}$:

$\mu Dmx_{mon}=\mu mx_{mon}+b_T*\frac{days_{wet}}{days_{tot}}*(\mu mx_{mon}-\mu mn_{mon})$ 1:3.4.16

Incorporating the modified values into equation 1:3.4.10, SWAT calculates the maximum temperature for a wet day using the equation:

$T_{mx}=\mu Wmx_{mon}+\chi_i(1)*\sigma mx_{mon}$ 1:3.4.17

and the maximum temperature for a dry day using the equation:

$T_{mx}=\mu Dmx_{mon}+\chi_i(1)*\sigma mx_{mon}$ 1:3.4.18