1:1.3.4 Water Temperature

Water temperature is required to model in-stream biological and water quality processes. SWAT+ uses an equation developed by Stefan and Preud’homme (1993) to calculate average daily water temperature for a well-mixed stream:

$T_{water}=5.0+0.75\overline T_{av}$ 1:1.3.13

where $T_{water}$ is the water temperature for the day ($\degree C$), and ${\overline T_{av}}$ is the average air temperature on the day ($\degree C$).

Due to thermal inertia of the water, the response of water temperature to a change in air temperature is dampened and delayed. When water and air temperature are plotted for a stream or river, the peaks in the water temperature plots usually lag 3-7 hours behind the peaks in air temperature. As the depth of the river increases, the lag time can increase beyond this typical interval. For very large rivers, the lag time can extend up to a week. Equation 1:1.3.13 assumes that the lag time between air and water temperatures is less than 1 day.

In addition to air temperature, water temperature is influenced by solar radiation, relative humidity, wind speed, water depth, ground water inflow, artificial heat inputs, thermal conductivity of the sediments and the presence of impoundments along the stream network. SWAT+ assumes that the impact of these other variables on water temperature is not significant.

Table 1:1-8: SWAT+ input variables that pertain to water temperature.

Definition	Source Name	Input Name	Input File
$T_{max}$Tmx: Daily maximum temperature ($\degree C$)	MAX TEMP	tmpmax	.tmp
$T_{mn}$: Daily minimum temperature ($\degree C$)	MIN TEMP	tmpmin	.tmp