How SWAT+ Computes Channel Water Temperature

In the SWAT+ engine we ship, each reach computes its own heat balance and passes the temperature it computes to the reach below.

Each day, for each channel, the engine combines the water entering the reach, finds the equilibrium temperature the water would reach with the atmosphere, and moves the water toward it over the reach's travel time.

Each step links to the line of the engine source that performs it, on our public engine fork.

The temperature model is upstream SWAT+ work: the mixing by Kristin Peters, the heat exchange by Efrain Noa-Yarasca. We ship it as part of the engine we build and maintain.

Our engine corrects three issues. The heat-balance result never reached the output: a simple air-temperature estimate was written over it. And the step toward equilibrium could overshoot when flow was low, so the storage temperature reached more than a thousand degrees Celsius; we replaced it with the exact solution. On a model of the White River in Michigan, the first two fixes took the channel-days above sixty degrees Celsius in the storage temperature from 55,113 to zero.

We have not validated the temperature output for general use, and we make no claim about how closely it matches a particular river. It runs the default temperature parameters, and calibration is against streamflow. There is no reservoir temperature, and no ice.

The method, its source lines and its references are on swatgenx.com.