# Evaporation Losses

Evaporation losses from the reach are calculated:

           $$E\_{ch}=coef\_{ev}*E\_o*L\_{ch}*W*fr\_{\Delta t}$$                                                              7:1.6.1

where $$E\_{ch}$$ is the evaporation from the reach for the day (m$$^3$$ H$$*2$$O), $$coef*{ev}$$ is an evaporation coefficient, $$E\_o$$ is potential evaporation (mm H$$*2$$O), $$L*{ch}$$ is the channel length (km), $$W$$ is the channel width at water level (m), and $$fr\_{\Delta t}$$ is the fraction of the time step in which water is flowing in the channel. 

                The evaporation coefficient is a calibration parameter for the user and is allowed to vary between 0.0 and 1.0. 

                 The fraction of the time step in which water is flowing in the channel is calculated by dividing the travel time by the length of the time step.

Table 7:1-6: SWAT+ input variables that pertain to evaporation losses.

| Variable Name | Definition                                          | File Name |
| ------------- | --------------------------------------------------- | --------- |
| EVRCH         | $$coef\_{ev}$$: Reach evaporation adjustment factor | .bsn      |
| CH\_L(2)      | $$L\_{ch}$$: Length of main channel (km)            | .rte      |