Dear Dr. Drew Birkhead :

I think that you are hitting a limitation of the HEC-RAS model.

However, one item you can try is to specify lateral pumps in your model. You can define pumps along the river, setting the pumps to turn on and off, as well as pump at various rates as the water surface elevation rises. In that way, you can simulate the evaporation and infiltration at various rates depending upon how much water is flowing at a cross section. This solution is not perfect, but will allow you to work around this limitation.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com