I’m trying to model first-order decay in the water quality module of HEC-RAS. I’ve been assuming that the decay was exponential, i.e., a concentration equation of the form C = Co*e^(kt). So, I put in a k value of -2.3/day (based on data for the constituent I’m modeling), but the model does not seem to be simulating any decay at all. Now, looking more carefully at the user manual, it strikes me that the concentration equation including decay may be more like this (though it isn’t very clear): C = Co*(1-k). If that is the case, maybe k would need to be less than 1 to make sense. Is that true? Does k need to be less than 1?