A computer-implemented method includes controlling water separation in a hydrocarbon stream flowing through a separator train including one or more separator vessels located upstream of a dehydrator by manipulating a demulsifier flowrate added to the separator train by: receiving data from a real-time process test of the separation train, estimating model fit parameters to the data to generate a water separation profile (WSP) correlating water draw-off and demulsifier flowrate for the separation train, determining a maximum and a minimum demulsifier flowrate from the WSP, receiving, from an operator, a separation performance for a target water separation value entering the dehydrator downstream from the separator train, and adjusting the demulsifier flowrate according to the WSP to achieve the target water separation entering the dehydrator.