A well drilling system can include a hydraulics model which determines a modeled fluid friction pressure and a calibration factor applied to the modeled friction pressure, and a flow control device which is automatically controlled in response to a change in the calibration factor. A well drilling method can include drilling a wellbore, a fluid circulating through the wellbore during the drilling, determining a calibration factor which is applied to a modeled fluid friction pressure, and controlling the drilling based at least in part on a change in the calibration factor.