Skyhawk 172/Cutlass 172
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach
Human Factors; Procedure
I had just relieved local 1. When I had taken position there was one aircraft on upwind instructed to continue straight out; this aircraft requested closed traffic.  There was on aircraft on final a Cub; this aircraft had been cleared to land RWY 21L.I completed scan of RWY 21L for landing traffic and was instructing upwind traffic to make left closed traffic.  The controller I had relieved was conducting overlap and alerted me to an aircraft encroaching into safety area for RWY 21L at [Taxiway] Delta 3. I quickly instructed the Cub on about a quarter mile final to go around and parallel RWY 21L.  The Cub pilot complied thus preventing a loss of separation on RWY. I then initiated coordination with ground control to continue the C172 across RWY 21L at [Taxiway] Delta 3.This runway incursion occurred when the ground controller decided at the last second to change the taxi route of the C172 as the aircraft was stopping at the hold bars.  The ground controller then instructed the C172 to continue via Taxiway delta; then realized that the aircraft was crossing the hold bars and instructed the aircraft to hold position. Skill enhancement training for this individual to improve pre-planning vs reacting. Improved communication skills and better scanning.
Tower Controller is advised by the Controller he is relieving that a runway incursion is about to occur. As a result; a Cub on short final is sent around. The incurring C172 was confused by the Ground Controller into thinking that a clearance to cross Runway 21L was being issued when it was not.