A method of operating a selective catalytic reduction (SCR) system includes detecting at least one of an ammonia level and a nitrogen oxide level within a cross-sectional area of a chamber at a downstream end of a SCR catalyst. The cross-sectional area is divided into sections. Ammonia flow is automatically regulated from an ammonia distribution grid at an upstream end of the SCR catalyst based on at least one of the ammonia level and the nitrogen oxide level detected.