I am using high sodium vapor lights in an agricultural setting and require that a certain amount of lumens be available. I am wondering if the amount of light emitted declines over time and if the bulbs should be replaced before they burn out to ensure that the amount of lumens emitted remains constant.

The characteristic is called Lumen Depreciation. HPS has one of the best ratings or factors of the amount of lumen output at or near the end of life.

Other factors that affect lumen output is dirt in the fixture or on the surface of the reflector and refractor or lens, along with reflector degradation, ballast losses, temperature, and voltage fluctuations. This is called Light Loss Factor, and is applied to a light fixture and lamp combination.

Generally, HPS lamps should be relamped at 70% of their rated life and clean the fixture to maintain light output.