Changes over time in sodium hydroxide concentration in a hot water bath used for pretzel manufacture
You need both. 


Adding water, as water evaporates, but hydroxide does not.
Adding the hydroxide solution, as it is taken away by pretzels


But I suppose the evaporation is the major effect.

Probably the most convenient way is measuring density, with areometer $\pu{1.0-1.2 g / ml}$

Additionally, hydroxide gradually absorbs carbon dioxide, becoming sodium carbonate. pH paper strips may do good here, if solution looses it's alkality.