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We here show that dual-band GPS measurements of precipitable water vapor(PWV) at KPNO predict the overall per-image sensitivity of the Mayall z-bandLegacy Survey (MzLS). The per-image variation in the brightness of individualstars is strongly correlated with the measured PWV and the color of the star.We use synthetic stellar spectra and TAPAS transmission models to predict theexpected PWV-induced photometric errors and find good agreement with theobservations. We also find that PWV absorption can be well-approximated by alinear relationship with PWV_eff and present an update on the traditionaltreatment in the literature.  Within the range of reasonable observing conditions, the MzLS zero pointvaries with a standard deviation of 127 mmag. This variation is dominated by agray secular trend with time, consistent with a gradual accumulation ofcontamination on optical surfaces that accounts for ~114 mmag of variation.Correcting for PWV based on a suite of stellar spectra and detailed PWVabsorption models accounts for another 47 mmag of zero-point variation. TheMzLS per-image sensitivity is decreased by ~40 mmag per effective mm of PWV.The difference between blue (r-z < 0.5 mag) and red (1.2 mag < r-z) starsincreases by 3.25 mmag per effective mm of PWV.  These results show the need for high-precision photometric surveys tosimultaneously monitor PWV. We find that this GPS system provides more precisePWV measurements than using differential measurements of stars of differentcolors and recommend that observatories install dual-band GPS as alow-maintenance, relatively low cost, auxiliary calibration system. We extendour results of the need for well-calibrated PWV measurements by presentingcalculations of the PWV photometric impact on three science cases of interest:stellar photometry, supernova cosmology, and quasar identification andvariability.

GPS Measurements of Precipitable Water Vapor Can Improve Survey Calibration: A Demonstration from KPNO and the Mayall z-band Legacy Survey