A new, compact, spatially scanning, open-path $2.7\mathrm{\mu m}$ tunable diode laser absorption spectrometer with short absorption path lengths below $10\mathrm{cm}$ was developed to analyze the spatiotemporal dynamics of one-dimensional (1D) spatial water vapor gradients. This spectrometer, which is based on a room- temperature distributed feedback diode laser, is capable of measuring absolute, calibration-free, line-of-sight averaged, but laterally resolved 1D ${\mathrm{H}}_2\mathrm{O}$ concentration profiles with a minimum fractional optical resolution of $2.1\times {10}^{-3}$ optical density (OD) ( $2.5\times {10}^{-4}$ OD after a background subtraction procedure), which permits a signal-to-noise-ratio of 407 (3400) at $10,000\mathrm{parts}\mathrm{in}{10}^6(\mathrm{ppm})$ ${\mathrm{H}}_2\mathrm{O}$ , or normalized sensitivities of $2.6\mathrm{ppm}\cdot \mathrm{m}$ ( $0.32\mathrm{ppm}\mathrm{m}$ ) at $0.5\mathrm{Hz}$ duty cycle. The spectrometer’s lateral spatial resolution (governed by the $500\mathrm{\mu m}$ sampling beam diameter) was validated by analyzing a well-defined laminar jet of nitrogen gas in humidified air. This scanning setup was then used to (a) quantitatively investigate for what we believe to be the first time the ${\mathrm{H}}_2\mathrm{O}$ boundary layer from 0.7 to $11\mathrm{mm}$ beneath the stomatous side of a single, undetached plant leaf, and (b) to study the temporal boundary layer dynamics and its dependence on stepwise light stimulation of the photosynthetic system. In addition the $2.7\mathrm{\mu m}$ diode laser was carefully characterized in terms of spectral purity, beam profile, as well as quasi-static and dynamic wavelength tuning coefficients.