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A new, compact, spatially scanning, open-path 2.7 [micro]m tunable diode laser absorption spectrometer with short absorption path lengths below 10 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 [H.sub.2]O concentration profiles with a minimum fractional optical resolution of 2.1 x [10.sup.-3] optical density (OD) (2.5 x [10.sup.-4] OD after a background subtraction procedure), which permits a signal-to-noise-ratio of 407 (3400) at 10, 000 parts in [10.sup.6] (ppm) [H.sub.2]O, or normalized sensitivities of 2.6 ppm.m (0.32 ppm m) at 0.5 Hz duty cycle. The spectrometer's lateral spatial resolution (governed by the 500 [micro]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 [H.sub.2]O boundary layer from 0.7 to 11 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[micro]m diode laser was carefully characterized in terms of spectral purity, beam profile, as well as quasi-static and dynamic wavelength tuning coefficients. OCIS codes: 120.6200, 300.6260, 300.6340.