A conceptual design and simulations are presented to explore the feasibility for an economical, eye-safe water vapor lidar. The design is based on using GaAlAs laser diode arrays in the lidar transmitter and photon counting silicon avalanche photodiodes (APD's) in the lidar receiver. Simulations are presented to demonstrate that a lidar using a laser diode powered transmitter and an APD photon counting detector should yield quite good signal-to-noise ratios out to ranges of a few kilometers by averaging no more than approximately 10 minutes, even for daylight conditions. A novel broad band DIAL approach is considered to take advantage of the relatively broad spectral output produced by multi-mode GaAlAs laser diode arrays. It is shown that the broad band water vapor transmittance to be measured by the lidar should permit water vapor profile retrievals through the lowest few kilometers of the atmosphere with resolutions of a few hundred meters or less.