A few of the available options for Raman and differential absorption lidar measurements of the vertical profile of the water vapour mixing ratio in daytime are compared through numerical simulation. In ENEA's project, such an instrument is to be deployed at the island of Lampedusa for measurements in the Planetary Boundary Layer. The numerical model shows good agreement with independent lidar system simulations from the literature. The results show that DIAL yields a definitely better signal-to-noise ratio than Raman lidar. However, the Raman lidar technique is retained because of the system complexity that would arise with DIAL. The 'solar blind' region has been discarded because ranges larger than 1–2 km cannot practically be reached, and a wavelength of 355 nm has been chosen due to the availability of powerful laser sources that can be kept simple to operate. A dual-receiver system has been found to be necessary for viewing the entire 0.1–3 km range interval.