Since the first stoma appeared about 400 million years ago, moisture exchange between lands and the atmosphere extends into the root zone. However, due to its invisibility from the surface, root distribution and its temporal variation are difficult to estimated, which greatly hinders investigation of soil-plant-water relations and transpiration modelling. Plant water potential reflects dynamic water condition in vegetation, which is determined by moisture supply in the root zone, atmospheric demand, and plant physiological control. Thus, dynamic water potential can provide a “periscope” to observe root zone hydraulic conditions. Based on this hydraulic connection in the soil-plant-atmosphere continuum (SPAC), plant individuals work very likely as “observation wells” to the whole root zone at predawn, and as “pumping test wells” in daytime. They provide information to estimate root-zone and plant hydraulic states, and hydraulic properties. In this presentation, we will show how this root-zone periscope concept, based on continuous monitoring of plant water potential, has been used in SPAC model development, root water uptake model improvement, transpiration model parameterisation, as well as investigation of plant drought responses.