The study of internal water relations or water balance within the plants and crops in dry zones is relevant to understand the physiological processes involved in the production and adaptation to the environmental conditions where a water deficit prevails. Since some time ago, several authors have proposed different concepts to describe the water status within a plant. The aim of this work is to analyze some of the popular methods commonly used to study plants or crops under water stress. Such methods are based on the measurement of the water potential and its components, as well as the transpiration process. An analysis of the pressure-volume curve is useful to calculate the osmotic potential and the turgor pressure. This possibility is relevant to get a deeper undersanding into the study of plants and crops growing under different water-stressed environmental conditions, and to compare genotypes, genetic lines or varieties of a plant species. Several sources and possibilities of error must be considered in order to collect useful information to propose valid conclusions, in basic or applied research. Such considerations must include a precise estimation of the physiology of the plant or crop under study, taking into account other features as plant age, season and time of measurement, health status and overall performance at the whole plant level.

....