Improving crops against water deficits requires a better understanding of plant root system functioning. This requires a better knowledge of the water uptake process and to address the influence of root system architecture or root physiological properties on the uptake efficiency. To this end, we describe here a non-destructive system that enables a dynamic, quantitative, functional imaging of the soil water and of the root system, from the single root to the whole root system scale. This system is based on plants grown in sandy rhizotrons and relies on the modulation, by soil water content, of the intensity of light transmitted through the rhizotron. Images of the transmitted light during plant water uptake (or release) phases are recorded with a CCD camera and water content can be related to the grey level of image pixels with a calibration. This system is affordable and can be implemented relatively easily without specific equipment. It is scalable and quick to allow the phenotyping of a range of plant genotypes relative to their water uptake pattern. This pattern can then be related with root system properties (soil colonization, root architecture) at different plant stages. Combined with modeling, imaging results help in getting parameters such as root hydraulic conductivity, distributed root water uptake rates or root xylem water potential. Combination of modeling and experiment further helps in testing biological and physiological assumptions and in predicting the uptake behavior of plants in the field.