Water uptake of plant roots is monitored using non-invasive imaging by nuclear Magnetic Resonance Imaging (MRI). The evaluation of observed patterns and conclusions on the underlying mechanisms require further comparison with numerical simulation using basic physical principles on water fluxes in soils. This also requires an unambiguous reconstruction of the entire root system from the partly noisy and discontinuous 3D information. In this paper we compare two methods for doing this : a manual method using 3D virtual reality system with manual tracking of root strands and a automated image processing method.