More diverse plant communities are generally more productive than monocultures. This benefit of species diversity is supposed to stem from resource partitioning of species in mixtures where different species use the resources spatially, temporally, or chemically in distinct ways. With respect to water, the simultaneous cultivation of crops with distinct water uptake patterns might reduce niche overlaps and thus result in higher productivity. However, little is known about whether and how spatial water uptake patterns of crop species differ among different planting arrangements and whether these changes result in increased niche partitioning and explain overyielding in mixtures.Stable isotopes of water and a Bayesian model were used to investigate the spatial water uptake patterns of six different crop species and how these patterns change depending on the planting arrangement (monocultures vs mixtures). Niche overlaps and niche widths in spatial water uptake were compared among the different crop diversity levels and linked to productivity. Furthermore, spatial water uptake was related to competition intensity and overyielding in mixtures.We found evidence for increased niche partitioning in spatial water uptake, and therefore complementary spatial root distributions of crop species, and higher expected productivity in mixtures compared to expected productivity in monocultures both due to inherent species‐level differences in water uptake and plasticity in the water uptake pattern of species. We also found a significant relationship of competition and overyielding with observed patterns in spatial water uptake. These results suggest that competition was most intense in shallow soil layers and enhanced overyielding was related to a gradual increase of water uptake in deeper soil layers. Thus, overyielding might be related to a more complete spatial exploitation of available water sources.Synthesis. Differences in spatial water uptake and niche partitioning of intercropped species, driven most likely by a complementary spatial root distribution, might explain why mixtures outperform monocultures. These findings underpin the potential of intercropping systems for a more sustainable agriculture with a more efficient use of soil resources and hence reduced input demands. [ABSTRACT FROM AUTHOR]