Although large amounts of potassium (K) are applied in tropical crops and planted forests, little is known about the interaction between K nutrition and water supply regimes on water resources in tropical regions. This interaction is a major issue because climate change is expected to increase the length of drought periods in many tropical regions and soil water availability in deep soil layers is likely to have a major influence on tree growth during dry periods in tropical planted forests. A process-based model (MAESPA) was parameterized in a throughfall exclusion experiment in Brazil to gain insight into the combined effects of K deficiency and rainfall reduction (37% throughfall exclusion) on the water used by the trees, soil water storage and water table fluctuations over the first 4.5¿years after planting Eucalyptus grandis trees. A comparison of canopy transpiration in each plot with the values predicted for the same soil with the water content maintained at field capacity, made it possible to calculate a soil-driven tree water stress index for each treatment. Compared to K-fertilized trees with undisturbed rainfall (+K+W), canopy transpiration was 40% lower for K deficiency (-K+W), 20% lower for W deficit (+K-W) and 36% lower for combined K deficiency and W deficit (-K-W) on average. Water was withdrawn in deeper soil layers for -W than for +W, particularly over dry seasons. Under contrasted K availability, water withdrawal was more superficial for -K than for +K. Mean soil water content down to 18¿m below surface (mbs) was 24% higher for -K+W than for +K+W from 2¿years after planting (after canopy closure), while it was 24% lower for +K-W and 12% lower for -K-W than for +K+W. The soil-driven tree water stress index was 166% higher over the first 4.5¿years after planting for -W than for +W, 76% lower for -K than for +K, and 14% lower for -K-W than for +K+W. Over the study period, deep seepage was higher by 371¿mm¿yr-1 (+122%) for -K than for +K and lower by