This thesis investigates the impact of a phenology model on conceptual hydrologic model. In conventional conceptual hydrologic models the evapotranspiration is partitioned into evaporation and transpiration by a combination of the potential evaporation and the availability of water. This way the seasonal differences in vegetation dynamics are not taken into account. These vegetation dynamics represent the presence of transpiring leaves in summer, changing to an almost complete absence of transpiration in winter for some vegetation types. Including information on these vegetation dynamics could potentially improve the way streamflow and evaporation is modelled by conceptual hydrologic models. To investigate the impact of a phenology model on conceptual hydrologic models the FLEXmodel is adjusted with a phenology model based on the ’Kc-ETo’ approach of the Food and Agricultural Organisation (FAO) of the UN. With the conventional FLEX-model and the FAOadjusted FLEX-model, 28 catchments from the USA are simulated in terms of streamflow and transpiration. The 28 catchments differ in terms dominant vegetation type and climate indices. The conventional model and the adjusted model are calibrated with streamflow observations and transpiration data from NASA Global Land Data Assimilation System (GLDAS) both separately and together. The conventional FLEX-model and the FAO-adjusted FLEX-model are compared to each other in order to determine under what climate conditions and for which vegetation types the phenology adjustment improves the performance of the model in terms of the ability to simulate streamflow and transpiration and the predictive capacity of the model. The streamflow simulation of the FLEX-model does not improve by the FAO-adjustment. The FAO adjustment does cause an improvement in the mean seasonal sum of the discharge in the spring months, which is structurally underestimated by the conventional model due to an overestimation in spring of the transpiration caused by the lack of information on vegetation dynamics. However the transpiration simulation of the FLEX-model does improve drastically by the FAO-adjustment. The large overestimation of the transpiration in early spring by the conventional FLEX-model is solved by the FAO-adjustment, improving the NS- and NSlogefficiencies for all catchments, regardless of their vegetation type of climate conditions.