Forecasting the evolution of water resources in response to a changing climate in California is essential for not only sustaining its economy and agriculture but also building resilient communities. Projections show that extremes conditions will characterize the climate of California at the end of the century (EoC). Although crucial for water management, understanding the impacts of these changes in climate on the integrated hydrologic cycle, especially groundwater, has yet to be done. In this work, we utilize cutting-edge dynamical and thermodynamical global climate and integrated hydrologic models at unprecedented resolutions to assess the impacts of EoC extreme conditions (i.e. both wet and dry). We model a representative and relatively unmanaged Californian watershed to isolate the impacts of climate separate from those of water management decisions. Results highlight both an increase and change in phase (from snow to rain) in precipitation in addition to an increase in temperature. As a result, simulations show that snowpack will decrease by 90%, and groundwater and surface water storages will increase, and will do so sooner in the water year. However, because evapotranspiration demand in the EoC is also higher, summer surface water and groundwater levels are much lower than historical ones. Comparisons between historical and EoC extremes show that EoC extremes will generally be wetter than historical conditions. As groundwater storages will change both in time and magnitude by the EoC, these changes will have strong implications for water management and ecosystems health.