In the semi-arid regions of temperate zones, actual evapotranspiration (AET) may exceed annual precipitation, posing risks to the hydrologic budget and groundwater depletion. Despite numerous studies on AET, the contribution of various water sources to AET remain poorly understood at a regional scale. In the hydrologic cycle, AET is sourced from surface water evaporation (SE), as well as porous media evaporation (PME) and transpiration (PMT), both of which are sourced from groundwater and soil moisture. We developed a HydroGeoSphere model to study AET spatiotemporal variations and source contributions in the semi-arid North Saskatchewan River Basin (NSRB) in central Alberta, spanning Mountains, Foothills, and Plains, as well as diverse land use types. Our results indicate that across most of the NSRB, PMT represents the largest water contribution to annual AET, followed by PME, demonstrating the importance of subsurface water, with SE making the smallest contribution. The fluctuation in surface and groundwater fluxes through PMT, PME, and SE that contribute to AET varies seasonally. PMT plays a more significant role during the warmer growing season, while in colder months, the flow of water through all three sources becomes more balanced. Furthermore, the Foothills exhibit the highest water loss via AET, followed by the Plains and Mountains. This study lays a strong basis for the assessment of water flows from groundwater and surface water under future climate change scenarios, and the results can inform surface and groundwater management and planning in the future.