Streamflow Generation is a synthesis of field-based understanding of how catchments store and release water. Where does water go when it rains? What flowpath does water take to the stream? And, how old is the water leaving the catchment by streamflow and transpiration?Observing catchments through field work and field discovery are an essential part of hydrological science. This book is a new equation-free narrative of what we know about the flow pathways, sources, and travel times of water through headwater catchments.Streamflow Generation lowers the barrier of entry into the world of process hydrology, covering the period from the first field-based measurements in the early 20th Century to the development of early rainfall-runoff concepts. It covers the 1960s' great expansion of studies during the International Hydrological Decade, and the subsequent use of isotope tracers in catchment hydrology that brought about radical shifts in our understanding of the science.McDonnell explores our modern understanding of how catchments store and release water at the point-, hillslope- and catchment-scales, and how bottom-up and top-down measures have been used to develop perceptual models. The opportunities and challenges for catchment science are discussed, as well as what remains to be discovered about hydrological connectivity, the sources, residence times, and transit time distributions of streamflow and plant transpiration. What remains to be discovered is vast in comparison to what is known – early career hydrologists must continue field-based discovery science in streamflow generation.