Jeffrey J. McDonnell

This last chapter synthesizes the core principles and future directions in catchment hydrology, emphasizing the processes governing streamflow generation. It critiques the field’s over-reliance on models, which often reproduce hydrographs without accurately representing underlying mechanisms. The chapter calls for a renewed focus on perceptual models derived from direct field observation, highlighting the importance of connectivity, thresholds, hysteresis and long-term water storage. A potential diagnostic framework is proposed, combining top-down measures—such as runoff ratios, event water fractions and streamwater age—with bottom-up field interrogation guided by looking for where filling and spilling occurs in the headwater catchment. It helps the reader to reframe subsurface heterogeneity as a “permeability architecture,” stressing the critical role of hydrogeophysical mapping. It also underscores the need for dialog between field hydrologists and modelers, integrating field-based “soft data” into model evaluation to ensure process realism. Finally, the chapter highlights the urgency imposed by climate and land-use changes, which are altering hydrological regimes and challenging traditional assumptions. It notes that success in advancing the field will depend on fostering discovery-driven field research, standardizing measurement approaches, and training the next generation of hydrologists to combine curiosity, persistence and creativity with discovery field science.

This chapter synthesizes recent advancements in understanding how vegetation regulates streamflow generation through ecohydrological processes. Traditional paired catchment studies are discussed and their black box nature noted. To open up the black box, the use of dual stable isotope analysis with 18O and 2H is discussed and it is noted how this has transformed the field by revealing how plants and streams often access isotopically distinct water sources. The resulting ecohydrological separation shows that plants transpire evaporatively enriched, and more often tightly bound soil water, while mobile water generating streamflow moves through preferential pathways. This separation is now recognized across diverse climates and ecosystems. It discusses how new research is showing how plants dynamically adjust their water sources—often drawing from deep soil and rock moisture reservoirs. Emerging studies quantifying tree water transit times reveal residence times from days to decades. It ends with a recognition that future progress requires standardized water extraction techniques and integration of biological processes into process-based models to unify hydrology and ecology within a shared framework of water storage, flow and age.

This chapter discusses the advances in isotope hydrology and critical zone science that have transformed our understanding of catchment storage and streamwater age since 2010. New perceptual models and technologies have revealed that catchments actually consist of two distinct water domains: active (dynamic) storage, which contributes to streamflow, and passive (residual) storage, a much larger and older reservoir that interacts with tracers but rarely releases water. This framework has helped resolve the long-standing “age paradox” of groundwater being older than the streams it feeds. Comparative studies have shown that while topography and climate influence hydrologic behavior, it is ultimately geology that governs water age and transit-time scaling. The transition from static to time-varying transit time distributions, supported by StorAge Selection functions, has shown how streamwater becomes younger during wetter periods—a phenomenon known as the inverse storage effect. The concept of the young water fraction has further quantified these dynamics, revealing that approximately one-third of global streamflow is less than three months old, while the remaining two-thirds consists of a complex mixture of older waters stored deep within the critical zone.

This chapter discusses advancements in the past two decades where hydrological research has shifted from linear runoff models to a non-linear framework focused on connectivity and thresholds. This perspective sees catchments as dynamic networks of components that connect and disconnect in response to changing storage conditions, rather than as uniformly responding systems. Connectivity refers to the hydrological linkages among landscape elements, while thresholds represent the abrupt “on–off” behavior of runoff once storage surpasses critical limits. The fill-and-spill mechanism illustrates this behavior, where water accumulates in depressions on the soil surface or subsurface bedrock interface until connected flowpaths form. Experimental studies have shown that subsurface topography, rather than surface form, dictates the onset of lateral flow where flowpath losses often prevent complete hillslope–stream connection. At larger scales, riparian zones and groundwater dynamics influence these thresholds. This synthesis integrates various runoff processes under a storage–excess paradigm applicable from humid forests to arid landscapes, emphasizing that runoff occurs when connectivity exceeds system capacity.

This chapter examines how isotope tracing techniques ushered in a radical shift in thinking regarding the role of groundwater in storm runoff. Early isotope tracing with oxygen and hydrogen isotopes revealed that storm hydrographs were influenced primarily by pre-event, or “old,” water rather than event rainfall or snowmelt. This discovery challenged the long-held assumption that groundwater played a minimal role in stormflow generation. It also led to the development of several new perceptual models to explain the rapid mobilization of stored subsurface water. The chapter goes through four of the key early perceptual models (1) the substitution hypothesis introduced the idea of pressure-driven displacement of old water by new infiltration, distinguishing between hydraulic celerity and flow velocity, (2) groundwater ridging as a perceptual model to describe capillary-fringe induced activation near streams, (3) transmissivity feedback where non-linear groundwater discharge occurs when water tables intersect highly conductive layers, and (4) pipeflow of perched groundwater as a perceptual model for how macropores can quickly drain mixed, pre-event water. Together, these models reshaped our understanding of storm runoff as a dynamic release of catchment storage. They laid the groundwork for modern discoveries of water age, connectivity and flowpath complexity in catchments.