Exploring celerity–velocity differences in headwater catchments across scales

Hydrologists have long recognized that stream discharge responds almost instantaneously to rainfall or snowmelt events in headwater catchments, even though the water that comprises the discharge may be years or decades old. This rapid mobilization of old water arises from the difference between the celerity (or rate of pressure propagation) and velocity (or rate of water movement) of a wetting front through the subsurface. The ratio of celerity to velocity, known as the kinematic ratio, can vary multiple orders of magnitude between catchments — and across different storm events within the same catchment — but the underlying mechanisms that control variations in kinematic ratio remain poorly understood. 

To address this knowledge gap, we present a series of experimentally constrained hydrologic simulations that investigate how watershed properties (e.g., depth to the water table and aquifer transmissivity) and system states (e.g., antecedent soil moisture) control differences in celerity and velocity at the plot, hillslope, and catchment scales. Simulation results are validated against rainfall–runoff experiments that use isotopic tracers to measure residence time. Global sensitivity analyses reveal that, at the plot scale, the kinematic ratio of a wetting front through the vadose zone is predominantly controlled by antecedent water content. At the hillslope and catchment scales, this relationship becomes more complex and largely depends on depth to the water table and aquifer transmissivity. This work provides new insights into the subsurface controls on subsurface flow and pressure propagation, with implications for understanding the hydrologic behavior of catchments during storm events and resultant impacts on water quality.