A Lyne–Hollick filter-derived threshold framework for event and base flow separation in subsurface drainage systems

Accurate separation of event flow and base flow in subsurface drainage
systems is essential for nutrient transport assessment; however, this task
remains challenging because both components are conveyed through the same
tile network, producing a composite hydrologic signal. This study develops
a standardized, objective, and reproducible framework for delineating
event and base flow across sites, seasons, and temporal resolutions. The
approach is built on the Lyne–Hollick recursive digital filter, from which
a seasonally representative mean baseflow is derived and scaled using a
dimensionless factor to define the seasonal event threshold. Flow
distribution characteristics are evaluated to support threshold
interpretation and ensure consistency across varying hydrologic
conditions. An event is defined when discharge exceeds the seasonal
threshold, increases by at least 20 percent relative to the previous time
step, and ends when flow returns to or below the threshold for two
consecutive time steps. A web-based tool was developed to implement the
framework, compute event-scale metrics, and generate reproducible event
identifiers. The method produced consistent and repeatable separation of
event and base flow while reducing subjective judgment inherent in
existing approaches. Overall, this structured and physically grounded
procedure enhances event-scale hydrologic assessment in subsurface
drainage systems and supports improved evaluation of nutrient losses under
variable climatic conditions.