Introduction
Hydrological processes describe the movement of water through
watersheds, as part of the hydrological cycle. Any hydrologist is
familiar with these processes, such as infiltration, evapotranspiration,
or groundwater flow. Through field experiments, hydrologists have named
more specific processes, such as interflow, macropore flow or
fill-and-spill. These processes fall within the broader term “watershed
function”, defined as “the actions of the catchment on the water
entering its control volume” (Wagener et al., 2007). Building on
earlier frameworks (Black, 1997; McDonnell and Woods, 2004; Soulsby et
al., 2006), Wagener et al. (2007) classify catchment function into
partitioning, storage and release of water.
Previous frameworks largely have the goal of catchment classification:
determining clusters of catchment function that can be predicted using
physical characteristics such as soils, land-use, topography, etc.
Recently, community interest in open data has led to further work on
organizing hydrological information, for example in the CUAHSI
Hydrologic Information System (CUAHSI-HIS). A significant development is
HY_features, a “Surface Hydrology Features Conceptual Model”, and
part of the Open Geospatial Consortium WaterML 2.0 standard for online
water data (Almoradie et al., 2013). HY_features describes hydrological
and hydrographic features (e.g. waterbodies, observations) and their
relationships. It can be used to describe river networks for GIS and
modelling applications (Blodgett et al., 2021) and builds on previous
work on ontologies for hydrology (Stephen and Hahmann, 2017).
These previous frameworks, however, do not explicitly list the types of
processes (e.g. macropore flow) that hydrologists use when describing
runoff generation. There are many applications that would benefit from a
standard list of processes, such as labelling and searching hydrological
descriptions to compare processes and synthesize knowledge across sites,
collating watershed information for machine learning ground-truth, and
providing descriptors in perceptual models to share hydrological
knowledge and identify gaps (Wagener et al., 2020). These needs echo
those of the biological sciences to provide systematic nomenclature for
plants and animals (Linnaeus, 1758). Writing in Nature for Linnaeus’
300th anniversary, Godfray (2007) reminds us that
“to understand anything in science, things have to have a name
that is recognized and is universal ”.
Therefore, this technical note describes a taxonomy (hierarchical
organization) of hydrological processes, including primary and
alternative names. The taxonomy is designed to augment textual watershed
descriptions with a summary of the constituent processes. As in
biological taxonomy, future revision and expansion is expected. For
example, to limit the scope, we focused on processes in natural
watersheds that might be included in a typical runoff generation model.
We do not include process definitions, and direct the reader to
appropriate glossaries and encyclopaedia sources (WMO, 2012; NSIDC,
2021; Anderson and McDonnell, 2005). These glossaries do not however
include all the specialist terms used by catchment hydrologists. Future
work might include more detailed treatment of specific environments
(e.g. cold regions) and expansion to additional environments and domains
(e.g. wetlands, water quality, deep groundwater, human influences on
hydrology).