Results
Our investigation returned over 120 named processes; see the
supplementary material for a complete list. Processes could be
satisfactorily arranged into a hierarchical structure, with an initial
division into surface, subsurface and channel processes, and a maximum
of five underlying layers. Some processes had ambiguity in their
position, for example transient saturation at the bedrock interface
could fall into soil or groundwater processes categories. Processes
often had many alternative names, for example runoff generation by
displacement of groundwater is also referred to as pipeflow, pistonflow,
old water mobilization, pre-event water mobilization, translatory flow,
pressure wave and hydraulic displacement.
Each process was assigned a unique identifier, similar to a hashtag. The
identifiers use dots to signify hierarchical level, with each level
given a text tag of at most six characters. Thus, change in grain size
during snowpack aging is classified as Surface - Snow and Cold Region
Processes – Snow Storage – Snowpack Aging – Change in grain size,
with the identifier Surf.Cold.Snow.Age.Grain . This provides a
short reference string for any process. If a different hierarchical
structure is used in future, it would be relatively simple to remap the
identifiers.
Some names could be disputed as representing stores rather than
processes, and might be classed as a physical entity in an ontological
framework. However, water storage has long been regarded as part of
watershed function, and was therefore included in the taxonomy.
Functional classes are listed in Table 1, and give more detail than
previous classifications, for example differentiating between ‘store’,
‘filling of store’ and ‘release from store’. For example, snowmelt is
designated as ‘release from store’ as only the origin store is
specified. We created an interactive version of the taxonomy that can be
accessed at
http://mcmillanhydrology.org/ProcessTaxonomy/ProcessTaxonomyDiagram.html
. A view of the interactive taxonomy with overland flow processes
expanded is given in Figure 1.
When hydrologists described watershed function, a variety of information
types were provided. Information could relate to the existence or
magnitude (of stores, fluxes), spatial variation (location), temporal
variation (seasonal variation or wetness conditions for processes to
occur or stores to fill), or response time. These types could be used as
an additional descriptor of hydrological information when stored in a
database or similar.