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).