Catchment characterization: current descriptors, knowledge gaps and future opportunities

An ability to describe hydrologically relevant differences between places is at the core of our science. A common way to characterize hydrological catchments is to use descriptors that summarize important physical aspects of the system, often by aggregating heterogeneous geospatial data into a single number. Such descriptors aim to capture various facets of catchment functioning and structure, identify similarity or dissimilarity, and transfer information among them. However, so far there is no agreement on how catchment descriptors should be selected, aggregated and evaluated. Even worse, little is known about the existence of potential biases in the current practices to characterize catchments. In this systematic review, we analyze 742 research articles published between 1967 and 2021 to provide a categorized overview of current and historical practices of catchment characterization (i.e., data sources, aggregation and evaluation methods) in hydrological science and related disciplines. We uncover the existence of substantial biases in catchment characterization: (1) only 16% of the analyzed studies are in dry environments, even though such environments cover 42% of global land surface; (2) only 30% of studies use subsurface features for catchment characterization; (3) only 4% and 9% of descriptors are aggregated in spatially- and vertically-differentiated way respectively, while the absolute majority of descriptors are simple averages and do not account for hydrologically-relevant variabilities of features within catchments; (4) 25% of analyzed studies do not evaluate the usefulness and none of the analyzed studies quantify the uncertainties of catchment descriptors. We demonstrate the potential effects of these biases on our ability to effectively characterize catchments and identify functional similarity of catchment behavior with illustrative examples. Finally, we suggest possible ways to derive more robust, comprehensive and hydrologically meaningful catchment descriptors.