Which scales matter for water resources management?

Contributed by Andreas Hartmann, Axel Bronstert, Bettina Schaefli

The discussion about which scale is the most relevant for water resources management is an increasingly important debate of hydrological modelling principles over the last decade.

The session “(Ir‑)relevant scales in hydrology: Which scales matter for water resources management?” convened at this year’s EGU General Assembly 2017 tried to put some new light (and fire) into this ongoing debate.

Solicited speakers representing the plot scale, hillslope and catchment scale, and the large-scale provided valuable insights into their research experience and provided opinions about the session topic.

• Hans-Joerg Vogel provided a list of questions that may be answered at the plot scale. For instance, to what detail do we need to know the basic soil hydraulic properties? And how useful are sophisticated lab measurements to predict what is happening in the field? Also, methods to handle the highly non-linear change in flow paths as a function of the hydraulic state and its history may be developed at the plot scale. However, how soil water dynamics integrate over the scales of hillslopes or catchments is still an open question cannot solely be answered at the plot scale. How far can approaches like Richard’s equation that were developed at the plot scale be transferred to those larger scales?
• This question was picked up by the following presenter, Theresa Blume, who advocated for more research for understanding the link between plot and catchment scales. Nested monitoring programmes with monitored catchments that envelop different monitoring plots, as applied within the Catchments As Organised Systems (CAOS) project, may provide promising advances in our understanding how plot scale dynamics integrate to hillslopes and catchments. When such measurement design is applied in a comparative approach, generalized knowledge about hydrological processes across different types of hydrological landscapes could be obtained. But how to integrate the derived understanding into models that can be applied at test sites with less information?
• The modeller’s point of view was picked up by Jens Christian Refsgaard. Stating that it It is recognised that there is a mismatch of spatial scales between our process knowledge, the modelling grids of our distributed catchment models (50 – 500 m) and the water management problems, where the relevant scale often is claimed to be the catchment (e.g. 10 – 5000 km²). Groundwater pollution may even occur on larger scales (several 10,000 km²) while mitigation measures have to be applied locally. Hence, relevant scales vary from one issue to another (plot to large scale) and for some issues (e.g. mitigation measures) are in the order of 100 m when for instance the European Water Framework Directive (EWFD) is applied. Although distributed models are technically able to simulate the hydrological behaviour on such small scales, how can we evaluate them for their realism if now operations are available?
• The challenge of evaluating hydrological models that operate on scales even larger than the catchments scale was also picked up by Thorsten Wagener. We increasingly build and apply hydrologic models that simulate systems beyond the catchment scale. Such model can provide opportunities for new scientific insights, for instance, the consideration of intercatchment flow. Also, large-scale models can help us to understand changes to water resources from larger scale activities like agriculture or from hazards such as droughts. However, these models also require us to rethink how we build and evaluate them given that some of the unsolved problems from the catchment scale have not gone away. So what opportunities for solving these problems are there? Are there possibilities that have not yet been utilized?
• An increasing source of information for large-scale model applications are global archives of hydrological observations, as stated by Lena Tallaksen. These may allow a more detailed development and evaluation of hydrological models for the purpose of water resource assessments and climate change impact studies at the global and continental scale. Recent research has been providing improved knowledge of the present state of global water resources and variability across large spatial domains, the role of terrestrial hydrology in earth system models, the influence of climate variability and change on continental hydrology (including extremes), and the representation of subsurface hydrology and land-surface atmosphere feedback processes. Large-scale models are more and more adapted to include multiple types of input data such as remote sensing processes. However, a lack of ground truthing especially in less developed regions, the representation of hydrological variability below the modelling scale and uncertainties in downscaling large-scale climate forcings to the model scale still limit the applicability of large-scale models. Despite these challenges, large-scale models may represent a useful source of information for continental-scale hydrological assessments and evidence-based policy making. To increase their reliability, transfer of knowledge across scales is essential to improve hydrologic predictions at different spatial scales in an ever-changing world.

Overall, by the excellent presentations and discussion during the session, we found that each scale has its own relevance for water management. Experimental research at the plot and catchment scales brings advances in hydrological process understanding that can improve our simulation tools, while comparative hydrology and large scale modelling can provide quantitative information at larger scales to support water governance and policy making.

The key question in the near future is certainly how to further improve collaboration and foster discussions across all hydrological scales, especially in the context of ever more complex models at all scales. One option would certainly be to organize regular cross-cutting conference sessions. Another interesting initiative that has been presented at EGU2017 is the data and model sharing platform https://www.hydroshare.org/.