Abstract. Today there is a great consensus that water resources research needs to become more holistic, integrating perspectives of a large variety of disciplines. Groundwater and surface water (hereafter: GW and SW) are typically identified as different compartments of the hydrological cycle and were traditionally often studied and managed separately. However, despite this separation, these respective fields of study are usually not considered to be different disciplines. They are often seen as different specialisations of hydrology with different focus, yet similar theory, concepts, methodology. The present article discusses how this notion may form a substantial obstacle in the further integration of GW and SW research and management. The article focusses on the regional scale (areas of approx. 103 to 106 km2), which is identified as the scale where integration is most greatly needed, but ironically the least amount of fully integrated research seems to be undertaken. The state of research on integrating GW and SW research is briefly reviewed and the most essential differences between GW hydrology (or hydrogeology, geohydrology) and SW hydrology are presented. Groundwater recharge and baseflow are used as examples to illustrate different perspectives on similar phenomena that can cause severe misunderstandings and errors in the conceptualisation of integration schemes. It is also discussed that integration of GW and SW research on the regional scale necessarily must move beyond the hydrological aspects, by collaborating with social sciences and increasing the interaction between science and the society in general. The typical elements of an ideal interdisciplinary workflow are presented and their relevance with respect to integration of GW and SW is discussed. The overall conclusions are that GW hydrology and SW hydrogeology study rather different objects of interest, using different types of observation, working on different problem settings. They have thus developed different theory, methodology and terminology. Yet, there seems to be a widespread lack of awareness of these differences which hinders the detection of the existing interdisciplinary aspects of GW and SW integration and consequently the development of truly unifying, interdisciplinary theory and methodology. Thus, despite having the ultimate goal of creating a more holistic approach, we should start integration by analysing potential disciplinary differences. Improved understanding among hydrologists of what interdisciplinary means and how it works is needed. Hydrologists, despite frequently being involved in multidisciplinary projects, are not sufficiently involved in developing interdisciplinary strategies and do usually not regard the process of integration as such as a research topic of its own. There seems to be a general reluctance to apply (truly) interdisciplinary methodology because this is tedious and few, immediate incentives are experienced.
HESS Opinions "Integration of groundwater and surface water research: an interdisciplinary problem?"