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Hydrology is the bloodstream of the terrestrial system. The terrestrial system is alive, with the ecosystem as its active agent. The ecosystem optimises its survival within the constraints of energy, water, climate and nutrients. The key variables that the ecosystem can modify are the controls on fluxes and storages in the hydrological system, such as: the capacities of preferential flow paths (preferential infiltration, recharge and subsurface drainage); and the storage capacities in the root zone, wetlands, canopy and ground surface. It can also adjust through evolution, the efficiency of carbon sequestration and moisture uptake. Some of these adjustments can be made fast, particularly rootzone storage capacity, infiltration capacity, vegetation density and species composition. These system components are important controls on hydrological processes that in hydrological models are generally considered static and are determined by calibration on climatic drivers of the past. This leads to hydrological models that are dead and incapable to react to change, whereas the hydrological system is alive and will adjust.

The physical law driving this evolutionary process is the second law of thermodynamics with the Carnot limit as its constraint. This physical limit allows optimisation techniques to explore the reaction of the hydrological system and its components to change in climatic drivers. This implies a new direction in the theory of hydrology, required to deal with change and Unsolved Problems in Hydrology.

How to cite: Savenije, H. H. G.: The hydrological system as a living organism, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-548, https://doi.org/10.5194/iahs2022-548, 2022.

We would like to use models that are fit for a particular purpose in making predictions.   Traditionally, models have been calibrated against historical data and then some sample of those calibrated models used in prediction.   There has been very little consideration of the aleatory and epistemic uncertainties of the pertinent hydrological process and just how that might affect the way we assess models as hypotheses about how catchment systems work.   We suggest that a more Popperian approach is required to assess when models should be considered as NOT fit for purpose.   Model invalidation is, after all, a good thing in that it means we need to do better; that some improvements are required, either to the data, to the auxiliary relations or to the model structures being used.   The question is what is an appropriate methodology for such hypothesis testing when we KNOW there are epistemic uncertainties associated with the observations?   We consider this issue for the case of flood hydrograph simulation using Dynamic Topmodel, making use of a strategy of limits of acceptability for model simulations set prior to making model runs.

How to cite: Beven, K., Kretzschmar, A., Smith, P., and Chappell, N.: Problem 20.  Reducing uncertainty in model prediction: The role of model invalidation, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-452, https://doi.org/10.5194/iahs2022-452, 2022.

We develop a holistic approach of watershed hydrology, focusing on the interfaces between critical zone components and relying on interdisciplinarity. We mainly explore the first Unsolved Problem in Hydrology (UPH): Is the hydrological cycle regionally accelerating/decelerating under climate and environmental change, and are there tipping points? and 5 other UPH (3,4,7,12,19). As feedbacks exist between the critical zone components, forming a complex system, it can lead to the emergence of tipping points and to shifts between hydrological regimes.

A hydrological regime shift is indeed observed since 1950 in the Sahel (semi-arid region, Africa). Stream flows continuously increased in this region despite a severe regional drought in the 70s-90s. This suggests that a tipping point could have been surpassed, because of changes in climate and/or human practices. To explore this hypothesis, we develop a new modeling approach, relying on long term observations (1950-present) of hydrology, geomorphology and land used/land cover. We use System Dynamics modeling to capture feedbacks between water, soil structure, vegetation and flow connectivity between hillslope, channel and aquifer.

The modeling results accurately represent the observed decade-scale evolution of the hydrological regime at several watershed scale (1-10000 km²). This shows that tipping points exist in semi-arid land hydrology and allows to explore which processes are at play. We identified domains within watersheds where regime shifts occurred at small scale, propagated across scales and led to large-scale shifts. Ultimately, we aim at identifying areas where the risk of an irreversible hydrological regime shift is high under various climatic and socio-economical possible futures.

This study illustrates that using system dynamics to represent the critical zone is complementary to classical hydrologic modeling based on hydrodynamics. While the system-based approach represent correctly the decadal trends of the hydrological regime, the hydrodynamics approach did so for floods and drought at minute/year time scales.

How to cite: Wendling, V., Peugeot, C., Grippa, M., Panthou, G., Rajot, J.-L., Mora, O., G. Mayor, A., Lawin, E., Bouzou-Moussa, I., and Ba, A. and the TipHyc project: Tipping points in continental hydrology : developing a system dynamics approach to represent the interfaces in the critical zone and built a decade-scale hydrologic model., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-63, https://doi.org/10.5194/iahs2022-63, 2022.

Following the 23 Unsolved Problems in Hydrology paper (Blöschl et al. 2019), we launched the "23 UPH in Africa" initiative in collaboration with the UNESCO/IHP FRIEND-Water Programme, the ICIREWARD UNESCO Cat 2 Centre of Montpellier and the IAHS-ICSW commission. This consultation aims to identify and review the considerable research coming out of Africa lately, and put forward the contribution of the African academic community to advancing key unsolved problems in hydrology.

So far, more than 400 colleagues working in 26 countries registered their interest, and filled in a brief online questionnaire, created to gather and synthesise key information about interested participants. A call for volunteers led to selecting 42 colleagues to coordinate discussions within the seven UPH themes and 23 questions. Using online platforms and collaborative tools, these working groups seek to highlight essential research and results from Africa contributing to the 23 UPH. The synthesis also underlines the specific insights and advances related to the African physical and socio-economic environment, as well as priority and emerging questions from the continent.

How to cite: Mahé, G. and Ogilvie, A.: 23 UPH in Africa, process and progress, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-295, https://doi.org/10.5194/iahs2022-295, 2022.

Inspired by the 23 Unsolved Problems in Hydrology – 23 UPH (Blöschl et al., 2019) and the following “23 UPH in Africa” (Ogilvie & Mahé, 2021), we launched the “23 UPH in South America”. Our original aim was to review, identify and aggregate the contributions of the South American community to advancing key unsolved problems in hydrology. After a first round of discussion, we decided to expand the initiative to all Latin America. We initially advertised the initiative in August 2021 by using several mailing lists (e.g., Brazilian Association of Water Resources, AboutHydrology Google Group) and a Google Forms questionnaire. We collected responses from August through September 2021, with participants from 20 countries. Most participants are university professors and researchers with Engineering as the main background. We presented the first report to an audience of 27 in-person and 66 remote attendees at a 1-hour long meeting on November 24th during the XXIV Brazilian Water Resources Symposium. During the discussion, it became clear that we need to adapt the questions considering Latin America particularities, where water systems and their interaction with other components of the Earth and anthropic systems may be unique. We identified possible contributors to all the original 23 UPH and proposed a follow up workshop where each question will be assessed in depth by groups organized according to topics of interest. The discussions will result in a synthesis paper presenting the vision of the Latin America water community. We hope our effort to develop a common research agenda will strengthen the water community in Latin America as well as contribute with hydrological knowledge to the international water community's common problems.

How to cite: Chaffe, P. L. B., Paiva, R. C. D., and Fernandes, C. V. S. and the UPH Latin America: Unsolved Problems in Hydrology (UPH) – a Latin American perspective, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-602, https://doi.org/10.5194/iahs2022-602, 2022.