Unsolved Problems in Hydrology: Overarching session

Unsolved Problems in Hydrology have been identified through a community consultation and prioritization process, and formalised by Blöschl et al., 2019. Twenty-three Unsolved Problems in Hydrology (UPH) – a community perspective. Hydrological Sciences Journal, 64, 10, 1141-1158, https://doi.org/10.1080/02626667.2019.1620507. Several thematic sessions of the 2022 Assembly will articulate with some of these UPHs. This overarching session will welcome communications about the overall epistemic challenges and progress.

Co-organized by SP
Convener: Günter Blöschl | Co-Conveners: Berit Arheimer, Christophe Cudennec
| Mon, 30 May, 16:30–19:30|Room Auditorium Pasteur
| Attendance Mon, 30 May, 15:00–16:30|Poster area

Orals: Mon, 30 May | Room Auditorium Pasteur

Chairperson: Berit Arheimer
Hubert H.G. Savenije

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, 2022.

Rain-on-Snow Flooding: A Review
Sinan Rasiya Koya and Tirthankar Roy
Keith Beven, Ann Kretzschmar, Paul Smith, and Nick Chappell

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, 2022.

Valentin Wendling, Christophe Peugeot, Manuela Grippa, Geremy Panthou, Jean-Louis Rajot, Olivier Mora, Angeles G. Mayor, Emmanuel Lawin, Ibrahim Bouzou-Moussa, and Abdramane Ba and the TipHyc project

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, 2022.

Gil Mahé and Andrew Ogilvie

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, 2022.

Pedro L.B. Chaffe, Rodrigo C.D. Paiva, and Cristóvão V.S. Fernandes and the UPH Latin America

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, 2022.


Posters: Mon, 30 May, 15:00–16:30 | Poster area

Chairperson: Christophe Cudennec
Bertille Loiseau, Simon Carriere, Cédric Champollion, Chloé Ollivier, Nicolas Martin-StPaul, Nolwenn Lesparre, Albert Olioso, Jacques Hinderer, and Damien Jougnot

Estimating evapotranspiration (ET) is a primary challenge in modern hydrology. Hydrogravimetry is an integrative approach that provides highly precise continuous measurement of gravity acceleration. However, large-scale effects (e.g. tides, polar motion, atmospheric loading) limit the fine time-scale interpretation of this data and processing leads to residual signal noise. To circumvent this limitation, we exploited the difference between two superconducting gravimeters located 512 m apart on the same vertical. The difference calculation makes it possible to remove shared large-scale effects. Daily variation of this gravity difference is significantly correlated with daily evapotranspiration as estimated using the water balance model SimpKcET (p-value = 4.10-10). However, this approach is effective only during rain-free periods. In the future, comparison with direct ET measurements (e.g. eddy-covariance, scintillometer) may confirm and strengthen our interpretation. Improved hydrogravimetric data processing will allow to extend this approach to other experimental sites equipped with a single superconducting gravimeter.

How to cite: Loiseau, B., Carriere, S., Champollion, C., Ollivier, C., Martin-StPaul, N., Lesparre, N., Olioso, A., Hinderer, J., and Jougnot, D.: Analysis influence of evapotranspiration on superconducting gravity signal at daily time step, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-366, 2022.

On the possible mechanisms of paleofloods in the ancient Volga River basin as a trigger of the Caspian Sea transgression in the Late Glacier Era: checking the hypotheses
Alexander Gelfan, Andrei Kalugin, Alexander Kislov, and Andrei Panin
Daniel Schertzer, Vijay Dimri, Klaus Fraedrich, Vijay Gupta, Witold Krajewski, and Renzo Rosso

We would like to address epistemic questions about how scaling is considered in the community roadmap "Twenty-Three Unsolved Problems in Hydrology" (UPH). Scaling is only explicitly mentioned once in the second group of problems (5-8), "Space variability and scaling". Therefore, scaling is viewed as limited to spatial variability, since it is not being mentioned at all in the first group titled "Time variability and change" (problems 1-4). The roadmap therefore neglects space-time scaling, while the ubiquitous "scale problem" in hydrology is that hydrological fields exhibit extreme variability over a wide range of space-time scales and that their observations are therefore strongly scale-dependent, again in space and time. A similar problem arises with the third group of problems (9-11) "Variability of extremes", because extremes are particularly scale-dependent observables. In fact, the scaling question is only explicit for problem 6, that is "the classical scaling question of how point-scale equations relate to catchment-scale equations”, while the same questions arise, again in space and time, for precipitation, glaciers, lakes, river flows, water and energy fluxes at the surface, aquifers etc., and for changes in their scaling properties induced by and participating in global change.

Hydrology has thus been an important source of inspiration for scaling methodologies, from precipitation to groundwater, from short-term to climate prediction. Scaling techniques have consequently become increasingly sophisticated and diverse, e.g., ranging from scalar self-similarity to multivariate multifractals or multifractal operators. These methods are primarily subject of academic research, but pioneering applications in operational hydrology are worth to be mentioned. They provide practical means to understand, analyse and simulate highly variable fields over a wide range of scales. However, there are still many limitations to overcome, such as a fully consistent way to treat precipitation zeros or ephemeral flow patterns.  The community roadmap for future research (UPH) should identify the hydrological problems where advances in scaling approach have the potential to be most useful and the most challenging methodological developments to be achieved. 


How to cite: Schertzer, D., Dimri, V., Fraedrich, K., Gupta, V., Krajewski, W., and Rosso, R.: How Scaling Helps Solve Problems in Hydrology, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-538, 2022.

Vazken Andréassian, Tewfik Sari, Léonard Santos, and Alban de Lavenne

In our search for emergent catchment-scale hydrological laws, we discuss here how a class of widely-used water-balance formulas (Turc-Mezentsev and Tixeront-Fu) were conceived by their authors more than 60 years ago. Because these water balance formulas deal with long-term quantities, we discuss various attempts to downscale them temporally, and progressively integrate mathematical formulations able to account for both the short-term (seasonnal) and long-term (multi-year) hydrologial memory observed in catchment systems.


Fu, B.: On the calculation of the evaporation from land surface, Atmos. Sin., 5, 23–31, 1981. 

Mezentsev, V.: Back to the computation of total evaporation, Meteorologia i Gidrologia, 5, 24–26, 1955. 

Tixeront, J.: Prediction of streamflow (in French: Prévision des apports des cours d'eau), in: IAHS publication no. 63: General Assembly of Berkeley, IAHS, Gentbrugge, 118–126, available at: http://hydrologie.org/redbooks/a063/063013.pdf (last access: 1 May 2019), 1964. 

Turc, L.: The water balance of soils: relationship between precipitations, evaporation and flow (in French: Le bilan d'eau des sols: relation entre les précipitations, l'évaporation et l'écoulement), Annales Agronomiques, Série A, IV, 491–595, 1954.

How to cite: Andréassian, V., Sari, T., Santos, L., and de Lavenne, A.: In search of catchment-scale hydrological laws: learning from the long-term water balance formulas, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-692, 2022.

Anna Lamacova, Radek Vlnas, and Pavel Kram

Norway spruce monoculture stands often represent a typical vegetation cover in Central Europe and these stands impact the hydrological pattern on a catchment scale significantly. The processes of canopy interception, throughfall, snowmelt and land surface evaporation represent an important part of the water cycle in forested catchments. This study uses a headwater catchment Lysina Critical Zone Observatory located in the western part of the Czech Republic. The catchment with a long-term monitoring (since 1989) has an area of 25 ha and vegetation cover is a Norway spruce (Picea abies) monoculture. We applied two versions of a deterministic, process oriented, lumped parameter hydrological model Brook90 that runs on daily time step. The original version and the upgraded version LWFBrook90R was fully transformed to R environment. Our primary focus was the ability of the models to capture the interception. Both model versions performed well on historic streamflow and in agreement with each other according to the catchment water budget. The annual interception on catchment was almost 30% (260 mm) of the open area precipitation in the period of 1994–2019, this was calculated based on precipitation collected on catchment in an open area (bulk precipitation) and under the forest canopy (throughfall) in monthly interval. Both model versions showed notably lower interception than observed. However, the LWFBrook90R performed much better with an interception of 15% compared to only 1% of interception in the original Brook90 version. The difference was in evapotranspiration components, especially transpiration, that compensated the lower interception loss compared to observed and thus the total evapotranspiration of LWFBrook90R and Brook90 did not differ notably. These preliminary results show a necessity to focus on better model parametrization in order to correctly capture complex hydrological processes such as interception loss in forested catchments.

How to cite: Lamacova, A., Vlnas, R., and Kram, P.: Interception loss as an underestimated component of evapotranspiration modelling in a forested catchment, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-724, 2022.