Unsolved Problems in Hydrology

Working Group on Unsolved Problems in Hydrology (UPH)


On 10 July, 2023 the IAHS Bureau established a Working Group on Unsolved Problems in Hydrology (UPH). The goal of the UPH community initiative, promoted by the Working Group, is to increase the coherence of the scientific process in hydrology (thus overcoming fragmentation) through providing common research subjects; energise the hydrological community through increasing the awareness that we do not fully understand many hydrological processes; and to enable the community to speak with one voice to increase public awareness and enhance funding opportunities for community projects.

Purpose and general activities

The purpose of the Working Group is to capitalise on the initial momentum of UPH and fully bring to fruit its potential. In particular, the working group will encourage the following activities: 

  • Organising conference sessions according to UPH
  • Organising projects according to UPH
  • Community meta-analyses (beyond review papers) for individual UPHs
  • Exploiting synergies across hydrology journals, e.g. using UPHs to this end
  • Building of community data sets geared at shedding light on particular UPHs.

The Working Group will closely collaborate with other bodies within IAHS, in particular the Commissions, as well as with other hydrological associations such as EGU, AGU and IAH. 

Chair and Members

Günter Blöschl, Past IAHS President and leader of the UPH initiative, was appointed Chair of the working group. Members can potentially be chosen by a number of criteria, e.g. drawing from IAHS International Commissions. Given the cross-cutting nature of the UPH initiative, it is suggested to appoint members that represent regional UPH activities. In particular the following members are proposed:  

  • Ankit Agarwal, IIT Roorkee, India
  • Long Yang, Nanjing University, China
  • Pedro Chaffe, UFSC Florianópolis, Brazil
  • NN, representing UPH Africa

Members representing other regions could be added later.

Roadmap of activities 2023-2027

Greatly simplified, the goals the UPH are twofold: (a) Identifying UPH and (b) Solving UPH. The future activities are therefore organised under these two themes. 

(a) Identifying UPH

Regarding the identification of global UPH much has already been done. A broad international campaign was conducted in 2017-2018 to identify a set of 23 UPH. These UPH, as well as the process of their identification, were published in 2019 in HSJ. A similar process was followed in Latin America and the results will be published soon. The Latin American group adopted most of the original (global) UPH, but replaced some of them by more regionally specific UPH, e.g. on tropical and Andean hydrology. These UPH activities have already stimulated communication and collaboration among the Latin American countries and have led to the formation of a Latin America Regional Committee by the IAHS on 12 July, 2023. Similar activities are underway in Africa. UPH campaigns are being planned in India and in China, potentially involving neighbouring countries.  

Planned outcomes:

  • Sets of UPH for specific regions around the world
  • Enhanced collaborations within these regions and between disciplines, stimulated by UPH activities

(b) Solving UPH

The simplified term “solving” (in order to rhyme with “unsolved” in UPH) relates more specifically to elaborating a community consensus opinion in response to the UPH. This consensus opinion is intended to be as specific as possible. For example, the response to “UPH 9: How do flood-rich and drought-rich periods arise, are they changing, and if so why?” could (partly) read as follows:  

Flood-rich periods arise as a result of climate modes (moderate- to high-certainty evidence). In South America the main climate mode is ENSO, while in South Asia it is IOD (moderate- to high-certainty evidence). 

Flood-rich periods have become more prevalent in XX and less prevalent in YY (low-certainty evidence). 

The increased prevalence of flood-rich periods in the region of ZZ is a result of AA (low-certainty evidence). 

This type of assessment has been inspired by meta-analyses in hydrology, climatology and beyond (Blöschl et al., 2013; de Amorim et al., 2019, 2020; IPCC, 2021; WHO, 2021). The planned methodology will similarly follow these meta-analyses. A suitable procedure will be agreed upon at the community level before starting the analyses to maximise its representativeness and credibility. The approach will be two-pronged. On the one hand, a region for which UPH have been identified in (a) takes the lead on one global UPH and involves the rest of the world in order to provide a global community perspective. On the other hand, the relevant IAHS International Commissions will be involved in the assessment to take advantage of core IAHS expertise.

Planned outcomes:

  • Community consensus assessment reports for a small number of global UPH
  • Enhanced coherence within the scientific community on these particular UPH

Job description of Chair and members 

The Chair of the working group is envisaged to

  • Strategically coordinate the activities of identification of UPH and meta-analyses in and between the regions (LATAM, Africa, China, India) and beyond
  • Coordinate with the IAHS International Commissions and Regional Committees
  • Report to the Bureau

The members of the working group are envisaged to

  • Coordinate the identification of UPH in their region
  • Coordinate the meta-analyses in their region and with the rest of the world
  • Liaise with other regions and IAHS International Commissions


Blöschl et al. (Eds.) (2013) Runoff Prediction in Ungauged Basins - Synthesis across Processes, Places and Scales. Cambridge University Press, 465 pp. 
de Amorim et al. (2019) Towards a comprehensive characterization of evidence in synthesis assessments. Climatic Change, 155, 37-57. 
de Amorim et al. (2020) A web-based tool for synthesis assessments of the impacts of climate change on water resources. Environ. Model. Software, 133, 104848. 
IPCC (2021) Climate Change 2021: The Physical Science Basis. Cambridge University Press. 
WHO (2021) Guideline for the pharmacological treatment of hypertension in adults. World Health Organization. 

To all hydrologists of the world: A Call to Arms! What are the 23 Unsolved Problems in Hydrology that would revolutionise research in the 21st century?

At the IAHS Scientific Assembly in Port Elizabeth in July 2017 the then President of IAHS, Günter Blöschl, outlined his vision for a new initiative to generate a list of unsolved problems in hydrology. New Read more here.



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