A global hydrology research agenda ﬁ t for the 2030s

Global assessments show profound impacts of human activities on freshwater systems that, without action, are expected to reach crisis point in the 2030s. By then, the capacity of natural systems to meet rising demands for water, food, and energy could be hampered by emerging signals of anthropogenic climate change. The hydrological community has always been solution-orientated, but our generation faces perhaps the greatest array of water challenges in human history. Ambitious programmes of research are needed to ﬁ ll critical data, knowledge, and skills gaps. Priorities include ﬁ lling data sparse places, predicting peak water, understanding the physical drivers of mega droughts, evaluating hyper-resolution models, managing compound hazards, and adjusting water infrastructure designs to climate change. Despite the opportunities presented by big data, we must not lose sight of the deep uncertainties affecting both our raw input data and hydrological models, nor neglect the human dimensions of water system change. Community-scale projects and international research partnerships are needed to connect new hydrological knowledge with most vulnerable communities as well as to achieve more integrated and grounded solutions. With these elements in place, we will be better equipped to meet the global hydrological challenges of the 2030s and beyond. (Vörösmarty ). Evolving and/or intensifying threats to freshwater biodiversity also come from changing climates; global internet commerce and spread of non-native species; infectious diseases; harmful algal blooms; expanding hydropower; emerging contaminants; engineered nanomaterials; microplastic pollution; light and noise; freshwater saliniza-tion; declining calcium; and cumulative stressors (Reid et al.  : 849).

. Evolving and/or intensifying threats to freshwater biodiversity also come from changing climates; global internet commerce and spread of non-native species; infectious diseases; harmful algal blooms; expanding hydropower; emerging contaminants; engineered nanomaterials; microplastic pollution; light and noise; freshwater salinization; declining calcium; and cumulative stressors (Reid et al. : 849).
As the 'perfect storm' of the 2030s forecasted by Beddington () closes in, there appears to be greater urgency about tackling the climate-food-energy-water nexus through policy and planning at various levels (e.g.  (Harmancioglu ). Goal 6 is to ensure availability and sustainable management of water and sanitation for all.
Yet, more impetus has been added by the realization that global mean warming of 1.5 C could be reached as soon as 2030 with concomitant risks to human and natural systems (IPCC ). Governments and institutions now find themselves under pressure to acknowledge a 'climate emergency' and to commit to much more ambitious mitigation pathways to achieve net zero emissions.
How should the hydrological community respond to such major imperatives? What new information and research are needed to support a concerted global effort to manage the water challenges of the 2030s and beyond?
Traditionally, we have arranged our knowledge within thematic silos that have changed little over the last 25 years (Table 1). However, greater emphasis is now needed on preparing for hydro-system changes that could fall outside historic variability (Wagener et al. ). This Comment Paper makes a case for a more integrated, solutionorientated approach to the global water-related challenges (5) managing compound hazards; and (6) adjusting engineering standards (under non-stationary conditions). These are followed by a section that reflects on ways of working together that strengthen multidisciplinary integration and research impact. Throughout, the emphasis is on supporting the most vulnerable communities. It is hoped that these suggestions will inform wider discourse about the future direction and priorities for hydrological science, including associated training needs. There is also scope for the closer alignment of our research and development programmes with complementary disciplines.

FILLING DATA SPARSE PLACES
Quality assured hydrological data are needed to adaptively manage resources, calibrate remotely sensed observations, build, and test models. Nonetheless, there are vast tracts of Earth lacking ground measurements of fundamental water balance components (i.e. precipitation, evapotranspiration and changes in ice, lake/wetland, soil, or groundwater storage). Network densities are particularly sparse in the Arctic, sub-Saharan Africa (apart from South Africa), central Asia, the Pacific Islands, and South America. High-altitude regions and fragile states are especially under-represented.
Even where there are data, records may be incomplete due to lack of resources for personnel and equipment or because  Research is still required on how best to convey highly uncertain model outputs to different stakeholders, in consistent ways, recognizing that preferred formats will be context-and decision-maker-dependent.  Perhaps the most technically demanding option surrounds the adjustment of engineering standards to reflect evolving and projected hydrological conditions. This is especially contentious because of the methods of economic discounting applied to costs and benefits, as well as the low confidence in regional climate projections over the design life times of new infrastructure (Kundzewicz & Stakhiv ). New ways of working with non-stationary information must also be deployed (Serinaldi & Kilsby ).

MANAGING COMPOUND HYDRO-HAZARDS
Nonetheless, a few agencies are already providing look- have to be expressed with reference to an agreed baseline, then aggregated spatially and rounded mathematically.
This is necessary to avoid the impression of undue precision.

;
Beven a), nor the human dimensions to water system changes. With these elements in place, we will be better equipped to meet the unprecedented hydrological challenges of coming decades.

ACKNOWLEDGEMENTS
This commentary is dedicated to the memory of Geoff Petts.