This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. ARTICLE HISTORY

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An ICP‐MS, equipped with an ultrasonic nebulizer and active‐film multiplier detector, is used to attempt to determine 54 trace elements directly in ground water. Lithium, arsenic, rubidium, strontium, barium, and antimony are found in the microgram‐per‐liter (part‐per‐billion = ppb) range. Most of the other elements are present at nanogram‐per‐liter (part‐per‐trillion = ppt) concentrations. Ion exchange preconcentration is utilized in order to improve the sensitivity for measuring the rare earth elements that exist at concentrations as low as 0.05 ppt for lutetium, thulium, and terbium. The formation of molecular species in the plasma produces false positive results for some of the elements. The presence of silicon or carbon dioxide interferes with the measurement of scandium, strontium interferes with rhodium and palladium, and barium interferes with europium. Correction procedures for these interferences are discussed. All together, the concentrations of the 54 elements in water from four Nevada springs span almost seven orders of magnitude.

A tracer experiment was conducted at the commercial low‐level nuclear waste disposal site near Barnwell, South Carolina, to test a new method for determining the tortuosity and sorption‐affected porosity for gaseous diffusion transport of materials in the Unsaturated zone. Two tracers, CBrClF2 and SF6, were released at constant rates of 105 and 3.3 ng/s, respectively, from permeation devices, which were placed in short screened sections in access holes. Soil gas was sampled from 15 piezometers located at various distances from the sources by sequentially pumping 60–160 mL of gas from the piezometers into a dual‐column gas chromatograph located at the test site. The CBrClF2 concentration data obtained from several of the piezometers were analyzed by use of type curves for a continuous point source in an areally extensive medium bounded above and below by planar no‐flow boundaries. The tortuosity of the geologic unit tested, an eolian sand, was determined to be about 0.4, and the sorption‐affected porosity to be 0.22. The tortuosity value is plausible, but the sorption‐affected porosity value is substantially less than that computed from the drained porosity, particularly if adjustments are made for retardation due to solution of the tracer in the liquid phase and sorption on the solid phase. The SF6 data could not be reliably analyzed.

The rare earth element (REE) signature of ground waters from both felsic volcanic rocks on the Nevada Test Site and from the regional Paleozoic carbonate aquifer of southern Nevada resemble the REE signature of the rocks through which they flow. Moreover, the REE signatures of Ash Meadows ground waters are similar to those of springs in the Furnace Creek region of Death Valley but different from shallow ground waters from predominantly tuffaceous alluvial deposits in the Amargosa Desert, perched ground waters from felsic volcanic rocks, and ground waters that have only flowed through the regional Paleozoic carbonate aquifer. The similar REE patterns of Ash Meadows and Furnace Creek ground waters support previous investigations that suggested ground waters discharging from the Furnace Creek springs are similar to the ground waters emerging from the Ash Meadows springs. The REE patterns indicate that the contribution of ground water from the Amargosa Desert to the Furnace Creek springs is of minor importance. Our REE analyses along with previous stable isotope, ground‐water potentiometric surface relationships, and geologic structure analyses support ground‐water flow from east to west in the fractured and faulted carbonate rocks beneath Ash Meadows, the Amargosa Desert, and the southern end of the Funeral Mountains. Our observations are contrary to some previous investigations that identified shallow ground waters from the central and northwestern Amargosa Desert as a substantial component of the ground water that discharges from the Furnace Creek springs.

Concentrations of 40 trace elements and other constituents in ground water from springs in Death Valley National Park were measured to investigate whether trace element composition of the ground water can be related to the aquifer materials. Samples from these springs were analyzed by inductively coupled plasma‐mass spectrometry (ICP‐MS) for the trace elements and by ion chromatography (IC) for the major anions. A Principal Component Analysis was performed on the data set. Surprise and Scotty's Springs formed one group; Texas, Nevares, and Travertine Springs formed another group; and Mesquite Springs did not group with any of the others. Scotty's and Surprise Spring issued from volcanic rocks; Texas, Nevares, and Travertine discharge from carbonate rocks; and Mesquite Spring is located in alluvial basin‐fill deposits. The first three components in each Principal Component Analysis accounted for approximately 95% of the variance in the data set. The Principal Component Analysis suggests that ground water inherits its trace element composition from the rocks or aquifer material with which it has interacted and may be used for the purpose of identifying ground‐water movement and source.

Two‐dimensional physical modeling of diesel fuel leaks was conducted in sand tanks to determine liquid and vapor migration characteristics. Mathematical modeling provided estimation of vapor concentrations at discrete times and distances from the vapor source and was compared to the physical experiment. The mathematical gaseous diffusion model was analogous to the Theis equation for ground‐water flow, accounted for sorptive effects of the media, and was calibrated using measured concentrations from the sand tank. Mathematically different positions of the vapor source were tested to better relate observed liquid flow rates and media configuration to gaseous concentrations. The calculated diffusion parameters were then used to estimate theoretical, three‐dimensional vapor transport from a hypothetical liquid leak of 2.0 1/hr for 30 days. The associated three‐dimensional vapor plume, which would be reasonably detectable by commercially available vadose zone monitors, was estimated to have a diameter of 8 m with a vapor concentration of 50 ppm at the outside edge of the vapor plume. A careful application of the method and values can be used to give a first approximation to the number of vapor monitors required at a field site as well as the optimal locations for the monitors.

Water stress and scarcity has affected, and will continue to affect, the stability of communities. An overview of global water security challenges indicates profound difficulties and potential flashpoints. There are many examples of struggles in supplying clean water throughout the world, and how water has been both a strategic tool and object of conflict in the past. Water has been an instrument of ethnic and religious conflict, and has recently been used in regional and local clashes. Transboundary water disputes are also potentially dangerous in several regions of the world and stresses from climate change and variability increase the uncertainty of clean water supplies. Potential ways to move positively forward and increase international security include: anticipating future regions of conflict over water, cooperation among water users, proper policy and regulatory structures, and infrastructure solutions.