Droughts are deeply intertwined with socio-economic and ecological systems across temporal and spatial scales. They are getting more severe and frequent under climate change and human pressures, affecting livelihoods, socio-economic sectors and the environment. Addressing these multifaceted impacts of droughts now and in the future will require actionable insights and a comprehensive research agenda. In this Perspective we draw on the interdisciplinary expertise of the International Association of Hydrological Sciences Working Group on ‘Drought in the Anthropocene’ and participants of the Drought Resilience +10 conference, a follow-up to the first High-Level Meeting on National Drought Policy in 2013. We synthesize ten key insights from drought research that have emerged in the past decade and highlight their relevance for policy and practice. Furthermore, we identify ten critical research gaps that must be addressed over the coming decade to further guide drought governance, strengthen drought resilience and effectively combat the water crisis.

Co-creation is seen as instrumental in bridging the gap between scientific innovation in climate services and their use in decision-making. However, there has been limited engagement with the different types of co-creation approaches that exist in practice, how they are executed, how they bridge the usability gap, and in what situations they would be most effective. This study aims to characterise climate service co-creation in practice, and develop typologies to explore how they bridge the usability gap. We conducted Thematic and Ideal Type Analyses of 33 case studies developed from Key Informant Interviews and Content Analysis of co-creation process documents. We show that i) co-creation approaches place a strong emphasis on the climate information (its usability and usefulness) to improve use of climate services, ii) co-creation in practice deviates from the theoretical approach, and iii) in addition to other contextual factors, the mode (research and commissioned) of co-creation has a strong influence on the execution of co-creation processes. We develop three typologies of climate service co-creation in practice; i) information-intensive (n = 21), concerned with producing useful information; ii) functional-use intensive (n = 5), concerned with the usability of the co-created information in decision-making; and, iii) innovation-oriented (n = 7), concerned with embedding new insights into innovative climate services. This study benefits researchers and practitioners implementing co-creation in the field of climate services to understand the types of co-creation that exist, the risks associated with each type, and the level to which each type may influence the use of climate services.

This paper presents a comprehensive and critical comparison of 10 disinfection methods of swimming pool water: chlorination, electrochemically generated mixed oxidants (EGMO), ultraviolet (UV) irradiation, UV/chlorine, UV/hydrogen peroxide (H₂O₂), UV/H₂O₂/chlorine, ozone (O₃)/chlorine, O₃/H₂O₂/chlorine, O₃/UV and O₃/UV/chlorine for the formation, control and elimination of potentially toxic disinfection byproducts (DBPs): trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), trihaloacetaldehydes (THAs) and chloramines (CAMs). The statistical comparison is carried out using data on 32 swimming pools accumulated from the reviewed studies. The results indicate that O₃/UV and O₃/UV/chlorine are the most promising methods, as the concentration of the studied DBPs (THMs and HANs) with these methods was reduced considerably compared with chlorination, EGMO, UV irradiation, UV/chlorine and O₃/chlorine. However, the concentration of the studied DBPs including HAAs and CAMs remained much higher with O₃/chlorine compared with the limits set by the WHO for drinking water quality. Moreover, the enhancement in the formation of THMs, HANs and CH with UV/chlorine compared with UV irradiation and the increase in the level of HANs with O₃/UV/chlorine compared with O₃/UV indicate the complexity of the combined processes, which should be optimized to control the toxicity and improve the quality of swimming pool water.

Chemical hydrograph separation using electrical conductivity and digital filters is applied to quantify runoff components in the 1,640 km² semi-arid Kaap River catchment and its subcatchments in South Africa. A rich data set of weekly to monthly water quality data ranging from 1978 to 2012 (450 to 940 samples per site) was analysed at 4 sampling locations in the catchment. The data were routinely collected by South Africa's national Department of Water and Sanitation, using standard sampling procedures. Chemical hydrograph separation using electrical conductivity (EC) as a tracer was used as reference and a recursive digital filter was then calibrated for the catchment. Results of the two-component hydrograph separation indicate the dominance of baseflow in the low flow regime, with a contribution of about 90% of total flow; however, during the wet season, baseflow accounts for 50% of total flow. The digital filter parameters were very sensitive and required calibration, using chemical hydrograph separation as a reference. Calibrated baseflow estimates ranged from 40% of total flow at the catchment outlet to 70% in the tributaries. The study demonstrates that routinely monitored water quality data, especially EC, can be used as a meaningful tracer, which could also aid in the calibration of a digital filter method and reduce uncertainty of estimated flow components. This information enhances our understanding of how baseflow is generated and contributed to streamflow throughout the year, which can aid in quantification of environmental flows, as well as to better parameterize hydrological models used for water resources planning and management. Baseflow estimates can also be useful for groundwater and water quality management.

The variability of rainfall and climate, combined with land use and land cover changes, and variation in geology and soils makes it a difficult task to accurately describe the key hydrological processes in a catchment. With the aim to better understand the key hydrological processes and runoff generation mechanisms in the semi-arid meso-scale Kaap catchment in South Africa, a hydrological model was developed using the open source STREAM model. Dominant runoff processes were mapped using a simplified Height Above the Nearest Drainage approach combined with geology. The Prediction in Ungauged Basins (PUB) framework of runoff signatures was used to analyse the model results. Results show that in the headwater sub-catchments of Noordkaap and Suidkaap, plateaus dominate, associated with slow flow processes. Therefore, these catchments have high baseflow components and are likely the main recharge zone for regional groundwater in the Kaap. In the Queens sub-catchment, hillslopes associated with intermediate and fast flow processes dominate. However, this catchment still has a strong baseflow component, but it seems to be more impacted by evaporation depletion, due to different soils and geology, especially in drier years. At the Kaap outlet, the model indicates that hillslopes are important, with intermediate and fast flow processes dominating and most runoff being generated through direct runoff and shallow groundwater components, particularly in wetter months and years. There is a high impact of water abstractions and evaporation during the dry season, affecting low flows in the catchment. Results also indicate that the root zone storage and the parameters of effective rainfall separation (between unsaturated and saturated zone), quickflow coefficient and capillary rise, were very sensitive in the model. The inclusion of capillary rise (feedback from the saturated to unsaturated zone) greatly improved the simulation results.

In previous studies we have ascertained that inflows and seawater intrusion in the Shatt al-Arab River (SAR) are two key physical factors behind fluctuating and sharply escalating salinities observed in recent years. Such levels require a series of countermeasures and investigative studies to translate physical factors into a salinity dynamics model to understand the problem and its impact as these factors vary in location, time and quantity. A one-dimensional hydrodynamic and salt intrusion numerical model was applied to simulate the complex salinity regime in the SAR based on hourly time-series data for the year 2014. The model was used to analyse the impact of different management scenarios on salinity under different conditions. The results show a high correlation between seawater intrusion and river discharge. Increased use of water upstream and local water withdrawals along the SAR will increase seawater intrusion and salinity concentrations. Improving the quantity and quality of the upstream freshwater sources could reduce salinity levels. Discharging the drainage water into the river could be used to counteract the salt intrusion, considering that its location affects both the salinity distribution and extent. A scenario analysis based on a numerical model constructed for the longitudinal salinity variation associated with different sources in a tidal regime, can efficiently screen alternative water management strategies.

ABSTRACT: Understanding the salinity variation caused by a combination of anthropogenic and marine sources is important for water resource management in heavily used rivers impacted by tidal influence. A quantitative analysis of intra-annual variability of salinity levels was conducted in the Shatt al-Arab River. Based on hourly records during 2014, the results showed high spatiotemporal variability in the range of 0.2–40.0 ppt. Similarities in salinity dynamics were used to divide the river course into four distinct spatial units to guide respective management actions. Salinity dynamics are influenced by different sources of saline water inflows and withdrawals associated with irrigation, industrial and municipal waste, marshes and by seawater intrusion. Adapting a simple interpolation approach, the measured distance of seawater intrusion was 80 km upstream the river mouth. Continuous monitoring of water quality can localize and assess the relative impact of the various salinity sources at different times. Managing seawater intrusion and any local effects should take into account variations in quantity and quality of irrigation return flows and wastewater discharges.