The International Atomic Energy Agency (IAEA) has convened quadrennial symposia on isotope hydrology since 1963. Over the decades these symposia have provided opportunities to present and review analytical developments as well as applications and research trends in isotope hydrology. Nowadays, isotope hydrology is an established discipline within the field of Earth Sciences and makes important contributions to our understanding of hydrological processes, particularly in water resources assessment and management in the context of UN Sustainable Development Goal 6. The stable isotopes of the water molecule are key recorders of changes in the Earth’s climate. More recently, scientists used environmental isotopes in areas such as forensic sciences and ecology, food source traceability, and in water cycle studies.
This symposium is the 15th in the series and will be held at the IAEA’s Headquarters in Vienna, Austria, from 20 to 24 May 2019. The symposium aims to facilitate exchange of information and knowledge among water and environment professionals from developed and developing countries in order to advance scientific understanding, promote collaboration, and demonstrate new capabilities to respond to pressing global water challenges .
Across southern Africa, an understanding of the inter-relationship between climate change and available water resources, and the implications of this relationship on socio-economic factors, is critical to the long-term sustainability of the region’s economy. The most profound aspect of this inter-relationship is the predicted change in climate linked to significant changes (both positive and negative) in regional, national and transboundary water budgets. This outlook has significant socio-economic implications for the region including the ability to support urban centres, to reduce poverty, to protect food and energy supplies and to develop skills and capacity within the population. In recent years, these issues came to head as severe drought affected the city of Cape Town, an urban centre at the bottom of Africa, with a population of ~ 3.8 million people. Below average rainfall between 2014 and 2017 and water management failures, resulted in the City of Cape Town experiencing extreme water stress over the 2017-2018 summer. The bulk of Cape Town’s municipal water supply is derived from six surface water storage facilities with a combined total capacity of 828 991 Ml. By March of 2018, the total storage in these facilities reached its lowest recorded level, at less than 20 %, with the largest of the reservoirs, the Theewaterskloof reservoir, at only 13.5 % of its 480 188 Ml capacity. Given that the bottom 10% of dams is regarded as “unusable”, this left only 3.5% usable water in the largest dam supplying the City of Cape Town. All residents were required to reduce water consumption to 50 L per person per day and various pronouncements of an imminent "Day Zero" were made, the day the city would turn off the municipal water supply in order to maintain critical infrastructure.
In the end “Day Zero” in Cape Town did not eventuate. The collective effort of the Cape Town population to save water allowed the city’s water storages to last until the arrival of the winter rains. However, the possibility of the municipal water network being shutdown, focused people’s minds on how we use and value water and led to a real change in water use patterns. Cape Town, like many large urban centres worldwide, based its water security on surface water storage systems. This practice leaves these urban centres at the mercy of fluctuating climates and increasingly erratic rainfall patterns. In response, many cities around the world are now introducing a diverse mix of water sources to supplement of municipal water supply networks and to protect supply security. These water sources include desalination, grey-water recycling, and direct rainwater harvesting but for many cities, including Cape Town, groundwater supplementation remains a strong focus.
Isotopes have been frequently used as tracers to study water flow and transport processes in the unsaturated zone. For example, they can give information about water transit times, groundwater recharge rates, evaporation rates or ecohydrological processes. In recent years, new analytical and technical developments have transformed the field of isotope hydrology and widened the scope of questions that can now be tackled. In particular, it is now possible to measure and parameterize isotope dynamics within the unsaturated zone at increasingly finer resolution. In addition, new sampling systems in combination with the new laser technologies can provide high-resolution and even in-situ measurements. In this talk, new possibilities of sampling and analyzing oxygen and hydrogen isotopes in pore water as well as challenges of these sampling and analytical methods are presented. The importance of isotopes and their contribution to advancing our understanding of water flow and transport processes in the unsaturated zone are highlighted, including different examples of using isotopes for numerical model calibration. Further, current challenges and future opportunities for including isotope approaches in solving open research questions in unsaturated zone hydrology are presented.
Calibration of hydrological models is challenging in remote, high latitude regions where hydrometry data are minimal. The vast water resources and complex hydrology of Canada necessitate the use of models to predict future changes in water supply, yet often with high amounts of uncertainty, in part, from poorly calibrated models. In this paper, we demonstrate the utility of isotopes for improving the amount and type of information available for model calibration using the isoWATFLOODTM model. We show that adding additional information to calibration does not hurt model calibration statistics, improves model validation, and offers additional feedback on internal flow paths and hydrologic storages that can be useful for informing model calibration. The inclusion of isotopes in model calibration reduces the number of realistic parameter combinations, resulting in more representative model calibrations and improved long-term simulation of large-scale water balance.