Invited Keynote -- Expanding the global view of water availability to encompass water quality
Authors: Alcamo Joseph 
Published: March 2024

By approving the Sustainable Development Goals (SDGs) more than 190 countries have made a commitment to &#8220;ensure availability ... of water ... to all&#8221;. Yet the implications of this high-profile pledge are unclear because of the lack of an internationally accepted definition of &#8220;water availability&#8221;. Moreover, definitions, where they exist, are usually incomplete because they neglect the important factor of water quality. This is a significant omission because degraded water quality impedes the usage of water resources for drinking water, hygiene, irrigation, habitat for plant and animal communities, and other important uses. It can be argued that contaminated water withdrawals can be treated to make more water available, but this is not an affordable option for much of the Global South except for drinking water. Hence, degraded water quality genuinely reduces water availability.Until recently water quality could not be included in large-scale (regional to global) assessments of water availability because of the lack of appropriate data and tools. The situation is changing, however, with the development of a new class of large-scale water quality models. These models have broad geographic coverage and a fine enough grid to simulate water quality gradients along river networks.As an example, as part of a UNEP study, the WorldQual model estimated that pathogen pollution hindered the usage of approximately one-third of the total river network in Latin America, Africa, and Asia for safe bathing and hygienic purposes. Organic pollution was estimated to impede the usage of about one-seventh of this river network for fish production, and salinity pollution about one-tenth of the network for irrigation water supply. These and other preliminary results from the modelling community suggest that water quality should not be overlooked as a potentially important factor in large-scale assessments of water availability.To improve the performance of large-scale water quality models, and make them more reliable for including in water availability assessments, researchers will have to contend with some difficult challenges including (but not limited to):The problem of representativeness &#8211; A wide range of water quality parameters are relevant to uses of surface water and groundwater, and the challenge is to find a manageable set of representative parameters for analysis that are both measurable and calculable on the large-scale.The problem of validation &#8211; There is a paucity of data available for validating large-scale water quality models, and in some parts of the world the coverage and frequency of data collection is declining rather than increasing.&#160;The problem of characterising anthropogenic fluxes of contaminants &#8211; The data necessary for determining fluxes of contaminants into the water environment (e.g. data on wastewater discharges) are either not accessible or very incomplete in many countries.&#160;Finally, a particularly high priority for going forward is to establish collaborations between researchers assessing the quantity of water available (for example, under climate change) and those assessing the quality of water. These collaborations are a prerequisite for developing a more comprehensive and realistic concept of water availability.