|Frequently, the principle of ‘equitable and reasonable’ utilisation is used synonymously with simply ‘equitable utilisation’, raising the question about the actual role and meaning of reasonableness.211 In its Commentaries to the 1994 Draft Articles, the International Law Commission (ILC) took the view that there is a distinction to be made in the application of ‘reasonable’ and ‘equitable’. The former being applied in the judgement on the quality of the use; and the latter being employed in the balancing process of the various uses between states in case of a ‘conflict of use.’212
The second sentence of Art 5 (1) elaborates upon the concept of equitable and reasonable utilisation, providing that watercourse states shall develop an international watercourse ‘with a view to attaining optimal and sustainable utilisation thereof and benefits therefrom’ consistent with adequate protection of the watercourse. The term ‘with a view to’ indicates that the achievement of optimal utilisation and benefits is the objective international watercourse states should ultimately aim for. The terms ‘optimal utilisation’ and ‘benefits’ do not indicate the goal to achieve the ‘maximum,’ the most efficient, or even the most (economically) valuable use. Nor do they indicate that the state in the position of utilising the watercourse most efficiently (e.g. due to economic or technological advantages) should have a superior claim to the use of the waters. It rather implies attaining maximum possible benefits for all riparians and achieving the greatest possible satisfaction of all their needs, while minimising potential detrimental impact. The term ‘sustainable use’ reflects the need to balance economic, social, and environmental values in the use of natural resources and to take into account the long-term carrying capacity of international watercourses – in line with the principle of sustainable development.213 The closing phrase of the second sentence stresses that the efforts to attain optimal and sustainable utilisation must be in line with the ‘adequate protection’ of the watercourse – which not only includes the conservation measures and activities tackling water-related diseases, but also technical and hydrological ‘measures of control.’214
In determining what constitutes a reasonable use, the ‘reasonable man’ test can be applied to create an objective standard against which conduct can be measured.215 By using this test it becomes clear that the requirement of reasonableness does not demand the most efficient use available, nor does it call for utilising the most advanced technology. Reasonableness differs from the concepts of ‘beneficial’ or ‘best possible’ use. It encompasses the contemporary conception of rationality and takes factors like a state’s stage of development into consideration. Yet, even if a use of an international watercourse has been identified as reasonable, it might still be challenged when compared with other uses through the lens of equity. The principle of equitable and reasonable use, then, recognises equity as a broader umbrella within which the concept of reasonableness becomes relative. This means that what may be considered to be perfectly reasonable by one state can be inequitable when looked at within the broader picture of the whole watercourse and the various needs and interests of co-riparian states. Hence, ‘reasonable’ uses are still subject to an ‘equitable’ allocation.
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