|The level of harm in order to qualify as ‘significant’ has to be higher than merely perceptible or trivial (which would be considered insignificant), but can be less than severe or substantial.223 What constitutes ‘harm’ has to be more than just an ‘adverse effect’ – it engenders a real impairment of a use, with a detrimental impact of some consequence upon the environment or the socioeconomic development of the harmed state (e.g. public health, industry, property, agriculture). Significant harm, then, has to be established by objective evidence and determined on a case by case basis.224
Since the allocation approach of ‘limited territorial sovereignty’ serves as the framework for the two main substantive principles of present international water law – ‘equitable and reasonable utilisation’ (Article 5) and ‘significant (transboundary) harm’ (Article 7) – the relationship between these rules has been at the root of extensive debates and the question which takes precedent is probably the most crucial one in the application of both Articles 5 and 7. Downstream states tend to favour the no harm rule, as it protects their existing uses from adverse effects caused by upstream developments, while upstream states tend to favour the principle of equitable and reasonable utilisation, as it allows for a broader use of the shared resource for developments that may impact co-riparians.225 The UN Convention tries to avoid the potential difficulties between those two rules by affording the principle of equitable and reasonable utilisation priority (Article 5) while giving the no significant harm rule special status (Article 7 (1)). Pursuant to the Convention, and only in certain limited circumstances, taking measures to prevent significant harm may be inappropriate, or lead to an inequitable result, and some level of harm may therefore be tolerated (Article 7 (2)). The reference to Articles 5 and 6 support the UN Convention, states are therefore not legally responsible for causing significant harm if they can show that they have taken all appropriate measures to prevent such harm, and their use of an international watercourse is equitable and reasonable. Thus, the scope for a state to cause significant harm becomes limited. See also Article 20 with regard to the protection of ecosystems, and Article 10 relating to vital human needs, where the principle of preventing significant harm to other watercourse states is further limited.
The burden of proof for establishing that a particular use of an international watercourse is still equitable and reasonable lies with the state whose use of the watercourse is causing significant harm.226 According to the ILC, ‘[t]he plaintiff state starts with the presumptive rule in its favour that every state is bound to use the waters of rivers flowing within its territory in such a manner as will not cause substantial injury to a coriparian state. Having proved such substantial injury, the burden then will be upon the defendant state to establish an appropriate defence, except in those cases where damage results from extra-hazardous pollution and liability is strict. This burden falls on the defendant state by implication from its exclusive sovereign jurisdiction over waters flowing within its territory’.227
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