It is not intended to be a comprehensive inventory, but should provide a summary of how the core uses of an international watercourse in one state can impact upon another state and whether or not these waters and water uses will be covered under the scope of the Convention.

The watercourse scope diagram (Figure 1.1) is explained as follows: The transboundary watercourse system above begins with natural precipitation in the mountainous headwaters of state A. The system of tributaries flows downstream, often collecting in one main river channel which may be dammed and the water stored in reservoirs. The dammed water can be used to generate hydropower for state A and potentially state B, but the presence of the dam can reduce water flow and increase the loss of stored water through evaporation, which could ultimately result in less water for downstream state B.

The water continues to flow downstream where it may again be dammed, stored and diverted to supply domestic, municipal, agricultural, energy or industrial uses. Some of the diverted water may be lost from irrigated fields and canals through evapo-transpiration98 which could result in less water returns to the stream for downstream state B, or return flows may become polluted. The return flows from the various uses will continue to flow downstream as surface water and may enter the groundwater depending on the groundwater table. Groundwater can flow both ways – replenishing or receiving surface water flows.
If the groundwater table is below the bed of the stream, the water in the river bed feeds the groundwater and is an influent stream; such rivers can easily dry up if their flow is reduced (e.g. an irrigation channel or an ephemeral river). When the groundwater table is above the surface elevation in the river, the groundwater feeds the river, creating an effluent river which flows all year round.

An aquifer containing confined groundwater99 exists in state A, fed (very slowly) through precipitation from a recharge zone located in state A, but this aquifer replenishes at such a slow rate that it is considered non-renewable and it is not related to any immediate transboundary surface water stream. State A may decide to drill a well into the aquifer to supply irrigation water for nearby agriculture.

A significant gap may exist in scientific understanding surrounding the recharge relationship between surface water and groundwater and the impact of different land uses on this relationship, and despite the fact that this aquifer has been classified as ‘confined’, it is possible that the confined aquifer may also be remotely connected to a distant transboundary wetland situated across the border between states A and B. The withdrawal of water for agriculture may therefore seriously threaten this eco-system by lowering the groundwater table, causing the wetland to dry out, and could even cause subterranean peat fires. This is of course just one scenario and it may also be that the confined aquifer has no significant connection to the transboundary wetland and therefore state A’s water withdrawal would not have this transboundary impact. The use of this confined or unrelated groundwater is not covered by the UN Watercourses Convention. State A may also choose to utilise groundwater from a second aquifer for irrigation which is recharged and recharges the nearby surface water. If the aquifer is unconfined and connected to surface waters, some return flow – for example from irrigation – will occur but the water may be contaminated and affect plant and aquatic life downstream in state B, and some water will be lost to evapotranspiration. Groundwater from the second aquifer also supplies the springs in state B and excessive pumping in state A may affect spring flows to the point where they cannot reach ground surface and feed rivers and lakes.

If state A decides to cut down native forests the groundwater level rises towards the ground surface. At a certain depth high soil evaporation causes the soluble salts in the local geology to migrate to the soil surface causing problems of high soil salinity. This has caused major problems to agriculture, and these problems worsen when there is an increased pumping/overuse of local groundwater.

Floodplains are flat areas of land adjacent to rivers that can stretch from the banks of rivers to the base of valley slopes. As a river travels to the sea, the floodplain becomes an estuary which connects the river to the sea. Damming rivers and overuse of water can cause a river to lose its connectivity with the sea; this has devastating ecological effects and also destroys local coastal economies and communities. Floodplains also need to receive sediments transported by the river to provide fertile soil for agriculture and nutrients for ecosystems. Dams can reduce sediment loads downstream causing eutrophication in reservoirs. Eutrophication can decrease oxygen in the water resulting in loss of fish and diversity of macrophytes. In coastal areas, if freshwater aquifers near the coast are over pumped, saltwater intrusion can occur in the aquifer. Generally aquifers near the coast have a layer or lens of freshwater near the surface and then denser seawater under the freshwater. If the ground-water near the coast is pumped too much, saltwater can intrude into the freshwater aquifer and cause contamination of potable freshwater supplies. An interesting point here is that some aquifers situated near coastal areas which have been classified as confined can become unconfined by this over pumping.

The legal scope of the UN Convention covers all the above mentioned interactions, where they occur across state borders and where the components of the watercourses system are related to the international watercourse. One major exception to this legal coverage is the utilisation of confined groundwater which is not covered by the UN Convention.

To understand how much water is available on earth and in what form, see Figure 1.2 on the right.

98 Evapotranspiration is a term used to describe the loss of water to the atmosphere from the earth’s surface by evaporation and by transpiration through plants. US Geological Survey, (USGS 2008 <http://wa.water.usgs.gov/projects/evapo/> accessed 1 October 2011.

99 Confined groundwater was understood by the ILC during the drafting of the Convention to mean ‘groundwater which is unrelated to surface water’. See UNGA ‘Report of the International Law Commission on the Work of its Forty-Sixth Session’ (2 May-22 July 1994) UNGAOR, 1994 Draft Articles at 135. The words ‘confined aquifer’ and ‘fossil aquifer’ are often (mis-)used interchangeably. A confined aquifer can sometimes be a fossil aquifer but not always. Whether an aquifer is confined or not is a matter of pressure and not of the age of the water, whereas fossil aquifer refers to age of the groundwater e.g. an aquifer which is very old. See Section 2.1.4 for further explanation.

	Figure 1.1 Scope Defines the Legal Coverage of a Treaty– Waters and Water Use (Source Authors)
	Figure 1.2 Status of World Water (Source UNESCO)

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