Human water use involves changes to natural patterns of runoff and river discharge, through water withdrawals, consumption and return flows, land use change, irrigation, and inter-basin water transfers.
Compared to water stress and shortage, or the need to share water, it is not necessarily obvious how changes to flows relate to water scarcity. Fundamentally, it is a question of mass balance: even if water is a renewable resource, there is only a limited amount at any given place and time; using water – and being able to use water – affects and depends on how water is distributed in a landscape, and what other users have done to influence this.
Key means of altering flows:
Water withdrawals involves diverting water from rivers, lakes or groundwater that would otherwise have flowed elsewhere. This is typically because water is scarce where it is being taken, and it may result in water scarcity for those who would otherwise have received the water.
Water consumption refers to water that will no longer be available for other users within this loop of the water cycle (e.g. until it rains again). Part of the water withdrawn will become available again (often at a different time and place) as “return flows”. The rest of the water either evaporates, is incorporated into products, or discharges to the ocean.
Land use change affects runoff and evaporation. More (or less) water may flow into rivers, lakes and groundwater, or instead will be evaporated.
Irrigation is the most prominent water cycle change globally. It involves substantial land use change, affecting both runoff and evaporation on site, and additionally consumes most of the water withdrawn (with return flows affected by irrigation efficiency).
Inter-basin water transfers are a high profile type of water withdrawal because they transfer water between areas that are not otherwise connected. The change to flows is generally more obvious than within a river basin where it is normal for some water to evaporate (be consumed) locally, and for some to continue on to downstream users.
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Water scarcity is closely tied to how we change the way water flows in a landscape
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