Global distribution varies dramatically in space and time
At the global scale, surface water is a dynamic resource1. While surface water is only one part of the global water resource, it is the most readily accessible for human use and provides wide-ranging ecosystem services. Over the past three decades, 3 % of the Earth’s landmass (4 476 571 km2) has been covered by water for at least some period of time. Permanent lakes and rivers comprise about 60 % of surface water, while the rest occurs as episodic events, including seasonal (e.g. monsoons) and sporadic (e.g. once every few years) events. Most surface water is freshwater, but there is no accurate data available on the proportions of fresh/brackish/saline waters.
When and where surface water occurs on the planet is critically important. Water availability is relevant to almost all socio-economic and environmental impacts of climate and demographic changes and has far-reaching implications for sustainability. The presence or absence of surface water on the planet’s landmass influences the physical climate system because it affects the exchange of heat, gas and water vapor between the planet’s surface and atmosphere. Surface water sustains ecological systems on which terrestrial life depends. It affects our capacity to grow crops and raise animals, underpins industrial processes, influences the movement of diseases and toxins, generates energy, can cause loss of life and damage to property and infrastructure and also has immense spiritual, recreational and cultural value in our lives. Surface water is therefore a fundamental land resource.
North America accounts for 52 % of the planet’s truly permanent water resources and 15 % of contemporary seasonal water, but has just 5 % of the human population. In contrast Asia, with 60 % of the human population, has only 9 % of the truly permanent and 35 % of the contemporary seasonal water. Africa and Latin America have almost the same share of the world’s permanent water at around 9 % each, though their populations are very different, with Africa (16 % of the total) supporting nearly twice as many people as Latin America (8.6 %). Europe, including Russia, with 10 % of the global population has 22 % of the permanent water and 18 % of the contemporary seasonal water.