Surface Water Changes
Every land-use decision is a water-use decision
Patterns of Change
Surface water is crucial because it is the most accessible form of available water to human populations. High-resolution mapping of data from three million Landsat satellite images has shown that over the period from 1984 to 2015, permanent surface water contained in lakes, rivers and reservoirs has fluctuated greatly across the globe. Once considered permanent, about 90 000 km2 of surface water has been lost – overwhelmingly (70 %) in the Middle East and Central Asia – while about twice as much (184 000 km2) of new surface water has formed across the globe. All continents show a net increase in permanent water, except Oceania, which has a 1 % net loss. Overall, 24 countries have gained at least 1 000 km2 of new surface water. The underlying drivers of these fluctuations have been drought, climate change and human actions, the latter of which includes river diversion or damming and unregulated withdrawals. Much of the global increase is from the formation of new reservoirs, although some is due to climate change, such as in high-elevation lakes throughout the Tibetan Plateau. These lakes have expanded in size and number over the past decade – an increase in area of 20 % – as a result of accelerated snowand- glacier melt caused by higher temperatures and changing precipitation, which is caused by global warming.
The bulk of increases in surface water is the direct result of dam construction and the subsequent filling of reservoirs. Dams are considered to be essential components of economic development and indeed have many economic and societal benefits, including flood control, hydroelectric power generation, and storage of water for irrigation, tourism and recreation. But dams also come with heavy costs to the environment and local human societies. This includes significant alteration of biogeochemical cycles (such as those of silicon and phosphorus), the invasion of exotic species, the upstream flooding and loss of rich agricultural land, negative impacts on human health (such as creating favourable environments for disease vectors, e.g. of malaria), displacement of human populations and their loss of livelihoods and the destruction of terrestrial and aquatic ecosystems and subsequent loss of biodiversity. Diminished downstream flows also cause fragmentation and the disappearance of fluvial systems, which has a devastating impact on ecosystems and the services they provide. Dams impact downstream estuaries, deltas and coastal zones via increased salinity, the loss of nutrients and decreased sediment, which is essential for the maintenance of deltas, beaches and sandbars. The Three Gorges Dam in China is a recent, high-profile example of the massive scale that such disruptions can have. Downstream, in the irrigated cropland areas served by dams, there are challenges to manage and maintain soil structure and fertility, prevent soil salinity, minimise fertiliser movement into groundwater and surface waters and protect riparian and adjacent ecosystems. The magnitude and complexity of managing large river basin developments as coupled human and environmental systems are well documented and illustrated in examples around the world, including the Indus River in Pakistan, the Nile in Egypt and the Colorado in the USA. North America contains about 52 % of the planet’s permanent surface water. Between 1984 and 2015, its permanent surface water area increased by 17 000 km2, largely because of the construction of new reservoirs. Other notable increases in surface water are associated with increased runoff from glaciers and snowpack surrounding the Himalayas and the restoration of the Garabogazkol Aylagy lagoon along the Caspian Sea in Turkmenistan.
Reductions in surface water can be attributed to numerous factors, including drought, climate change and increasing human demand (mainly for agriculture). The Aral Sea in central Asia and Lake Chad in Africa are two prominent examples of where upstream diversions have reduced surface water, although climate change has perhaps exacerbated these losses. Another cause of the disappearance of surface water is the drying up of reservoirs due to the construction of upstream dams, as in Syria and Iraq where the Greater Anatolia Project in Turkey has constructed new reservoirs to bring new agricultural land into production. The rate of loss of global wetlands is alarming. Although wetlands are significant for water security, food security and human health, there is as yet no agreed global map of wetlands, and global estimates on conversion are mainly based on satellite data. Due to uncertainties in distinguishing man-made features, such as rice fields from natural features such estimates have not been reliable. Based on alternative and combined approaches, the loss of global wetland extent has recently been estimated at around 30 % during the 1970-2008 time period (based on the Wetland Extent Trends (WET) index) and to be “at least 33 %” in 2009. Wetlands are not only global biodiversity hotspots, they are an intrinsic chain in the water flow cycle providing vital ecosystem services such as hydrological regulation, flood prevention, water supply, erosion control and nutrient retention. Their degradation or loss, at an annual average decline of around 1 % globally, is an important aspect to be considered in terms of water resource change and land degradation.
Water Trends in some Key Areas.
Surface water bodies altered due to climate variability and human activities.