"Freshwater shortages are likely to trigger increased environmental damage over the next 15 years," notes the UN Environmental Program's (UNEP) Global International Waters Assessment (GIWA) report, based on the input of 1,500 experts worldwide.

Inadequate potable water is an immediately critical problem for billions of people, the study points out.

But freshwater shortages caused by massive damming and depleted aquifers are provoking a chain reaction of environmental problems as well, beginning with falls in river flows, rising saltiness in biologically-rich estuaries, and the reduction in coastline sediment. The knock-on impact of these changes, the study predicts, will be a serious loss of fish and aquatic plant life, shrinking farmland, damage to fisheries and food insecurity by the year 2020.

At the end of the chain of consequences, it says, are increases in malnutrition and disease.

The problem is aggravated by changes in the pattern of human food consumption.

"Globally, there has been an increased demand for agricultural products and a trend towards more water-intensive food such as meat rather than vegetables and fruits rather than cereals," the study concludes as nations mark World Water Day on Wednesday.

Irrigated agriculture now accounts for 70 percent of freshwater withdrawals, with only 30 percent returned to the environment, studies have shown.

Industry and households, by comparison, return up to 90 percent of the water used.

The fact that many developing countries do not have adequate scientific or technical information about their water supplies is also an aggravating factor.

Such nations are "operating in the dark on the size of their water resource, and the precise patterns of supply and demand," the study says.

"Aquifers represent the largest information gap, which is an increasingly significant hindrance for effective water management," it continues.

The study also points to "market failures" as contributing the problem, noting that many of the factors contributing to environmental degradation and pollution -- including use of pesticides and herbicide, water for irrigation, dam construction -- are heavily subsidized by governments.

Topping the list of medium-term concerns related to water is pollution.

by 2020, the environmental impacts of pollution "are predicted to increase in severity in over three quarters" of the areas studied, "making this the most negative future outlook for any of the GIWA concerns," the study says.

The report says that suspended solids, mainly resulting from deforestation and agriculture, have already severely affected coral reefs and river habitats in the Caribbean Sea, the Brazil Current, the east African Rift Valley lakes and all the regions of Southeast Asia.

It also cautioned against increasing the impact of eutrophication, a process which triggers oxygen deficiencies as a result of excessive richness in nutrients, frequently a result of fertilized agricultural run-off, untreated sewage and air pollution.

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