Amid late-arriving rains and increasingly unpredictable weather patterns, groundwater supplies are shrinking, a fact that could leave 1.5 million Cambodian farmers unable to water their crops within 15 years, according to a study published last month in the Journal of Hydrology.

The study by Laura Erban and Steven Gorelick of the department of earth system science at Stanford University found that a growing reliance on groundwater use – which has grown by 10 per cent annually in recent years – may drop the water table below the “lift limit” of suction pump wells.

“Extensive groundwater irrigation jeopardises access for shallow domestic water supply wells, raises the costs of pumping for all groundwater users, and may exacerbate arsenic contamination and land subsidence that are already widespread hazards in the region,” the study authors wrote.

Even if the Kingdom starts drawing more irrigation water from rivers and lakes, its options are limited, the study found.

At the moment, 96 per cent of rice fields are left fallow in the dry season, due to lack of irrigation.

Expanding irrigation networks to allow countrywide dry season rice production would require a volume of water equal to 31 per cent of dry season Mekong River flow to Vietnam.

Vietnamese media has repeatedly warned about a shortage of fresh water entering the country in recent months, partly due to the El Niño weather cycle, long-term climate change, and dams and water diversions by upstream countries. Recently, Thailand began diverting 47 million cubic metres of Mekong water for its own irrigation projects, a number expected to grow.

Bun Hean, secretary of state at Cambodia’s Ministry of Water Resources and Meteorology, confirmed that groundwater depletion was an issue in parts of Cambodia, especially in Svay Rieng and Prey Veng provinces.

He said that the water table in those areas dropped to low levels typically for three months during the dry seasons. However, he could not confirm how many farmers this pattern threatened.

Hean said that groundwater use was the work of individual farmers. Government irrigation projects relied on water from the Mekong and the Tonle Sap lake.

The government was looking to develop 20 major agricultural reservoirs in the Tonle Sap region, though Hean could not give a target completion date.

Ian Thomas, an adviser with the Mekong River Commission, said that groundwater depletion was worst in areas that didn’t get recharged by Mekong or Tonle Sap floods.

The Tonle Sap didn’t flood last year due to El Niño and lack of typhoons entering Cambodia, which exacerbated the shortages, according to Hean.