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On Jun 16, 2012 03:01AM ET in Green 101
The study, featured in the NG article, was conducted by a team of Dutch scientists spearheaded by Yoshihide Wada, Ph.D. researcher at Utrecht University, theNetherlands. According to the study’s findings, groundwater may be a major contributor to sea level rise in the future more than melting of the polar ice sheets.
Groundwater is water (not only in liquid form) that collects and/or flows under the surface of the earth. This includes permafrost and soil moisture, but groundwater that is usually extracted comes from accessible water tables. Once pumped out of the ground, groundwater is used for irrigation, municipal and industrial uses, and for drinking water. Groundwater is recharged through rainfalls, snowmelt, and partly by surface water like rivers. However, when the rate of usage exceeds the rate of recharge, groundwater depletion occurs. Water quality problems such as salt water intrusion and water quality degradation, may result from groundwater depletion. Subsidence, reduced surface water flows, and food security threats are also associated with groundwater depletion.
According to the study groundwater depletion has more than doubled in the past decades, making it a major contributor to sea level rise and likely to dominate other terrestrial water sources in the near future.
The team measured the effect of groundwater on sea level rise by dividing the Earth’s total land surface into 50-by-50 km. squares grid and then calculating present and future groundwater usage. The researchers used current as well projected statistics based on economic and development projections for each nation. Climate change-induced effects on regional water needs were also considered. Taking into account groundwater recharge rates, the team calculated the net rate of groundwater depletion.
They found that net groundwater depletion is bringing an additional 0.6 millimeters to sea level every year. This is predicted to increase to 0.82 millimeters annually by 2050 due to population increase, economic development, and changing irrigation needs due to warming temperatures.
Wada states that groundwater depletion is set to add approximately 25% to the projected rates of sea level rise, effectively making it as the most significant contributor aside from the melting of polar glaciers.
Above ground reservoirs that hold water and prevent it from reaching the ocean are no longer very significant in offsetting the net groundwater loss, Wada adds. This is because many of them are already built and new ones are difficult to add in many locations.
Groundwater depletion is not the only way land-based water sources are lost. Draining of wetlands, deforestation, and increasing stormwater runoff levels contribute to sea level rise as the land becomes increasingly unable to hold them and they end up in the sea.
Though other scientists challenge the study’s findings on several points, including lack of data on how much groundwater seepage into rivers and difficulty of deep aquifer pumping offset the overall contribution to sea level rise, they agree its still an important consideration in predicting sea levels in the future.
Wada puts forth a possible solution: a search on how water usage in agriculture can be made more efficient, translating to more growth and productivity with less resources, particularly water. In doing so not only groundwater depletion’s contribution to sea level rise can be lessened, but land mismanagement can be addressed as well.