吳昕華翻譯;吳佳靜、瞿涵審校

美國加州大學爾灣分校(University of California, Irvine)去年發表兩份研究報告(註),美國國家太空總署(NASA)重力反演與氣候實驗(Gravity Recovery and Climate Experiment , GRACE)衛星的資料顯示,地球上最大的地下水庫,水量正以令人不安的速度消耗中。
 

圖片來源:NASA Goddard Space Flight Center(CC BY 2.0)。
 
GRACE衛星藉著測量2003年到2013年十年間的引力,觀測地球上37個最大地下水含水層的水量,發現其中21個含水層已經超過了永續的臨界點,這意味著它們每年流失的水量,比天然補注的降雨和融雪來得多。而這21個含水層中,其中八個的情況為「嚴重透支」,恢復水資源的自然補注量很少。另外五個含水層則被認定是「極度或高度透支」,表示它們處於困境,但仍然有一些水會流回去。
 
全世界地下水水源透支最嚴重的是阿拉伯含水層系統,供水給超過6000萬人。位於印度西北部和巴基斯坦的印度河盆地含水層,透支情形排第二,第三名是非洲北部的Murzuk-Djado盆地。
 
雖然並非透支前三名的含水層,加州中央谷含水層系統仍然被認為是「高度透支」。加州46%的水是由地下水源供應,對農業極為必要。2015年是加州第四年處於乾旱,由於缺乏降雨,人們在乾旱期間更加依賴地下水。亞馬遜河流域和北美大平原的情況則稍微好一些,因為人口稀少且農作物大多利用雨水灌溉。
 
不幸的是,我們完全不知道這些地下水庫還剩下多少水。科學家是可以鑽入這些水層,但這是一個昂貴的過程,而且目前並沒有經費能資助這樣的計畫。
 
然而,我們必須想辦法知道剩下的水量,因為大約有20億人仰賴地下含水層作為淡水資源。水資源的消耗已經造成生態損害,包括河流縮減、水質下降和地層下陷,而氣候變遷和人口增長,也預期會加劇水資源的枯竭。
 
好在還是有些減少個人用水的事情你可以做,例如安裝省水馬桶和省水蓮蓬頭。更簡單的方法還有:刷牙、刮鬍子、或抹肥皂時關上水龍頭。如果你要幫草坪澆水,最好選在一大早或是晚上晚一點,並挑比較涼快的日子,減少水分蒸發。種植原生種植物,以減少植物存活所需的水量。如果你有水池,不使用的時候就加蓋,也能避免水分蒸發。
 
盡你可能節約用水,因為令人難過的是,在進一步研究完成之前,沒人能知道我們的地下水層究竟還剩多少水。
 
 
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Our World's Water is Diminishing, But How Fast?

The University of California, Irvine (UCI) recently released two studies (here and here) using data from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites showing that the earth's biggest groundwater basins are being depleted at troubling rates. The GRACE satellites observed the 37 largest aquifers on the planet by measuring their gravitational pull over the course of ten years, from 2003 to 2013. Out of these 37 aquifers, 21 were found to have exceeded their sustainability "tipping points," which means they lose more water every year than can be naturally replenished by rainfall and snow melt. Out of those 21 aquifers, 8 are "overstressed" and are receiving little, if any, "natural replenishment" to restore their water resources. An additional five aquifers are designated as "extremely or highly stressed," meaning they are "still in trouble" but have "some water flowing back into them."
 
The most overstressed groundwater source in the world is the Arabian Aquifer System that supplies water to more than 60 million people. The second-most overstressed aquifer is the Indus Basin aquifer in northwestern Indian and Pakistan, followed by the Murzuk-Djado Basin in northern Africa. While not in the top three most stressed aquifers, the California Central Valley Aquifer System is still considered "highly stressed." In California, 46% of the state's water is supplied by groundwater sources, which are necessary for agriculture. California is currently in its fourth year of drought and people rely on aquifers more heavily during droughts because of a lack of rainfall. Places like the Amazon Basin and the Great Plans are doing slightly better because of their sparse populations and use of rainwater for crops.
 
Unfortunately, it is completely unknown how much water is left in these groundwater basins. Estimates range from decades to millennia. It is possible for scientists to drill into the basins to determine how much water remains, but this is an expensive process and funding for a project like this does not currently exist. However, it is necessary to find out how much water remains because around two billion people rely on underground aquifers for their freshwater. There is already ecological damage from the water depletion, including diminished rivers, a decrease in water quality, and subsiding land. Climate change and population growth are expected intensify the depletion.
 
Luckily there are some things you can do to reduce your individual water use like installing low-flush toilets and low-flow showerheads. Even simpler practices include turning off the water while brushing your teeth, shaving, or soaping in the shower. If you water your lawn, it is best to do so early in the morning or late in the evening, and on cooler days, to reduce evaporation. Planting indigenous plants reduces the amount of water necessary to keep them alive. If you have a pool, cover it when it is not in use to once again reduce evaporation.
 
Do what you can to help conserve water because sadly, there's no telling how much we have remaining in our aquifers until further research is done.
 
Written by Marisa Barley
Published on Earth Day Network, June 25, 2015