There’s another water crisis happening in the Colorado River Basin — and this one’s invisible

Kaylee Greenlee/Reuters

The United States’ two largest reservoirs have dropped to unprecedented lows this year as drought grips the Southwest, but below the ground there’s another water crisis playing out — one that’s much less visible, but no less alarming.

The amount of groundwater in the Colorado River Basin is declining rapidly, and it’s doing so in unexpected ways, according to a new study from scientists at Virginia Tech, published in the journal Water Resources Research.

Lake Mead and Lake Powell are the visible faces of the Southwest’s critical water problem, as the Colorado River — which supplies water to nearly 40 million people — shrinks due to a combination of climate change and human demand.

“However, they represent only part of the region’s water resources,” said Esther Oyedele, a study author and a geosciences graduate student at Virginia Tech. “Groundwater is another essential component.” It is a vital alternative source in the Colorado River Basin, especially when surface water in rivers, lakes and reservoirs dries up.

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To build a picture of what’s happening to the region’s groundwater, the scientists compared measurements from NASA satellites with estimates of surface water, soil moisture and snow-water storage between 2004 and 2023.

They found declines across the basin during those two decades, with losses accelerating after 2019. A severe drought between 2020 and 2022 led to particularly significant falls — and the researchers found a surprising twist.

Historically, groundwater has typically declined more in the Lower Colorado River Basin, which encompasses California, Arizona and Nevada. The Upper Basin, which includes Colorado, New Mexico, Utah and Wyoming, has tended to be more resilient as its groundwater benefits from melting snow coursing down the Rocky Mountains.

However, from 2020 to 2022, groundwater in the Upper Basin declined by about 618 cubic feet, nearly twice the rate of the Lower Basin, which fell by an estimated 332 cubic feet.

“The system has hit a turning point: The Upper Basin is now experiencing rapid groundwater losses,” the report authors wrote .

‘Uncharted territory’: Second-largest reservoir in US plunges to a record low

Surface water and groundwater are linked in a delicate dance: Rain and melting snow seep into the ground and recharge aquifers, meanwhile groundwater can also make its way into rivers and lakes, for example through springs.

There are big differences in how these relationships play out between the Upper and Lower Basins, the study found. Snowmelt plays a much more important role in the Upper Basin, which receives more precipitation than the generally hotter, drier Lower Basin. Prolonged drought, higher temperatures and reduced snow appear to be placing the Upper Basin region under additional pressure.

The research follows another study published last year, which found that the amount of groundwater pumped out of the Colorado River Basin over the previous two decades was enough to fill Lake Mead .

It takes thousands of years to refill these aquifers, “and we as humans have more or less been burning through it over the last century,” Jay Famiglietti, an Arizona State University professor who was an author on last year’s study, said at the time.

The messages of this paper and the new study “have been that while surface water gets all the policy (and) management attention, groundwater dominates the losses from the entire basin,” Famiglietti told CNN recently.

Shrinking groundwater is going to be a problem as Arizona, California and Nevada are being forced to make cuts to the water they draw from the Colorado River, Famiglietti said, referring to a new operating plan for the basin published in August.

Three Colorado River states must take big water cuts as biggest US reservoirs plunge to record lows

The study does not give details on the total amount of groundwater remaining in the basin, only how it has changed over time. It also doesn’t quantify the exact impact of climate change on the falling water levels compared to other factors at play, including human water demand and groundwater extraction.

What the study does show is that “groundwater storage has experienced substantial and persistent declines,” Oyedele added.

“Focusing only on reservoirs could therefore overlook an important part of the region’s overall water challenges,” she said, adding that groundwater should not be seen as an “unlimited reserve.”

Ella Nilsen contributed to this report.

凯莉·格林利/路透社

今年，由于干旱席卷美国西南部，美国最大的两个水库水位降至前所未有的低点，但在地下，另一场水危机正在上演——这场危机不太明显，但同样令人担忧。

根据弗吉尼亚理工大学科学家发表在《水资源研究》杂志上的一项新研究，科罗拉多河流域的地下水量正在迅速减少，而且减少的方式出乎意料。

由于气候变化和人类需求的双重影响，为近 4000 万人供水的科罗拉多河水位不断下降，米德湖和鲍威尔湖成为美国西南部严峻水资源问题的直观体现。

“然而，这些水资源仅占该地区水资源的一部分，”该研究的作者之一、弗吉尼亚理工大学地球科学研究生埃丝特·奥耶德勒说，“地下水是另一个至关重要的组成部分。”在科罗拉多河流域，地下水是一种重要的替代水源，尤其是在河流、湖泊和水库等地表水干涸时。

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为了了解该地区地下水的状况，科学家们将美国宇航局卫星的测量数据与 2004 年至 2023 年间的地表水、土壤湿度和雪水储存量的估计值进行了比较。

他们发现，在过去的二十年里，整个流域的水位都在下降，而且在 2019 年之后下降速度加快。2020 年至 2022 年的严重干旱导致了特别显著的下降——研究人员发现了一个令人惊讶的转折。

历史上，科罗拉多河下游流域（包括加利福尼亚州、亚利桑那州和内华达州）的地下水位下降幅度通常更大。而上游流域（包括科罗拉多州、新墨西哥州、犹他州和怀俄明州）则相对更具韧性，因为其地下水受益于落基山脉融雪的补充。

然而，从 2020 年到 2022 年，上盆地的地下水减少了约 618 立方英尺，几乎是下盆地减少速度的两倍，下盆地估计减少了 332 立方英尺。

报告作者写道：“该系统已经到了转折点：上游盆地现在正经历着地下水的快速流失。”

“未知领域”：美国第二大水库水位降至历史新低

地表水和地下水之间存在着微妙的联系：雨水和融化的雪水渗入地下，补充含水层；同时，地下水也可以通过泉水等途径流入河流和湖泊。

研究发现，上游流域和下游流域在这些关系中的表现方式存在显著差异。融雪在上游流域发挥着更为重要的作用，因为上游流域的降水量远高于通常更炎热、更干燥的下游流域。长期干旱、气温升高和降雪减少似乎正给上游流域地区带来额外的压力。

这项研究与去年发表的另一项研究相呼应，该研究发现，过去二十年间从科罗拉多河流域抽取的地下水量足以填满米德湖。

“这些含水层需要数千年才能重新注满，”亚利桑那州立大学教授杰伊·法米格列蒂（Jay Famiglietti）当时说道，他是去年这项研究的作者之一。

法米格列蒂最近告诉 CNN：“本文和这项新研究传达的信息是，虽然地表水得到了所有政策和管理方面的关注，但地下水在整个流域的损失中占据主导地位。”

法米格列蒂表示，地下水减少将是一个问题，因为亚利桑那州、加利福尼亚州和内华达州被迫削减从科罗拉多河抽取的水量。他指的是8月份发布的一项新的流域运营计划。

由于美国最大的几个水库水位降至历史新低，科罗拉多河沿岸三个州必须大幅削减用水量。

该研究并未提供盆地剩余地下水总量的详细信息，仅分析了其随时间的变化情况。此外，它也没有量化气候变化对水位下降的具体影响，并将其与其他因素（包括人类用水需求和地下水开采）进行比较。

奥耶德勒补充说，这项研究确实表明“地下水储量出现了大幅且持续的下降”。

“因此，只关注水库可能会忽略该地区整体水资源挑战的重要组成部分，”她说道，并补充说，地下水不应被视为“无限的储备”。

Ella Nilsen对本报道亦有贡献。