Walking over water might sound like something unbelievable. In fact, people do it all the time. How? Almost all of the world's liquid fresh water lies underground. This stash beneath our feet is called groundwater.
Earth is a water planet, but most of its H2O is in the oceans. Only about 2. 5 percent of the planet's water is fresh water. Of that, nearly 69 percent is frozen in glaciers (冰川) and ice caps. And about 30 percent is groundwater —— much more than the 1. 2 percent that flows through rivers and fills lakes.
Groundwater is found almost everywhere on Earth. It hides under mountains, plains, and even deserts. Tiny gaps between rocks and soil grains take in and hold this water like a sponge, forming buried bodies of water called aquifers (地下蓄水层). Together, they hold about 60 times as much water as the world's lakes and rivers combined. Groundwater is a key part of the earth's water cycle. Rain and melted snow go down into the ground. The water can stay there for thousands of years. Some groundwater naturally runs out onto the earth's surface through. springs. It also flows into lakes, rivers, and wetlands. People get groundwater through wells for drinking, watering crops, and other uses. People draw more than 200 times as much groundwater from the earth as oil every year.
As human-caused climate change dries out parts of the planet, demand for groundwater may rise. At the same time, climate change may intensify (加剧) storms. Heavier rain is more likely to rush straight into streams and storm drains, instead of going into the soil. So, there may be less groundwater around.
Many of the world's aquifers already seem to be drying up. Twenty-one of the earth's 37 biggest aquifers are becoming smaller, satellite data show. The most dried-out aquifers are near big cities, farms, or dry regions. As groundwater stores dwindle, they hold less water to refill rivers and streams, thus threatening freshwater ecosystems. In California, drying up the ground may even be causing small earthquakes.
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