Soil and water

Let it flow – ScienceDaily

Besides hurricanes and forest fires, there is another important, but rarely mentioned, effect of climate change: toxic water and sinking land made worse by dry groundwater.

Water from snow and rain seeps deep into the ground between layers of soil and collects in sponge-like underground tubs called aquifers. Farmers rely heavily on this groundwater to irrigate their crops when they cannot get enough water from surface water sources.

About 85 percent of Californians also depend on groundwater for part of their water supply. Around the world, around two billion people depend on it. However, the excessive use of groundwater combined with droughts has caused the land surface to sink, damaging critical infrastructure including roads, buildings, sewers and water pipes.

New research from UC Riverside shows that groundwater takes an average of three years to recover from drought – if it ever does. In the largest study of its kind, scientists found that this recovery time only applies to aquifers that are not affected by human activity, and the recovery time could be even longer in areas with excessive pumping.

For groundwater levels to recover after a drought, new precipitation needs time to seep into the soil and recharge the depleted aquifer. Researchers show that this process can take several years longer in areas with deeper groundwater levels.

“If people pump groundwater without letting it recharge first, groundwater levels continue to drop, the cost of pumping increases and the land sinks,” said Hoori Ajami, UCR’s groundwater hydrologist. and co-author of the study and principal investigator on this project.

Posted in the Hydrology Journal, the new study is the first to examine the response of groundwater to droughts on a continental scale. Previous groundwater drought surveys relied mainly on model simulations and covered smaller areas. This study was based on 30 years of daily measurements of 600 wells across the country.

On average, researchers have determined that it takes about two years for drought in rainwater to turn into drought in groundwater, although in some cases it takes up to 15 years. Due to the long standby time, the effects are not felt or visible immediately. However, they can be severe.

Dry groundwater combined with pumping can cause a gradual and uneven lowering of the soil surface called subsidence. “This is a known problem in the Central Valley of California, exacerbated by climatic factors and excessive water pumping,” Ajami said. “The subsidence is causing irreversible damage to infrastructure, buildings and roads.

As the soil moves and the water level drops, contaminants in the soil, such as arsenic, can mobilize and poison the water. In coastal areas, aquifers depleted by drought and pumping can fill with salty seawater, rendering groundwater unusable for consumption or agriculture.

“You start with a water quantity problem and you end up with a water quality problem,” Ajami said.

“Excessive pumping lowers the groundwater level, creating a downward spiral in which aquifer restoration becomes increasingly difficult,” added study co-author Adam Schreiner-McGraw.

To reduce the damage from prolonged droughts that are inevitable as the planet continues to warm, researchers have several recommendations. Most climate models show that the rains are getting more intense. Storing rainwater could recharge aquifers, thereby speeding up the recovery process.

The researchers also suggest that farmers are improving irrigation efficiency and replacing perennials like almonds, pistachios and walnuts with less water-intensive annual crops in areas with severe groundwater depletion.

“We need to improve our climate projections to include groundwater, so that we can better assess what we have and how to protect it,” Ajami said. “There are ways to better manage what we have. “

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