Critical groundwater supplies may never recover from drought

Groundwater refers to the soil/rock saturated zone below the surface of the earth. Credit: USGS
Other than hurricanes and wildfires there is another, less-discussed, effect of climate change: sinking land and toxic water. This is made worse by groundwater shortages.

Snow and rain water seep deep into the soil between layers of soil, where it accumulates in underground bathtubs called aquifers. This groundwater is used by farmers to water their crops when surface water sources are insufficient.

Groundwater is used for a large percentage of Californians' water supply. Groundwater is used by around two billion people worldwide. Due to excessive groundwater usage and droughts, the land surface has been weakened, causing damage to critical infrastructure such as roads and buildings.

Groundwater recovery from drought takes on average three years according to new research by UC Riverside. Scientists found that the average recovery time for aquifers not affected by human activity is three years. Recovery times might be longer for regions that have experienced excessive pumping.

Groundwater levels must recover from droughts. New precipitation needs to percolate through soil to recharge the depleted well. This process may take longer for areas with higher groundwater levels, according to the researchers.

"If groundwater is pumped without letting it recharge, groundwater levels will continue to drop, the cost of pumping will go up, and land sinks," Hoori Ajami (UCR groundwater hydrologist, study coauthor, and principal investigator for this project) explained.

The new study, published in the Journal of Hydrology is the first to analyze groundwater response to droughts at a continental scale. Previous groundwater drought investigations have mainly relied on simulations of models and covered smaller areas. This study used 30 years of daily measurements taken from 600 wells throughout the country.

Researchers found that it takes approximately two years for rainwater droughts to turn into groundwater drought. However, in some cases, it can take up to 15 years. The effects of rainwater drought are not immediately felt because of this long lag. They can be very severe.

Subsidence is a combination of groundwater drought and pumping. Ajami stated that subsidence is a problem in California's Central Vaalley. It is also exacerbated due to climate factors and excessive water pumps. Subsidence can cause irreversible damage and destruction to infrastructure, buildings, roads, and other road infrastructure.

Ground shifting and falling water levels can lead to soil contaminants like arsenic that can be mobilized and poison the water. Aquifers can become unusable in coastal areas due to salty seawater and drought.

Ajami stated, "You start off with a problem with water quantity and end up with a water quality problem."

Adam Schreiner McGraw, study co-author, said that excessive pumping lowers groundwater levels and creates a downward spiral in the process of restoring the aquifer.

Researchers have several suggestions to reduce the impact of prolonged droughts, which are inevitable as the planet warms. Many climate models predict that rain intensity will increase. Storing rainwaters could help to recharge the aquifers and speed up recovery.

Researchers also suggested that farmers increase irrigation efficiency and change perennials such as almonds, walnuts, and pistachios to less water-intensive crops. This is especially important in areas with severe groundwater depletion.

Ajami stated that "We must improve our climate projections so that they include groundwater, in order to better assess what is available and how we can protect it." There are many ways to manage what we do have.

Continue reading Tracking water storage offers options to improve water management during droughts and floods.

Additional information: Adam P. Schreiner McGraw et al., Delayed groundwater response to multi-year meteorological dry years in the absence of anthropogenic control, Journal of Hydrology (2021). Adam P. Schreiner McGraw et al., Delayed groundwater response to multiyear meteorological droughts without anthropogenic management. (2021). DOI: 10.1016/j.jhydrol.2021.126917

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