Drought takes many forms. There’s meteorological drought, in which snow and rainfall are abnormally scarce. There’s hydrologic drought, in which rivers, lakes, and underground aquifers draw down or dry up; typically, a hydrologic drought is declared if water levels drop below the 25th percentile in a given region. And there’s agricultural and socioeconomic drought, which occurs when rain, surface water, and groundwater are not sufficient to sustain crops or other human activities. Now, a new study shows how different patterns of rainfall drive hydrologic drought when combined with human water use, such as groundwater pumping.
Meteorological droughts drive hydrologic droughts, but humans exacerbate water scarcity, too, by extracting water from rivers and underground aquifers faster than it can be replenished. To explore how humans and climate interact to impact drought conditions, Apurv et al. built a virtual watershed that included some fixed properties, such as silty loam soil, but allowed climate and human properties—like how much rain fell at different times of the year and how much water people removed from reservoirs and aquifers—to change.
The team used precipitation data from the Global Historical Climatology Network from 10 sites around the world, including Australia, the United States, China, India, and Poland, to mimic a range of different climates. Then they simulated a typical human response to changing water supplies: When rivers, lakes, and reservoirs were full, people drew on them for water to drink, bathe, water crops, and run businesses. When those sources dwindled, people started pumping water from the ground.
The researchers found that different annual patterns of rainfall played a major role in how people used water and how much water was available in aquifers long term. For example, in California, where the annual rain supply is highly variable, the model showed little groundwater depletion in response to drought. Although people pumped groundwater during dry spells, they stopped after sporadic, heavy rains filled the reservoirs, allowing the water table to recover. (The model is not intended to mimic real-life California, where overwhelming demand for water has indeed depleted aquifers, the authors note.)
By contrast, in a climate such as Mwinilunga, Zambia, where annual rainfall is less variable, the model predicted significant groundwater depletion in response to drought. When the reservoir dried up, people started pumping groundwater, which, in turn, depleted the flow in the rivers. No periodic deluges arrived with enough precipitation to refill the reservoirs, so people kept pumping, creating a positive feedback loop.
The results need to be validated with data from the modeled sites, including more accurate details about human responses. For example, intense, rising demand for water nearly always leads to groundwater depletion, regardless of the climate. But the study does shed light on how different precipitation patterns affect the human response to drought and could point to strategies for making dwindling resources last a little longer. (Water Resources Research, https://doi.org/10.1002/2017WR021445, 2017)
—Emily Underwood, Freelance Writer