California’s lengthy drought made mudslides more likely when the rains came.

After several years of intense drought, California is now being pummeled by weeks of heavy rain. As the state’s residents are discovering, the two opposite meteorological conditions can combine to make for severe mudslides.

In a prolonged drought, soils dry out, harden and become less permeable to water, said Dan Shugar, a geoscientist at the University of Calgary in Canada. When heavy rain falls on soil like that, less of the water soaks in.

“Any gardener who has forgotten to water their flower patch during a heat wave has firsthand experience of this,” Dr. Shugar said.

Water can pond quickly on the hardened soil, and, depending on the terrain, will eventually run off. On steep slopes, the water rushing downhill can accelerate, eroding soil in its path, picking up rocks and debris and joining with other rivulets of water to make a growing and potentially destructive mudslide.

California’s drought has also helped fuel major wildfires in recent years, and post-wildfire slopes are especially susceptible to mudslides. On Monday, concern about potential mudslides prompted evacuations in Montecito, Calif., where 23 people were killed five years ago in a slide that occurred a month after a wildfire on the hills nearby.

After trees and other vegetation are killed in a fire, their roots weaken over the next several years, making the soil they are in less stable. By vaporizing waxy compounds in vegetation that are then deposited in the soil, extremely hot fires can also make soils water-repellent, increasing surface runoff when the rains come and raising the risk of a mudslide.

Mudslides, which are also referred to as debris flows, tend to be shallow, eroding the topmost layer of soil and picking up rocks and other debris on the surface.

But heavy rains can also cause major landslides, when a large part of a slope becomes saturated with water to great depth, increasing the pressure between soil particles and making the slope unstable.

The rugged hills along California’s coast are especially susceptible to this kind of landslide. In 2005, houses in the small town of La Conchita, just below the coastal highway, Route 1, in Southern California, were buried when the hillside gave way, killing 10 people.

Some major landslides may be moving in slow motion for years before heavy rain triggers a much faster-moving failure. After the 2017 slide in Big Sur in Northern California that is considered the largest in state history, when about 6 million cubic yards of debris slid across Route 1, NASA researchers used radar data to study the slope. They found that it had been moving at a rate of about 7 inches a year for about a decade.