A new method to determine how resistant rivers are to drought has been developed by researchers at the University of California, Davis. The method uses satellite data to measure the amount of water stored in the soil and groundwater along a river's course. This information can then be used to calculate how long a river can withstand a drought without running dry.

The method is described in a paper published in the journal Water Resources Research. The authors say it could be used to help water managers develop plans to protect rivers during droughts.

"Our method provides a new way to assess drought vulnerability and identify rivers that are most at risk," said lead author Christopher Castro, a postdoctoral researcher at UC Davis. "This information can be used to develop more effective drought management strategies."

The researchers used satellite data from the Gravity Recovery and Climate Experiment (GRACE) mission to measure changes in water storage along 24 rivers in the western United States. They found that the amount of water stored in the soil and groundwater varied significantly from river to river. Some rivers, such as the Sacramento River in California, had a large amount of storage and were therefore less vulnerable to drought. Other rivers, such as the Colorado River, had a small amount of storage and were therefore more vulnerable to drought.

The researchers also found that drought vulnerability varied over time. During wet years, rivers had more water stored in the soil and groundwater and were therefore less vulnerable to drought. During dry years, rivers had less water stored in the soil and groundwater and were therefore more vulnerable to drought.

The researchers say their method could be used to help water managers develop plans to protect rivers during droughts. For example, water managers could release water from reservoirs to keep rivers flowing during dry periods. They could also work with farmers and other water users to reduce water use during droughts.

"Our method provides a valuable tool for drought management," said co-author Jay Famiglietti, a professor of hydrology at UC Davis. "It can help water managers identify rivers that are most at risk and develop strategies to protect them during droughts."