Harnessing rainfall’s changing nature to replenish our groundwater resources

A guest blog post by Sarah Beganskas

Rainy days, like people, come in many styles and temperaments. Some rainy days are mellow and drizzly, while others are harsh and drenching. At times there are a few calm puddles on the ground, at other times, water gushes down the street. Some storms last less than a day, while others deliver rain for weeks on end, and sometimes it doesn’t rain at all. And like people, the character of rainstorms in a particular area can change over time. Recent studies have observed that in our part of California (from the San Francisco Bay Area through the Central Coast), rainstorms are becoming heavier but less frequent. In other words, we’ve had fewer drizzles and more heavy downpours in recent years than in the past, as well as fewer rain events overall. These trends are expected to continue. While this has implications for how often you’ll be grabbing your umbrella, there is a less-visible, larger-scale impact: the changing character of our rainfall also significantly influences our water supply.

Here in the Central Coast region of California, more than 80% of the freshwater we use—for drinking, showering, growing food, and more—comes from beneath the ground. 

The percentage of total water usage that groundwater meets is shown for each region of California. The Central Coast meets 83% of its freshwater demand with groundwater—much higher than any other region. Groundwater overdraft occurs where groundwater is persistently used faster than it is refilled, and is very common in California. Overdraft can cause permanent loss of groundwater storage, land collapse, seawater intrusion into freshwater supplies, less water in local streams, and reduced water quality. The California Department of Water Resources has identified “critically overdrafted basins” as those at the highest risk, and they are highlighted in purple (right). Critical basins in the Central Coast region include the Pajaro Valley and Soquel Valley. Santa Cruz is shown in a red star and my field site location is shown with a yellow star. Both figures are modified from the California Department of Water Resources.

The percentage of total water usage that groundwater meets is shown for each region of California. The Central Coast meets 83% of its freshwater demand with groundwater—much higher than any other region. Groundwater overdraft occurs where groundwater is persistently used faster than it is refilled, and is very common in California. Overdraft can cause permanent loss of groundwater storage, land collapse, seawater intrusion into freshwater supplies, less water in local streams, and reduced water quality. The California Department of Water Resources has identified “critically overdrafted basins” as those at the highest risk, and they are highlighted in purple (right). Critical basins in the Central Coast region include the Pajaro Valley and Soquel Valley. Santa Cruz is shown in a red star and my field site location is shown with a yellow star. Both figures are modified from the California Department of Water Resources.

Our limited groundwater supply, invisible to us on the surface, is naturally replenished by rainfall. When it rains, water soaks into the ground and gravity pulls some of that water further down to become groundwater. But not all rain gets to become groundwater. As you know from walking around on a rainy day, a lot of rain becomes runoff, flowing along the ground and into storm drains. Our stormwater systems often aim to get runoff out of the area quickly to reduce flooding risk. Around here, we send it straight into the ocean. 

Stormwater collection system on a dry day in March 2013 (above) and on a rainy day in December 2014 (right).

Stormwater collection system on a dry day in March 2013 (above) and on a rainy day in December 2014 (right).

We often treat runoff as a nuisance, but it can actually be a valuable resource! Using cutting-edge scientific tools, I study how to take extra runoff and help it become groundwater by collecting the runoff in a basin, where it has the chance to seep into the ground (as illustrated in the photos above). While the concept is straightforward, you can imagine there are many considerations when designing a system that does this: Where should it be? How big should it be? How quickly will it drain? My research focuses on coming up with specific answers to these challenging and nuanced questions.

The author (middle) and two colleagues on a rainy day in the field.  We are standing at the entrance to the culvert through which runoff enters the basin, celebrating the first runoff of the season! 

The author (middle) and two colleagues on a rainy day in the field.  We are standing at the entrance to the culvert through which runoff enters the basin, celebrating the first runoff of the season! 

Addressing these questions requires understanding how much runoff is available. For the past four years, I have been working on a stormwater collection system near Watsonville in the Pajaro Valley, where the amount of runoff we collect each year varies widely. I have found that the most important factor is not how much it rains in total, but how hard it rains. For example, there were mostly light drizzles in 2012, while in 2014, most rain fell in a few heavy downpours. Despite there being less total rainfall in 2014, we collected nearly eight times as much runoff during the downpours that year compared to drizzly 2012. The amount of runoff collected in our basin during 2014 would be enough to supply 68 families of four with water for a year. These results mean that there are opportunities to collect runoff and improve groundwater supply even when we do not have a large amount of rainfall, like during the severe drought we are experiencing in California right now.

During a heavier rainstorm, more rain naturally becomes runoff and flows to storm drains, while less water naturally soaks into the ground. As rain in our area becomes heavier and less frequent, collecting extra runoff and directing it toward groundwater can help offset these changes. At a time when much of the world, including California, is struggling with groundwater management, the tools I am working on can be an important part of long-term water solutions. As a result, our water supply will be more resilient to rainfall’s changing temperament, and reliable and sustainable groundwater resources can be available for everyone.

Sarah Beganskas is a Ph.D. candidate in Earth and Planetary Sciences at UCSC.