California’s Hydrology and Drought Conditions: Explained

The rainy, snowy winter of 2018 and spring of 2019 gave California a well-deserved surface water recharge. Does this mean California is now drought-free?


What’s to blame for this wet winter?


Why was this past winter abnormally wet after years of drought conditions? Atmospheric rivers are to thank—or blame. An atmospheric river is a long, narrow region in the atmosphere that transports most of the water vapor outside of the tropics, and releases water vapor in the form of rain or snow as it hits land. Essentially, “atmospheric river” is a fancy term for heavy rain. Imagine a river of water vapor flowing above your head and suddenly dumping. The West coast is no stranger to atmospheric rivers; California depends on them and when they do not arrive, the state suffers and slips into drought. While some atmospheric rivers provide beneficial rain or snow, those that contain large amounts of water vapor can be disastrous, resulting in extreme rainfall that induces flooding, mudslides, and infrastructure damage. In places facing drought, like California, too much rain at once can hurt more than help. 


National Geographic explains that concentrated moisture in the atmosphere and strong winds to move it around combine to form an atmospheric river’s power couple. As air temperature increases, water is evaporated from the ocean and travels into the atmosphere; Air can hold seven percent more water vapor for each degree warmer it gets. So, warmer air makes wetter air that fuels intense atmospheric rivers.

NASA/JPL-Caltech I


After this rainy season, are we out of a drought?


How one categorizes California’s drought condition depends on the way “drought” is defined. In climatology, draught is defined as a decrease in annual precipitation in comparison to the historical average; in hydrology, the definition of draught has different parameters: persistently low water volumes in either the surface (rivers, lakes, reservoirs) or subsurface (groundwater). I, along with Dr. Jenkins, Paleoclimatologist and Chair of the Department of Environmental Studies at the U of R, believe that it is crucial to take the historical perspective of subsurface water into account. Considering the typical rainfall and water reserve amounts for this region, and thus following the hydrological definition, our subsurface water reserves have not been sufficiently recharged to be considered out of the drought.


On March 14, the National Integrated Drought Information System released the U.S. Drought Monitor: the below map showing drought intensity decrease in California. The U.S. Drought Monitor uses the climatology definition of drought. For the first time since 2011, California was declared drought-free, breaking a 376-week streak. 


News headlines celebrated: “California is drought-free for the first time in nearly a decade,” “California Is Drought-Free for First Time in over 7 Years; Snowpack, Reservoirs in Great Shape for Summer,” and “SHADOW OF A DROUGHT.” The U.S. Drought Monitor considers “how recent precipitation totals across the country compare to their long-term averages. They check variables including temperatures, soil moisture, water levels in streams and lakes, snow cover, and meltwater runoff.” Notice that only precipitation and surface water are being taken into consideration—groundwater is not included in the calculations. 


Groundwater is water found underground in the cracks and spaces in soil, rock, and sand. It flows through aquifers, an underground layer of rock that stores water. Surface water found in lakes and streams has been generously replenished by the recent increase in precipitation. Groundwater, however, has not been. While surface water can slowly percolate into the subsurface, the speed at which it does so is much slower than the rate at which California is extracting groundwater from aquifers for agriculture, industry, and drinking water. Following the hydrological definition, no, we are not out of the drought just yet.


The Future of California water


Over the past century, California has experienced an increase of 1.1 to 2 degrees Fahrenheit in mean temperature. Spring and winter temperatures are increasing at a faster rate than fall and summer. A warmer winter may mean less snowfall, but warmer springs definitely signal drier summers.

Western Regional Climate Center via California Climate Science and Data FOR WATER RESOURCES MANAGEMENT.


Average precipitation is projected to be below normal over the next century. In recent decades there has been a trend toward more rain than snow in the total precipitation volume. Under warmer conditions, there will be more intense dry periods. Due to more frequent warm, wet atmospheric river events and more rain than snow, wet conditions will intensify. These wet extremes impact the hydrologic system’s ability to handle flooding. Southern California will see drier conditions and Northern California with heavier and warmer winter precipitation. Overall, both the wet and warm ends of the spectrum will heighten, creating climate extremes. Data and trends from California Department of Water Resources.


Snowpack represents one-third of California’s water supply, making it an important factor in the future of the state’s drought conditions. Heavy snowfall in the Sierra Nevadas over a period of about four months provides water storage for millions of people in California and the West. Warmer springs and winters are causing a decrease in snowpack and therefore less snowmelt during the spring to feed rivers, lakes, and streams and replenish water reservoirs. The model below projects a 48-65 percent loss of snowpack from the 1961-1990 average.

Western Regional Climate Center via California Climate Science and Data FOR WATER RESOURCES MANAGEMENT.


How does recent snowfall this winter impact these projections? Similar to one intense rainy season not dramatically influencing an overall climatic issue, one snowy season alone cannot predict the future of California’s snowpack. It was reported that the Sierra Nevadas had a snow water equivalent of 146 percent of normal as of February 19th. Climate scientists are reluctant to pinpoint one event or a couple of weeks of huge snows on climate change, but they acknowledge that these erratic swings are getting more dramatic and intense.

NWS via


A deeper dive into California’s precipitation patterns


Topography, the physical features of a landscape, drives precipitation patterns. The Eastern United States is wetter than the western due to North American topography. The 440 million-year-old Appalachian Mountains drape over the east coast, standing at an average elevation of 3,000 feet. On the opposite side of the nation, the North American Cordillera rise even higher at an average elevation of 10,000 feet. The Appalachian Mountains are not as wide as the North American Cordillera and have significantly eroded from natural weathering over time. When moisture-laden winds blow east to west, they are not blocked by high mountain ranges and can therefore dump rain along most of the south, southeast, east, and northeast U.S.


The Pacific Coast Ranges and the Sierra Nevada mountains play a crucial role in California precipitation patterns and explaining asymmetrical water distribution across the state. These mountain ranges generate huge rain shadows. A rain shadow is a dry region on the leeward side of a mountain. As wind blows onto a mountain, moist air parcels rise along the windward side of the mountain, raining out moisture until it condenses into a cloud. Once the moist air parcels have reached the top of the mountain, they are dry and create a “shadow” of dryness.

Public Domain Image via


The eastern slopes of these mountain ranges have far less precipitation than the western slopes, generating a desert environment in the Central Valley and Death Valley. 

Department of Forestry and Fire Protection


The Jet Stream (or Westerlies), a strong surface air current blowing west to east, dictates precipitation patterns. Air traveling over the Pacific Ocean fills with water vapor and organizes itself into a stream of air. The Jet Stream meanders over the U.S. and brings precipitation to different areas of California depending on the season. In the summer, the Jet Stream flows over Canada and northern U.S. During winter months, the Jet Stream travels south and rains on the southern U.S. The displacement of the Jet Stream in the summer means California receives rain in the winter. 



California sees asymmetrical water distribution. Spatially, rain is falling in concentrated areas in northern California rather than being evenly dispersed over the state. 79 percent of California’s rain falls north of the San Francisco Bay Area. Thus, the other 70 percent of the state receives only 21 percent of rainfall. These southern two-thirds of the state contain the majority of the population, but the least amount of rainfall. Southern California, specifically, is experiencing a population and precipitation crisis, especially during the summer months when rain does not come until winter.

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The terms “weather” and “climate” need to be separated and used intentionally when thinking about atmospheric conditions. “Weather” is used to describe atmospheric conditions over a short period of time, while “climate” describes how the atmosphere behaves over long periods of time. With time scales being paramount, it is unreasonable to believe one season can predict the future or rapidly solve climatic issues. Just as one wet winter can not sufficiently replenish sub-surface water storage, one wet winter does not represent the region’s climate patterns as a whole. However, this season falls within a trend of changing atmospheric conditions, which is linked to climate change’s effect of intensifying climatic events. Overuse of natural resources, such as water, places a strain on the Earth’s natural systems. Like each season is an important building block for the climate, each person is an integral part of human society. Let us be aware of our water usage and environmental impact.

[hr gap=””] Header image credit: National Drought Mitigation Center at the University of Nebraska-Lincoln, with text added by Callie Roach