In addition to stored supply, water in California is obtained through several avenues. Precipitation supplies approximately 75 million acre feet of usable water to California each year, mostly from December to March. Additionally, snowpack in the Sierra Nevada mountains can account for up to 30% of California’s water supply– as temperatures warm in the spring, snow from the mountain range thaws and trickles down, collecting in reservoirs and basins. Lastly, the Colorado River is a large supply source for California water, especially for southern California. All three of these sources are potentially compromised by a changing climate.

There are two immediate sources of water in California – surface water, which is water found in things like rivers, lakes, and reservoirs, and groundwater like that found in aquifers. Water that comes from snow melt and precipitation can become surface water or groundwater, and groundwater can be naturally distributed to surface water when groundwater levels are high. Drawing water out of groundwater basins also changes the relationship between surface water and groundwater, as low supply in groundwater can alter how surface water flows or is “imported” in the state, and vice versa. 

Surface water, which accounts for 60-70% of the state’s supply, is not geographically dispersed – the majority of it is found in the northern part of the state, whereas groundwater is more evenly distributed throughout California. Hundreds of miles of water storage and delivery systems of reservoirs, aqueducts, and pumping plants stretching almost the entire length of the state convey water from its origin in the north to demand in the south.

The available water in California is used by three avenues – environmental, agricultural, and urban. Though use by sector can vary dramatically within the state and can change each year, approximately half the water goes to environmental uses, 40% goes to the agricultural segment, and 10% is urban use. There is constant interplay between these three, as they have competing interests over a commodity with limited supply. Let’s look at all three.

Agricultural use supports the largest agricultural producing U.S. state in terms of cash receipts. There are approximately 9 million acres of irrigated farmland in California, and that acreage required 24.5 million acre feet of water in 2018. Most agricultural water use occurs from June through September. Though agricultural production has increased with less intensive water inputs over the last few decades, California has seen growth in tree and vine crops which need to be watered each year. The reliance on these types of agricultural products makes California farms more vulnerable to any water shortages. Additionally, as populations increase both locally and globally, there is more demand for crops grown in California. 

Approximately half of urban water use is dedicated to landscape watering and half is used for human consumption. California’s water resources need to support about 40 million inhabitants – a number expected to grow over the coming years. The strongest urban water demand is seen, like with the agricultural sector, in summer months. A recent focus on water shortages and development of water-saving technologies has resulted in a decline in per capita urban water usage. In 1990, per capita water use was 231 gallons per day, which decreased to 146 gallons per day by 2015. However, an expected increase in population clearly means more demand for water.

Rising temperatures also mean less snowpack will be available once spring arrives. Estimates suggest that the Sierra Nevada snowpack can experience as much as a 48-65% loss over the remainder of this century. The change in temperature also means that the timing of the snowpack melt will be impacted, and more water will run off earlier in the year. This alters the balance of storage and use currently employed in California, and months with the highest water demand could be even drier.

In addition to climate change impacting the supply of water in California, warmer, drier days will increase demand. Crops will need more water input over a longer season, and human consumption for landscaping and personal use will rise. Ultimately, climate change makes it more difficult to manage the priorities of the California water system – storing water for droughts without being oversupplied in certain areas, managing the risks of flooding, satisfying total demand, and protecting the surrounding environment.