CA Again Saved by the Rains

In January this year the rain and snow have returned to California. Powerful storms brought snow in the Sierras, rain and flooding; but the atmospheric river saved California from running out of water. Only about 15% of California remains in drought and the reservoirs are fuller, but impact to groundwater from the drought and rains is still being tallied by the NASA GRACE follow-on project and will not be available for a month or two.

The dual-satellite GRACE Follow-On mission, a partnership between NASA and the German Research Centre for Geosciences (GFZ), is a successor to the GRACE satellites that ceased operations in 2017 after fifteen years of service. GRACE maps Earth's gravity field by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system. This allows scientists all over the world an efficient and accurate way to map Earth's gravity field.

The GRACE mission is able to monitor monthly water storage changes on the planet. Regardless of whether water is solid, liquid or vapor, visible or invisible, it has mass, which exerts a gravitational pull. By tracking the changing pull of gravity very precisely around Earth, the U.S./German Gravity Recovery and Climate Experiment, or GRACE, mission observed the movement of water around our planet from 2002 to 2017 -- from the top of the Himalayas to the depths of the ocean to deep underground. GRACE Follow-On continues that work. In the meantime we know that groundwater in the central valley reached a historic low in November.

California experiences the most extreme variability in yearly precipitation in the nation. The potential for wide swings in precipitation from one year to the next requires that California must be prepared for either floods or drought in any year and has extensive water infrastructure-aqueducts, bridges, dams and more.

California uses about 37 million acre feet of water a year, 26 million acre feet for agriculture and 9 acre feet for all other users. An acre foot is about 326,000 gallons. In non-drought years 30-40% of the water is supplied by groundwater. However, in a drought California draws more than 60% of its water from groundwater. This is more groundwater than can be naturally recharged.

The groundwater of the southern Central Valley of California has both an upper unconfined and deeper confined aquifer system. An unconfined, or water -table aquifer is an aquifer whose upper surface is the water table under atmospheric pressure. The water table rises and falls with moisture content that is contained in the soil, and water can be extracted or recharged easily with only seasonal compaction and rebound of the land in wet years.

However, water-table aquifers are usually shallower than confined aquifers and because they are shallow, they are impacted by drought conditions much sooner than confined aquifers. Thus, most water wells draw from the deeper confined aquifers. The water is drawn from the fine-grained confining layers called aquitards. Water enters these aquitards very slowly and the danger is that the compaction of the layers will become permanent. If the water levels are drawn too low, then an irreversible compaction of the fined-grained confining layer occurs and there is permanent subsidence, permanently reducing the storage capacity of underground aquifers, threatening future water supplies; and also lowering the level of the land surface.

Subsidence caused by groundwater pumping in the Central Valley has been a problem in California for decades. Subsidence is also a serious problem for California's water managers, and their infrastructure. The subsidence puts the state and federal aqueducts, levees, bridges and roads at risk of damage. In the past few years subsidence has damaged thousands of public and private groundwater wells throughout the San Joaquin Valley. In recent years California has begun to try and manage their groundwater realizing it is a limited resource. It just may be a little late in the game.