Groundwater Recharge

Reager, J. T., Molotch, N. P., & Famiglietti, J. S. (2026). Net groundwater recharge in California occurs primarily during above-average snowpack years. Geophysical Research Letters, 53, e2026GL122396. https://doi.org/10.1029/2026GL122396

The below is excerpted from the research letter cited above.

Dr. John T. Reager, a water scientist at NASA Jet Propulsion Laboratory, and colleagues analyzed the GRACE and GRACE FO* satellite data and estimate how much groundwater recharge has occurred annually in California since 2004.  They found that the state’s aquifers have had a net gain in only six of 18 years, and years with lots of snow led to the biggest rises in underground water.

In the Central Valley, groundwater has been dropping for about 100 years as wells and pumps pull up water to irrigate crops and make up the difference when water allocations are cut. The historical maximum of 28 to 30 feet was measured southwest of Mendota in a famous USGS photo. Since that time other areas of the central valley have experienced over draft of groundwater and subsidence. The all resulted in the Sustainable Groundwater Management Act (SGMA), which has yet to prevent the overdrawing of groundwater.

“Nonetheless, California's Central Valley is one of the most heavily managed groundwater systems in the world, supplying much of the state's water use during dry periods and supporting a highly productive agricultural region. Long‐term groundwater withdrawals have led to sustained declines in aquifer storage over the past several decades (e.g., Liuet al., 2022), particularly during multiyear droughts when surface water availability is limited.”

Despite the importance of groundwater in buffering the region from drought, basin‐scale monitoring of groundwater recharge the satellite data from GRACE and GRACE‐FO observations reveal basin‐scale net groundwater recharge events in California are very rare. The net positive recharge years (when more water is added to groundwater stores than is pumped out) are strongly associated with above‐average snowpack years. Thus, future warming may reduce the likelihood of recharge‐capable years.

The data used in this study came from GRACE (Gravity Recovery and Climate Experiment) and its successor, GRACE-FO (GRACE Follow-On), are joint satellite missions between NASA and German research partners (DLR and GFZ) designed to track the movement of mass across the Earth by measuring changes in its gravitational field over time. The original GRACE mission operated from 2002 to 2017. GRACE-FO was launched in May 2018 (yeah SpaceX) after the first satellite had failed.

Unlike typical remote sensing satellites that observe the planet using reflected light or to look down at the surface. Instead, GRACE uses two identical satellites flying in tandem to act as a highly sensitive scale. Because water is incredibly heavy, its seasonal and multi-year movement constitutes the largest variable source of changing mass on Earth. GRACE and GRACE-FO provide critical monthly global maps of these regional variations, unlocking insight across years.

This years of data creates a time series of water data allows the researchers to derive the groundwater over time. By subtracting the measured surface water. The first derivative of a time data series represents the rate of change of the measured variable with respect to time. In physical terms, it allows the scientist to observe the recharge rate of groundwater.

Groundwater storage changes were estimated as the residual of the terrestrial water balance:

GWA(t) = TWSA(t) – SWEA(t) – SWA(t) – SMA(t) + E(t),

where GWA represents groundwater anomaly, derived from terrestrial water storage changes (TWSA) and sub-tracting out snow water equivalent changes (SWEA), surface water storage changes (SWA), and soil moisture storage changes (SMA). (We’ll leave off the discussion of the fudge factor E in this short discussion). The first derivative of a time data series represents the rate of change of the measured variable with respect to time. Thus, in physical terms, it measures the recharge of the water in the system.

By separating groundwater changes from other water stores such as snow, soil moisture, and reservoirs, they estimate when net groundwater recharge occurs and how large itis. The researcher found that basin‐scale net groundwater recharge is rare and has occurred in only 6 years since 2004. Importantly, years with groundwater recharge are more strongly linked to unusually large snowpack in the Sierra Nevada than to high rainfall alone. Even during the exceptionally wet winter of 2023, which produced record snowpack, only a small fraction of the total precipitation was retained as groundwater, and this gain did not offset long‐term groundwater losses. These results suggest that future groundwater replenishment in California depends on substantial snowpack, which is expected to decline as the climate warms. As more precipitation falls as rain rather than snow, opportunities for large‐scale groundwater recharge may become increasingly limited.

Groundwater recharge is a complex Earth system process that has been identified by the National Academy of Sciences as critically important  to our source waters and poorly monitored. Distinguishing between mechanisms of groundwater recharge is essential for assessing how groundwater recharge potential may evolve under a warming climate and that is expected to shift precipitation from snow toward rain, and land use change that may alter the mechanics of recharge and snow melt.

Though, land subsidence does not directly cause the disparity between rainfall and snowfall recharge, which is primarily governed by the physical delivery timeline of the water. However, decades of compaction significantly compound the issue by reducing the aquifer system's vertical permeability and capacity to absorb rapid surface water inputs. When decades of groundwater over-extraction cause land subsidence, the fine-grained clay layers (aquitards) within the aquifer system undergo irreversible compaction. This structural collapse dramatically alters how the subsurface interacts with different water delivery rate.

Net groundwater recharge reflects a balance between climatic inputs and anthropogenic withdrawals. During dry years with limited surface water availability, continued groundwater pumping results in net depletion, whereas during wet years, reduced pumping and enhanced infiltration allow for net recharge (e.g., Famiglietti et al., 2011). Recharge pathways in California include diffuse infiltration, managed recharge, and recharge associated with prolonged snowmelt and runoff from the Sierra Nevada. While basin‐scale GRACE observations cannot resolve individual pathways, the strong association between net recharge and high snowpack is consistent with enhanced focused and mountain‐front recharge during years with sustained snowmelt.

Several high‐precipitation years did not correspond to the largest recharge events, whereas water year 2023 coincided with record‐high Sierra Nevada snowpack, indicating that precipitation alone may be insufficient to produce basin‐scale net recharge. The magnitude of recharge in wet years further emphasizes the limits of basin‐scale recovery. Across the six net‐positive recharge years since 2004, annual net groundwater gains amount to approximately 20%–45% of April 1 Sierra Nevada snowpack.  The researchers found that only a portion of the snow pack was converted into net groundwater gain.

These results suggest that groundwater recharge is not only through total water input, but also through its control on the timing and persistence of water availability. Snowmelt provides a delayed and sustained input of water that extends into spring and early summer, when evaporative demand remains relatively low and infiltration conditions are more favorable. In this sense, snowpack acts as a gating condition for basin‐scale recharge, enabling sustained infiltration beyond the peak precipitation period rather than serving as an independent water source.

This is an important piece of work, the methodology can be used in other climates and geologies to determine groundwater recharge and its drivers.