California's San Joaquin Valley (SJV) has experienced periods of land subsidence, triggered by the extraction of groundwater and subsequent compaction of subsurface clays, since the 1920s (Poland et al., 1975). The magnitude and extent of this subsidence have been vast: up to 9 m has been observed at some locations, while the footprint of subsiding regions exceeded 13,000 km 2 by the 1980s and has continued to grow since (Faunt et al., 2016;Ireland et al., 1984). The subsidence has been immensely damaging to long-range infrastructure crossing the Valley, in particular gravity-driven canals and aqueducts which have required remedial repairs costing 100s of millions of dollars (Borchers & Carpenter, 2014). This, in combination with a new wave of subsidence starting in the statewide 2012-2015 drought, led to subsidence being included in a groundbreaking piece of legislation, the Sustainable Groundwater Management Act (SGMA), which legally requires groundwater managers in California to make plans to reduce subsidence along with other negative impacts of groundwater extraction. Against this backdrop, it is a critical time for the study of subsidence in the Valley.Subsidence in the Valley is a complex process. The large-scale hydrostratigraphy is a three-layered alluvial aquifer system: an upper aquifer, a regional confining unit known as the Corcoran Clay, and a lower aquifer. Within the upper and lower aquifers is a mix of sediments, which are commonly described as a coarser-grained background containing many clay interbeds. Sediments throughout the aquifer system will compact, leading to land subsidence, if there is a drop in hydraulic head within those sediments. Drops in hydraulic head are triggered when there is a net loss of water from the aquifer system, which primarily occurs when the largest sink-groundwater pumping-exceeds recharge, a circumstance known as overdraft. Overdraft conditions have occurred repeatedly in the SJV since the early 20th century. Importantly, the head drops do not occur simultaneously in all of the sediments. Since they are hydraulically conductive, pumping and recharge both occur in the coarser-grained sediments, and head accordingly drops first in these materials. However, the head in the Corcoran Clay and clay interbeds only responds gradually, because clay has a low hydraulic conductivity. This gradual decline in head in clay layers means they experience so-called residual compaction, a process which can continue long after the overdraft which initiated it. The compaction of clays is the primary origin of subsidence, since clay is far more compressible than the coarser-grained materials. Together, these complexities mean that the subsidence is heterogeneous in space and time, thus challenging to study.

Convection in fluid layers at high Rayleigh number (Ra ∼ 10 6 ) have a spoke pattern planform. Instabilities in the bottom thermal boundary layer develop into hot rising sheets of fluid, with a component of radial flow toward a central upwelling plume. The sheets form the "spokes" of the pattern, and the plumes the "hubs." Such a pattern of flow is expected to occur beneath plate interiors on Earth, but it remains a challenge to use observations to place constraints on the convective planform of the mantle.Here we present predictions of key surface observables (gravity, topography, and rates of melt generation) from simple 3-D numerical models of convection in a fluid layer. These models demonstrate that gravity and topography have only limited sensitivity to the spokes and mostly reflect the hubs (the rising and sinking plumes). By contrast, patterns of melt generation are more sensitive to short-wavelength features in the flow. There is the potential to have melt generation along the spokes but at a rate which is relatively small compared with that at the hubs. Such melting of spokes can only occur when the lithosphere is sufficiently thin (≲ 80 km) and mantle water contents are sufficiently high (≳ 100 ppm). The distribution of volcanism across the Middle East, Arabia, and Africa north of the equator suggests that it results from such spoke pattern convection.

[copied directly from first paragraph of paper] Land subsidence, caused by groundwater extraction and subsequent subsurface compaction, is an issue of global concern. Since the 1920s, there have been numerous periods of subsidence in California’s San Joaquin Valley leading to widespread sinking of the land surface which has locally exceeded 9 m. The most recent period of severe subsidence, which was triggered by the 2012-15 drought, is now causing damage which threatens the long-term viability of critical water distribution infrastructure in the Valley. However, there is neither a continuous monitoring record of the subsidence nor high-quality records of the hydrologic head changes in the subsurface which have caused the subsidence, making it impossible to understand, and thus mitigate, the subsidence. Here, we leverage subsidence and hydraulic head data from a variety of sources to create and validate a one-dimensional model of subsurface compaction and subsidence over the 65 years between 1952-2017. This model, which simulated up to 7.5 m of subsidence since 1952, provides a complete record of subsidence in our study region by filling crucial gaps in the observed record. Our model reveals the long-term processes causing subsidence, which operated over decades-to-centuries and caused exceptionally high rates of baseline subsidence in 2017, resulting in a critical risk of future subsidence. This risk is exacerbated as the Valley moves into drought conditions again in Spring 2021. We demonstrated an approach which provided the understanding of subsidence in the Valley needed to directly inform sustainable groundwater management, and which is applicable in subsiding regions around the World.