Dataset:
Ojha, Chandrakanta; Shirzaei, Manoochehr; Werth, Susanna; Argus, Donald; Farr, Tom (2022): Land deformation estimates over Central Valley California associated with Water Resources Research Publication: Sustained Groundwater Loss in California’s Central Valley Exacerbated by Intense Drought Periods by Ojha et al. 2018. University Libraries, Virginia Tech. Dataset. https://doi.org/10.7294/19968413.v2 

Please cite the related publication:
Ojha, C., M. Shirzaei, S. Werth, D. F. Argus, & T. G. Farr (2018), Sustained Groundwater Loss in California’s Central Valley Exacerbated by Intense Drought Periods. Water Resources Research, 54(7), 4449-4460. https://doi.org/10.1029/2017WR022250

Dataset:
Ojha, Chandrakanta; Shirzaei, Manoochehr; Werth, Susanna; Argus, Donald; Farr, Tom (2022): Groundwater volume loss estimates over Central Valley California associated with Water Resources Research Publication: Sustained Groundwater Loss in California’s Central Valley Exacerbated by Intense Drought Periods by Ojha et al. 2018. University Libraries, Virginia Tech. Dataset. https://doi.org/10.7294/19968908.v1 

Please cite the related publication:
Ojha, C., M. Shirzaei, S. Werth, D. F. Argus, & T. G. Farr (2018), Sustained Groundwater Loss in California’s Central Valley Exacerbated by Intense Drought Periods. Water Resources Research, 54(7), 4449-4460. https://doi.org/10.1029/2017WR022250

Dataset:
Ojha, Chandrakanta; Werth, Susanna; Shirzaei, Manoochehr (2022): Groundwater volume change time series over San Joaquin Valley California associated with Journal of Geophysical Research: Solid Earth Publication: Groundwater Loss and Aquifer System Compaction in San Joaquin Valley During 2012–2015 Drought by Ojha et al. 2019. University Libraries, Virginia Tech. Dataset. https://doi.org/10.7294/19969019.v1  

Please cite the related publication:
Ojha, C., S. Werth, & M. Shirzaei (2019), Groundwater Loss and Aquifer System Compaction in San Joaquin Valley During 2012–2015 Drought. Journal of Geophysical Research - Solid Earth, 124, 3127–3143. https://doi.org/10.1029/2018JB016083

Dataset:
Carlson, Grace; Werth, Susanna; Shirzaei, Manoochehr (2021): Inversion Results associated with JGR Solid Earth Publication: Joint Inversion of GNSS and GRACE for Terrestrial Water Storage Change in California by Carlson et al.. University Libraries, Virginia Tech. Dataset. https://doi.org/10.7294/17192963.v3 

Please cite the related publication:
Carlson, G.; Werth, S. & Shirzaei, M. (2022). Joint Inversion of GNSS and GRACE for Terrestrial Water Storage Change in California. Journal of Geophysical Research - Solid Earth, 127(3), e2021JB023135. https://doi.org/10.1029/2021jb023135 

Dateset:
This dateset is currently under review and will be posted as soon as it is published.

Please cite the related publication:
This publication is currently under review and will be posted as soon as it is published.

Creators: S. Werth, G. Carlson

Video Link: The video is currently in editing and will be posted as soon as it is complete.

Video Script 

Part 1: Groundwater Storage and Flow

Scene I.1: Why is Groundwater important?

Groundwater, the liquid water that resides in sediment or rock under the surface of the Earth, is an important component of the global water cycle. Water is constantly circulating between the oceans, the atmosphere and the continents.. It is essential for many ecosystems, like habitats in wetlands or rivers. It is vital to people all around the world, as it provides a significant portion of freshwater used every day for drinking water, agricultural irrigation, industrial processing, and for energy production. The fact that groundwater resides below the surface, makes it also a hidden and less understood resource. But, what do we know about groundwater?

Scene I.2: Storage: Where does groundwater reside? 

Underground, water resides in pore spaces, the open spaces within any sediment or rock that can be filled with either air or water. But pore spaces of deep aquifer layers are only filled with water. The total volume of open spaces compared to the total volume is called porosity. It can range from ~50% for gravel to less than 1% for solid granite.

Another important medium characteristic is permeability. It describes the interconnectedness of the pores and the ease of water flow. For example, water flows several meters per day in gravel, a high-permeability material. While water flows only a few centimeters per year in clay, a low-permeability material. Other materials like fine-grained shale may have abundant pore space, but because the pores are tiny and poorly connected, it is difficult for water to flow. Regions containing such materials are considered aquitards, as they slow down water flow. Underground regions of high porosity and high permeability flowtransmit water easily and are called aquifers. Water flows faster through aquifers than aquitards, but overall much slower underground than on the surface.

Aquifers and aquitards are commonly interbedded. Shallow, unconfined aquifers are well connected to the surface. We distinguish unconfined aquifers from deeper confined aquifers that lie beneath an aquitard, where it takes much more time for water to flow in or out.

Scene I.3: Flow: What governs water flow under the surface?

When water reaches the Earth’s surface via precipitation, it infiltrates into the ground and percolates down through the soil until it reaches the water table. The water table is not flat: it resembles the shape of surface topography; it is high where the land is high - and low where the land is low. And a sloping water table indicates the groundwater is flowing.

Due to gravity, water flows from higher elevations to lower elevations. Groundwater usually recharges at higher elevations and it may discharge from underground at lower elevations.

The water flow also responds to pressure changes. For example, at two points of the same elevation, pressure is higher where more water weighs above them. Water flows from areas of higher pressure to those of lower pressure. Beneath aquitards, or confining layers, aquifers are under increased pressure because water cannot escape from the aquifer to low pressure regions above the aquitard.

In summary, groundwater flows more slowly underground than at the surface. It flows from high elevation to low elevation, eventually resurfacing to rejoin the water cycle after months to thousands of years. The transit time depends on the flow path, degree of confinement by aquitard layers, and the surrounding geology. In general, deeper flow paths take longer.

Part II: Groundwater use and its consequences.

Scene III.1: How can we access groundwater?

Groundwater is tapped either from springs, where groundwater naturally discharges from the ground, or it can be accessed at wells, excavated or drilled holes from where groundwater is pumped to the surface. Pumping groundwater affects the water table and if groundwater is extracted faster than it can be replenished, groundwater levels fall. Severe decline of the water table may dry up wetlands or streams, alter surface water flows, and lead to saltwater contamination near the coast.

Water managers, engineers, and scientists keep track of groundwater by measuring changes of groundwater levels in boreholes designated for such observations. If groundwater levels fall, water amount or pressure is reduced in an aquifer. And if levels rise, water amount or pressure has increased in the aquifer. 

It takes less effort to pump groundwater from shallow aquifers, which also replenish faster due to their proximity to water inflow from the surface. However, shallow aquifers are in closer contact with human activities, and therefore, easily contaminated. Deeper, and confined aquifers are less susceptible to human pollution, and drafted for their fresh water. However, if water pressure drops significantly in these deep layers, the land surface can begin to sink, or subside..

Scene II.2: Why does the land subside when groundwater levels change?

Deeper, confined aquifers often have comparably high water pressure, because water flows in from recharge areas but escapes only very slowly into surrounding units of low permeability. In confined units, groundwater in pore spaces acts to hold grains apart and support the granular matrix of the rock and the weight of the overlying water, rock, and soil. The pore space elastically expands if water is added and pressure increases. On the contrary, when groundwater is removed, sediment grains compress and the pores shrink. The expansion of pore spaces accumulates to inflation of the aquifer layer and is observed as uplift at the surface. The deflation of pore spaces leads to compression of the aquifer layer, which is observed as sinking, or subsidence at the surface. Severe loss of groundwater and associated subsidence can cause damage to human infrastructure like roads, water channels, or buildings. It can also lead to other hazards like earth fissures, or cracks in the shallow subsurface that can open at the surface in sudden, catastrophic failure. These can cause additional damage to roads, pipes and utility lines, and buildings, and can lead to contamination of shallow groundwater.

Part III: Monitoring Groundwater from Space

Scene III.1: How can we monitor groundwater from space? 

[InSAR material from Manoo] We can monitor groundwater volume change using sensors that are launched to space on board of satellites. They continuously orbit our planet Earth and collect more data over larger areas than sensors at the ground. One type of satellite sensor that is particularly useful for measuring changes in confined aquifers is called a Synthetic Aperture Radar or SAR satellite. It actively sends invisible light signals, namly radar, to the ground and measures the amount of time it takes for the signal to return. With a processing technology called interferometry or InSAR scientists can precisely calculate tiny changes in ground elevation between repeated observations. We can use these observations to quantify where and by how much land subsides or uplifts above groundwater aquifers. Hydrogeologists then combine this data with physical models to quantify water pressure and volume changes in the aquifer.

Another type of satellite that is useful to monitor groundwater volume change over large areas is the Gravity Recovery and Climate Experiment, in short GRACE. This pair of satellites follow the same Earth orbit 200 km apart from one another. The orbit of each satellite is controlled by the gravitational attraction of the satellites to the mass of the Earth. When one satellite approaches an area of higher mass concentration, the larger gravitational attraction causes the satellite to accelerate. When approaching a region of lower mass concentration, the satellite decelerates. By very precisely tracking the distance and speed between the two satellites, scientists can map changes in Earth’s gravity field. Over time, small changes in Earth’s gravity field are caused by changes in the amount of water stored on and below the ground. Therefore, repeated gravity observations in combination with other information about the water cycle allows hydrologists to track changes in groundwater storage over large areas, without drilling a single well.

Scene III.2: Example from California

During the last two decades, California experienced several droughts, meaning there was very little rainfall. In order to keep crops alive, farmers had to rely on groundwater resources. In response to increases in groundwater pumping, pressure and groundwater levels declined in California’s Central Valley aquifer, and subsidence, or a widespread, gradual drop in the elevation of the land surface occurred. Scientists were able to measure how much subsidence occurred during this drought using information from SAR satellites. During a drought that lasted from 2012-2015, the land subsided up to 1 m over 4 years in some areas of the Southern Central Valley. Using the GRACE satellites, scientists were also able to quantify that a total amount of 60 cubic kilometers of water was lost from the ground. This is equivalent to 60 Gigatons of water or more than 24 million Olympic-sized swimming pools.

The amount of groundwater loss estimated using the GRACE satellites was verified by independent estimates from scientists using SAR satellites. The scientists used geophysical models in combination with land subsidence measurements to estimate the amount of groundwater lost during the drought period. These studies provided new insights into the groundwater decline in California. Other studies using the same satellites also helped to track groundwater changes in further large aquifers around the world, for example the middle east, South America, India and in East Africa. 

A future and important goal of scientists is to make these satellite observations useful for a sustainable and successful management of groundwater resources.

Scene III.3: What are the most important facts to remember?

• Groundwater is an invisible resource vital for sustaining human and environments 

• It flows slowly through the pore spaces of rocks and sediments

• If pumping of groundwater from confined aquifer layers outpaces recharge, this can lead to land subsidence at the surface and damage infrastructure.

• Scientists can monitor groundwater resources from space using land elevation change or changes in the gravity field of the Earth

• Space based observations can help to improve our understanding of the availability of groundwater resources and our impact on them