1. Measuring Water Resources
Unit 3: Monitoring groundwater storage with GPS vertical position
Plate Boundary Observatory station P422 in Moscow Idaho
Image from UNAVCO

2. Outline Groundwater Mining: a Global Problem
GPS vertical position and hydrologic loading
Subsidence: compaction and poroelastic effects
Introduction to California physiography

3. 1. Groundwater mining: a global problem
Konikow, Geophysical Res. Letters, 2011
These figures are from Konikow, 2011
See also Figure 2 from Famiglietti, 2014
NASA EarthObservatory
A variety of water measurement methods show that groundwater aquifers from around the world are being depleted.
The left image from Konikow, 2011, ( was derived from a wide range of methods including water level, modeling, and pumpage.
The right image from NASA (also detailed in Famiglietti, 2014) was based on GRACE data (Gravity Recovery and Climate Experiment).
Left image: Konikow, 2011 – AGU journals allows use of published image for educational purposes
Right image: (J.T. Reager, NASA Jet Propulsion Laboratory)

4. 2. GPS Vertical Position
Solid earth responds elastically to changes in load, such as water loss from regional groundwater pumping or drought
instantaneous, reversible, linear
Extract groundwater
Total Water Storage decreases
The land surface moves up
This is not the same as the isostatic adjustment to surface loads (long time scale, spatial pattern affected by flexural characteristics of lithosphere, total magnitude depending on crust/mantle density contrast).
The reverse also happens: add groundwater, TWS increases, the land surface moves down
During the exercise in this unit, students will be analyzing data from nine GPS stations like this one, which are part of the Plate Boundary Observatory (PBO) run by the UNAVCO for the National Science Foundation.
Image from UNAVCO (
Plate Boundary Observatory station P037

5. GPS Networks Thousands of stations across continental United States
Data latency of ~1 day
GPS stations are widely distributed around the USA and most data are available with a day or so. Some are available with seconds.
Image from Nevada Geodetic Laboratory ( and used with permission. Base map by Google Maps.

6. GPS Data
GPS stations such as those in the Plate Boundary Observatory (PBO; were originally installed to measure things such as plate tectonic motion (horizontal motions shown in the left image). However, as the data came in, it became clear that elements of the vertical motion is related to changes in the terrestrial water storage (TWS) and there were much wider uses for GPS data than originally imagined.
Rear image: UNAVCO Velocity Viewer (base image from Google Maps)
GPS and GPS data from
Cycles in vertical position are related to the seasonal water changes.

7. How much vertical motion from TWS variations?
Response to loading depends on elastic parameter, Young’s modulus (E)
E = tensile stress / extensional strain
Units, are force/area (N/m2) or pressure (Pa).
Drawings by Kate Shervais, UNAVCO

8. How much vertical motion from TWS variations?
50 cm change in TWS  ~20 mm up/down
Noise in daily GPS position estimate ~2-3 mm
NLDAS is the National Land Data Assimilation System which provides estimates of hydrologic storage using meteorological forcing.
NLDAS TWS is the sum of the Noah model soil moisture (0-200 cm soil depth) and snow for the grid cell that contains PBO station P025. More information about NLDAS can be found here:
GPS vertical data records very similar pattern via a completely independent means from the methods used by NLDAS.
Image: modified from Larson, K.M., GPS Interferometric Reflectometry: Applications to Surface Soil Moisture, Snow Depth, and Vegetation Water Content in the Western United States, WIREs Water, Vol. 3, , /wat2.1167, 2016.

9. Spatial Scale of Sensing
Depends on horizontal scale of hydrologic load
For large aquifers, unloading effects of pumping may be sensed 10’s of km away
200 km wide
0.5 m water equivalent
40 km wide
2 m water equivalent
Displacement (mm)
Elastic response to surface loads extends beyond the area with the load. Two examples of he simulated 1-D response to surface loading are shown. The blue line shows the vertical displacement (downward for a positive load) resulting from a 40 kmi wide load that is equivalent to 2 m deep water. Red line is the same for a 200 km load with 0.5 m thickness.
Image by Eric Small
500
250
250
500
Distance (km)

10. 3. Subsidence: groundwater pumping may lead to subsidence
Two effects
Compaction of aquitard sediments; generally irreversible
Poroelastic effects; reversible

13. 3. Subsidence: Decrease in surface elevation may exceed several meters!

14. 3. Subsidence: groundwater pumping may lead to subsidence
Effects are local – not extending beyond the pumped area.
Unlike the elastic response to loading subsidence is localized to the pumping area.

15. 4. California Physiography

16. Central Valley Groundwater: pre and post development cross-section for central part of San Joaquin Valley
The groundwater movement patterns have modified significantly in the course of agricultural development.
Image modified by K. Shevais (UNAVCO) from Faunt, 2009, USGS Professional Paper 1766 (

17. Geologic Map of California
In the Unit 3 exercise, students will be asked to consider the type of soil/rock that lies beneath each of the stations they look at. They will be given this map as resource to answer those questions. You may want to help them see the areas of bedrock vs. sedimentary fill.
Images: (Courtesy of the California Geological Survey). Map Derived from the Simplified Geologic Map of California, Map Sheet 57.

18. Optional slides for during or after the student exercise

19. California Drought Severity and Area
It may help to show the students this graph showing the California drought duration and severity.
Image from US Drought Monitor ( via Wikipedia (

20. Animation Measuring Drought: A GPS Network Offers A New Perspective