This document discusses methods for characterizing groundwater storage, including traditional well measurements and satellite-based GRACE observations. It defines terrestrial water storage as all water on the land surface, and explains that groundwater often dominates variations in storage. Wells measure groundwater levels, with changes indicating replenishment or depletion over time. GRACE satellites detect changes in mass distribution and associated gravity field variations to infer changes in total water storage, including groundwater, at coarse spatial scales. The document provides examples of using both approaches to monitor groundwater in key aquifers.

1. This work is supported by the National Science Foundation’s
Directorate for Education and Human Resources TUES-1245025, IUSE-
1612248, IUSE-1725347, and IUSE-1914915. Questions, contact education-AT-unavco.org
MEASURING WATER RESOURCES
Unit 2: Characterizing groundwater storage
with well and GRACE data

2. OUTLINE
1. Terrestrial Water Storage (TWS) defined
2. Relationship between TWS and groundwater storage
3. Traditional measurement: Groundwater wells
– Depth to water table and groundwater storage
4. Geodetic measurement: GRACE (Gravity Recovery and
Climate Experiment) satellite
– Uses gravity to measure changes in mass

3. USGS GROUNDWATER WELLS
https://water.usgs.gov/ogw/

4. TERRESTRIAL WATER STORAGE (TWS)
Two options for units: P, ET, and R as km3/yr or mm/yr;
TWS as km3 or mm.
P = Precipitation ET = Evapotranspiration R = Loss to Rivers
1. Terrestrial water storage: All water stored on the land surface
 surface water, soil moisture, groundwater, snow, ice,
vegetation water content, and permafrost.
2. TWS: storage term in the basin scale water balance:

5. TERRESTRIAL WATER STORAGE (TWS)
AND GROUNDWATER
• In many locations, changes in groundwater
storage dominate variations in TWS.
• In some locations, seasonal variations in snow
or soil moisture are also important

6. 1350.62 m
Depth to water
7 m
Well depth
18 m
Measure groundwater levels
Water level =
(surface elevation) – (depth to water)
Depth to water table measured at a well

7. In > out replenished, water table/level goes up
In < out depleted, water table/level decline








porosityarea
d)replenishe(orvolumedepletion
increase)(ordeclinelevelwater
RELATING CHANGES IN WATER TABLE TO CHANGES
IN STORAGE
Note: Another term you will hear is “specific yield” which is the “drainable
porosity” in an unconfined aquifer – meaning the actual amount of water
that will leave an aquifer, somewhat less than the total water than may be
present.

8. Area
heightarea
volumespacepore
Porosity


Groundwater volume
change
Area
Groundwater level
change in aquifer
= height
heightarea
r volumegroundwate
Porosity


height
porosityarea
r volumegroundwate
height


Why divided by porosity?

9. POROSITY & SPECIFIC YIELD
Example: Sand with clay has lower porosity
(and hence lower specific yield) than sand
alone
Some end-member values for specific yield:
• Sandstone 20% (0.2)
• Clay 2% (0.02)

10. UNDERSTANDING SEASONAL CYCLES IN
GROUNDWATER DEPTHS
0
1
2
3
4
5
6
7
8
9
10
11
12
1/1/90 4/2/90 7/2/90 10/1/90 12/31/90 4/1/91 7/1/91 9/30/91 12/30/91
Depthtogroundwater(meters)
Groundwater depths well in SE Wyoming

11. GRACE MISSIONS:
GRAVITY RECOVERY AND CLIMATE EXPERIMENT
Original GRACE satellites 2002-17
GRACE Follow-on launched 2018

12. GRACE MISSION:
GRAVITY RECOVERY AND CLIMATE EXPERIMENT
• “GRACE consists of two identical spacecraft that fly about
220 kilometers apart in a polar orbit 500 kilometers (310
miles) above Earth. GRACE maps Earth's gravity field by
making accurate measurements of the distance between
the two satellites, using GPS and a microwave ranging
system. It is providing scientists from all over the world
with an efficient and cost-effective way to map Earth's
gravity field with unprecedented accuracy.
• The gravity variations studied by GRACE include: changes
due to surface and deep currents in the ocean; runoff and
ground water storage on land masses; exchanges between
ice sheets or glaciers and the ocean;……”

13. GRACE SATELLITES SENSING GRAVITY CHANGES

14. EXAMPLE OF DISTANCE & TOPOGRAPHY RELATIONSHIP

15. GRACE DATA CHARACTERISTICS:
• 2-6 month latency
• Monthly data (some months missing)
• Coarse spatial resolution (~100’s km)
GRACE original mission ran 2002-2017 (5-year expected duration
was far outlived although by late 2016 GRACE was under
modified operations to prolong battery life)
GRACE Follow-on mission launched May 2018, initially had some
issues with onboard equipment, but data flow was expected in
2019.

16. EXAMPLE OF GRACE GROUNDWATER DATA

17. GRACE APRIL 2015 ANOMALY FROM 2002-2015
AVERAGE

18. UNIT 2 STUDENT EXERCISE: TRADITIONAL AND
GEODETIC METHODS FOR MONITORING
GROUNDWATER CHANGE IN HIGH PLAINS AQUIFER
• Groundwater wells
• GRACE data