This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the Directorate for Education and Human Resources (DUE-1245025).

GEODESY AND WATER RESOURCES

Unit 2 Background

OUTLINE

1. Terrestrial Water Storage (TWS) Defined2. Relationship between TWS and groundwater storage3. Depth to water table and groundwater storage4. GRACE Mission: 5. GPS vertical position

ANNUAL HYDROLOGICAL CYCLE

Cockell, 2008 Fig. 1-38

ANNUAL HYDROLOGICAL CYCLE: MAIN RESERVOIRS

Oceans

Atmosphere

Land38, 000

Oceans

Atmosphere

Land38, 000

ANNUAL FLUX FOR THE OCEAN RESERVOIR

IN = 300+36=336

OUT

ANNUAL FLUX FOR THE ATMOSPHERE

336+64=400

RESIDENCE TIME FOR WATER IN THE ATMOSPHERE

Amount of water in the atmosphere (kg)Flux of water through atmosphere (kg/yr)tr =

tr =

tr =

336+64=400

RESIDENCE TIME FOR WATER IN THE ATMOSPHERE

Amount of water in the atmosphere (kg)Flux of water through atmosphere (kg/yr)tr =

tr =13 x 1015 (kg)400 x 1015(kg/yr)

tr = 0.033 years = ~12 days

ANNUAL HYDROLOGICAL CYCLE: MAIN RESERVOIRS

Oceans

Atmosphere

Land38, 000

TERRESTRIAL WATER STORAGE (TWS)

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:

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

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

1350.62 m

Depth to water7 m

Well depth18 m

Measure groundwater levels

Water level = (surface elevation) – (depth to water)

Depth to water table measured at a well

In > out replenished, water table/level goes upIn < out depleted, water table/level decline

porosity aread)replenishe(or volumedepletion increase)(or decline levelwater

RELATING CHANGES IN WATER TABLE TO CHANGES IN STORAGE

Area

height area volumespace porePorosity

Groundwater volume change

Area

Groundwater level change in aquifer

= height

height area r volumegroundwatePorosity

height

porosity area r volumegroundwateheight

Why divided by porosity?

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;……”

From: http://www.nasa.gov/mission_pages/Grace/overview/index.html#.VWcUAmRViko

GRACE MISSION: GRAVITY RECOVERY AND CLIMATE EXPERIMENT

http://photojournal.jpl.nasa.gov/jpegMod/PIA04235_modest.jpg

GRACE DATA CHARACTERISTICS:

2-6 month latencymonthly data (some months missing)coarse spatial resolution (~100’s km)

Castle et al., 2014, GRL

Colorado River Basin

GPS VERTICAL POSITIONSolid earth responds elastically to changes in load, such as water loss during drought

instantaneous, reversible, linear

Drought: TWS decreases Wet period: TWS increases

GPS NETWORKS

Thousands of stations across continental United StatesData latency of ~1 day

HOW MUCH VERTICAL MOTION FROM TWS VARIATIONS?50 cm change in TWS ~20 mm up/downNoise in daily GPS position estimate ~2-3 mm

NLD

AS T

WS

(cm

)

P025: Idaho

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).

SPATIAL SCALE OF SENSINGDepends on horizontal scale of hydrologic load

0 250 500 500 250 Distance (km)

Disp

lace

men

t (m

m)

40 km wide2 m water equivalent

200 km wide0.5 m water equivalent

2012 Drought: centered on High Plains

Long-term TWS and GPS position variationsVariations in GPS position consistent across region

GPS

positi

on (m

m)

Retrieve spatial pattern and magnitude of TWS variations

March 2014

Borsa et al., 2014

Borsa et al., 2014