Aquifers and Groundwater flow ATM 301 Lecture #16 (sections 9.1-9.2) Groundwater: Aquifers and Groundwater flow

Snowpack and snowmelt Groundwater Outline of the course The Basics The global hydrological cycle Precipitation Soil water (lab work) Surface energy fluxes Evaporation and transpiration Snowpack and snowmelt Groundwater Streamflow Floods & Droughts Water management

Hydrologic Soil Horizons (or layers): Vadose Zone (unsaturated zone) p=w g (z’-z’o) Phreatic Zone (saturated zone) p=w g (z’-z’o)

Groundwater is water stored in saturated soils and rocks. Pressure in groundwater is greater than atmospheric, increasing with depth (hydrostatic). http://waterinthewest.stanford.edu/groundwater/overview/index.html

About 30% of global freshwater is stored as groundwater Residence time of weeks to 1000’s of years!

Why care about groundwater? Stores much of Earth’s freshwater (30%) Used for irrigation and drinking water when other sources are unavailable About ¼ of world’s water resources are drawn from groundwater Subject to contamination and depletion Groundwater storage crosses property lines, town lines, state, and national boundaries Source of most water in many rivers and lakes Affects stream flow timing and magnitude

Ground water as a fraction of total freshwater use in US http://pubs.usgs.gov/circ/2004/circ1268/htdocs/figure13.html

Overview of groundwater Groundwater is water stored in saturated soils and rocks. Pressure in groundwater is greater than atmospheric, increasing with depth (hydrostatic). The water table is the upper boundary of the groundwater zone. Groundwater is increased by recharge and decreased by discharge Water table Unsaturated soil

An aquifer is a geologic unit that can store enough water and transmit it at a rate fast enough to be hydrologically significant. Unconfined aquifers Upper boundary is the water table, and the height of water in a drilled well. They have p=patm at upper boundary Confined aquifers Upper boundary is a confining layer of very low hydraulic conductivity. p>patm at the upper boundary, and water in a drilled well will rise above the confining layer. Fig 9-1

About ¼ of NYS population relies on groundwater http://www.dec.ny.gov/lands/36115.html

Principal Aquifers in the U.S. 10 3 11 1 13 50

Withdrawals from Major Aquifers in the U.S. in 2000

High Plains / Ogallala Aquifer Saturated Thickness (ft) High Plains / Ogallala Aquifer Very large aquifer that is central in sustaining a large fraction of US agriculture “About 27 percent of the irrigated land in the United States overlies this aquifer system, which yields about 30 percent of the nation's ground water used for irrigation. In addition, the aquifer system provides drinking water to 82 percent of the people who live within the aquifer boundary”- USGS http://co.water.usgs.gov/nawqa/hpgw/factsheets/DENNEHYFS1.html

Annual Withdrawal from Ogallala Aquifer for year 2000

Saturated Thickness (ft) in 2000 Water-level change (ft) from 1950 to 2005 ~9% decline since “pre-development” (~1950’s) …major concern, since recharge could take 100’s-1000’s of years!

US Groundwater Extraction from 1900-2008

in Eastern Australia

World’s Major Aquifers and their Footprint (GF) or Recharge Area

Monitoring Groundwater Measurements of water table by wells: very few long-term records (e.g., from Illinois State Water Survey).

Monitoring Groundwater from Space GRACE satellite measurements of changes in water storage over land by measuring changes in gravity. Launched in March 2002. GRACE=Gravity Recovery and Climate Experiment (GRACE), lauched in March 2002 GRACE=Gravity Recovery and Climate Experiment

Low spatial resolution ~300-400km for monthly data Base period: 2003-2007. From https://climatedataguide.ucar.edu/guidance/grace-gravity-recovery-and-climate-experiment-surface-mass-total-water-storage-and-derived- Low spatial resolution ~300-400km for monthly data GRACE Data portal: http://grace.jpl.nasa.gov/data/

GRACE Satellite Measurements (~7min.)

Groundwater flow For steady flow in an isotropic (K doesn’t depend on direction) and homogeneous (K doesn’t depend on location) medium: 3D version of Darcy’s Law can determine the distribution of hydraulic head, if given conditions at the boundaries. This can be used to calculate the groundwater flow Water flow directions (streamlines) are perpendicular to lines of constant hydraulic head (equipotential lines) “Flow Net” High h Low h streamlines Equipotential lines (constant h) Fig 9-5

Topographic effects on groundwater flow Gradual slopes lead to gradual gradients in h, with a single regional flow system with discharge distant from recharge regions Hills and valleys produce local variations in h, and create numerous local flow systems with discharge nearby to recharge regions Fig 9-7 regional flow system Local flow systems regional flow system

Local flow system Intermediate flow system Regional flow system

Geologic effects on groundwater flow Geologic material affects the hydraulic conductivity Sediment size Sediment or rock type Fracturing This influences flow paths and rates

Combined geological and topographic effects Fig 9-11 Low conductivity High conductivity

Landscape and Groundwater Flow The Fundamental Hydrologic Landscape Unit (FHLU)

Generalized Hydrologic Landscape Types and Groundwater Flow

Ground water links http://ny.water.usgs.gov/ http://groundwaterwatch.usgs.gov/ http://ny.water.usgs.gov/projects/eom/