3-8: Impact of Global Change on Water Resources and Water Use

Groundwater-"Nuts and Bolts"
Definitions:

Ground water: Subsurface water that occurs beneath a water table in soil and geological units that are fully saturated.

Hydrogeology: ".....interdisciplinary science of the study of water and its interrelation with rocks, soil, and humans, with an emphasis on groundwater" (Stephenson et al., 1991)

Course dealing with groundwater af lSU:
Geology 510, 511, 522, 534
CCE 573, 575, 579
Agronomy 577, 578, 677
ABE 520, 522

The hydrologic cycle

Withdrawal of freshwater in the US

Per capita freshwater usage in the US

Predicted Effects of Climate Change from Water in the 21st Century in Water in Crisis by Gleick, 1993).

• Change in precipitation and temperature
• Change in storm magnitude and frequency
• Change in snowmelt and snowfall
• Change in runoff
• Change in soil moisture
• Managing water under certainty

Water in the 21st Century (article by Peter Gleick, 1993, in Water in Crisis ).

• Precipitation and temperature:
  1. Doubling of CO₂ will result in 3 degree Celsius temperature rise and 3 to 15% precipitation increase.
  2. Alters distribution of:
  natural ecosystems
  crops
  human settlement
  water use for human development
  
• Storm frequency and intensity:
  1.More frequent extreme events
  2. Alters variability of the hydrological cycle
  
• Snowfall and snowmelt:
  1. Snowpack is a large reservoir that redistributes runoff
  2. Temperature increase will:
  increase ratio of rain to snow in cold months
  decrease length of snow season
  increase rate and intensity of snow melts
  
  3. Results:
  average winter runoff and peak runoff increase
  peak runoff occurs earlier
  faster and more complete depletion of soil moisture
  increased flooding in mid to upper latitude river basins
  
• Runoff:
  1. Water supply
  2. Energy generation
  3. Human health
  4. Commercial/industrial development
  5. Critical in areas where demand for water is near the limit of reliable supply

  Elasticity

  Percent change in mean annual runoff

• Soil Moisture:
  1.Determines what plants can grow in different regions and how much additional water should be applied for irrigation
  2. Generally overlooked in climate impact studies because it is difficult to model directly and is the leftover from precipitation and evapotranspiration (ET) (coupling GCM's to land surface)
  3. Increase in precipitation not equal to greater soil moisture (increase in ET may exceed increase in in precipitation and cause drying)
  4. Thus, droughts may increase as temperature rises despite the increase in precipitation. because ET also rises
  5. Soil moisture in mid-latitude continents will decrease due to higher temperatures. combined with insufficient precipitation, increases - based on 5 different climate models
  6. What about groundwater recharge?

• Groundwater Recharge
  1. Addition of mass to the water table by vertical infiltration through the unsaturated (vadose) zone or lateral inflow
  
  2. Recharge to groundwater is some percentage of mean annual precipitation:
  Midwest: 10 to 20%
  Arid areas: 0.25 cm/yr
  
  3. Recharge demonstrated by seasonal water table fluctuations
  4. Water in many aquifers has been recharged under wetter paleoclimates

  Hydrograph of water table fluctuations

  Water table fluctuations

  Areas with decreasing groundwater levels

• Mining Groundwater
  Definition: Overdraft occurs when water removal rates exceed the natural recharge rate
  • There are many areas of the U.S. where groundwater levels have dropped more than forty feet since predevelopement
  • Natural analog for global climate change; i.e. CO₂ doubling
    Raising the temperature or ET and/or decreasing precipitation leads to a decrease in the natural recharge rate and an increased need form pumpage.

Ogallala Aquifer

• A major source of water for the High Plains (from Texas to South Dakota)
  
• Course-grained stream sediments shed from mountains in the west
  
• Since the 1900's use has increased due to :
  
  • the 1930's drought
    
  • the 1950's drought
    
  • cheap energy
    
  • high food prices
    
  • a switch by farmers from growing cotton in the south to growing irrigated crops used for beef production
• In 1990, 15.6 million acre feet of water was removed from the Ogallala aquifer to irrigate 14 million acres of land
• In 1978, 170,000 wells were pumping 3X10^10 cubic meters of water per year
• These exceed the natural recharge rate by to 2 to 1000 times (the natural recharge rate is less than 0.5 cm per year on the Southern High Plains)
• The most critical saturated thickness occurs in Texas

Principle geologic units of the High Plains aquifer

Water-level changes in the High Plains aquifer

Water-Level Changes in the High Plains Aquifer, Predevelopment to 1994

Water management under certainty

• Great uncertainty about management and supply of fresh water resources
  
• Hydrologic processes are stochastic- we estimate the probability of future events, i.e., imply stationary climate
• Climate is not stationary, therefore the probability of future events are difficult to predict
• Governments unwilling to share water information or management schemes

Ames map showing locations of hydrogeologic interest