1. Global Water Cycle

• Why do we care? The water cycle is the largest chemical flux on earth. Water distributes heat around the globe and thus creates climate, and water is the single most important factor for land-plant productivity worldwide.
• Approach to understanding cycles: This general approach can be used to understand any element cycle, and cosists of three parts:

   

  • 1st - Accounting: "where things are" (distribution)
  • 2nd - Cycling: "where things are going"
  • 3rd - Controls: "what factors control the cycles and distribution"

  * Using this approach and gaining knowledge of each of these three components enables you to answer the question of "How will things change?". Gaining this kind of a predictive understanding of systems in the most important goal in basic scientific research.

Accounting for Water (distribution of water in km3 x 106)   Rocks (not usable) 25,000 Oceans (97.4% of usable water) 1,350 Ice 27.5 Groundwater 8.2 Lakes and Rivers 0.025 Atmosphere (vapor) 0.013

Cycling

There are 4 major pathways: precipitation, evaporation, vapor transfer from ocean to land, and return flow in rivers and groundwaters from land to oceans.

1. Total precipitation = 0.5 x 106 km3 / year
2. Evaporation from ocean = 0.425 x 106 km3 / year
3. Ocean Residence Time, Rt = (1,350 106 km3) / (0.425 106 km3/yr)
   = 3,176 years
   
4. Atmospheric water residence time - as part of your take home assignment, also calculate the atmospheric water residence time.

Controls

Human consumption - important today at local scales, may be important at a global scale in the future. "Regional vulnerability".

Temperature - increasing temperature increases the rates of evaporation and ice melting, and causes sea level to rise. Severe droughts, like in the Sahel, are caused by small changes in the geographical distribution of water.

1. Glacier melting in French Alps.

   
   
   

    

2. Sea level rise - due in part to thermal expansion of water.
   
   

1. Increased river flow of freshwater from ice to the arctic ocean will place a "cap" on the surface water, and prevent sinking of cold, salty water ("deep water formation") that drives ocean currents (see lectures on ocean circulation for review of this topic).

Land use changes - currently most deforestation is at a local scale. However, it may soon become important at regional scales and for the entire globe in the future.

• Hubbard Brook example (assigned reading). Run-off increased by up to 400% after deforestation. Nutrient cycles are strongly linked to hydrologic cycle, and so nutrient export was also increased.

2. Global Nitrogen Cycle

Forms

The nitrogen cycle is complex in part because of the many chemical forms of N, including gases: Organic-N; NO₃; NH₄; gases N₂, N₂O, NO + NO₂ (=NOx).

Accounting - distribution of N in grams times 10^15.

Rocks and sediments	190,400,120(deep, unavailable)
Atmosphere	3,900,000
Ocean	23,348
Soils	460
Land plants	14
land animals	0.2

Atmosphere
N2	3,900,000
N2O	1.4
NOx	0.0006 (less than 1 billionth %)

Cycling

• Pathways and Reactions:
  1. N₂ to organic-N; called "N-fixation" (plants and humans)
  2. Organic-N to NH₄+ ;"mineralization" (by bacteria and fungi)
  3. NH₄+ to NO₃- , producing NO and N₂O; "nitrification" (by bacteria)
  4. NO₃- to N₂ , producing N_2O ; "denitrification" (by bacteria)
  5. NO₃- & NH₄+ to organic-N; "photosynthesis" (uptake by plants)
  
• Fluxes - with respect to the atmosphere
  1. N₂ output from the atmosphere = 240 x 1012 g / year (N-fixation)
     * Rt of N₂ = 16.25 million years
  2. NO_x output from atmosphere = 60 x 1012 g / year
     * Rt = 0.01 yr = 3.6 days

Choose one chemical form to examine:

• NOx - produced by combustion of fossil fuels and by industry. Important in forming acid rain.
  1. NO + O₃ (ozone) = NO₂
  2. NO₂ + OH = HNO₃ "nitric acid"
  3. In water, HNO3 dissociates (breaks apart) to give H+ and NO₃- (charges must balance)
• Second important reaction is the formation of sulfuric acid in the atmosphere.
  • H₂SO₄ dissociates in water to give 2 H+ and SO₄2-

It is important to learn and understand that most biogeochemical questions must be solved by combining information about several element cycles. This is because most element cycles interact strongly with surrounding elements, and so for example to help solve the problems of acid rain we must first understand the controls on the elements that interact.

• H+ is neutralized by weathering reactions in the soil and plants. H+ exchanges for other positively charged elements.
• Buffering capacity of soils may be limited
• What effects does acid rain have?
  • Plants and trees can be damaged
  • As pH drops, aquatic life is negatively affected
    
  
• By measuring the "buffering capacity of soils, you can determine regions of sensitivity to acid rain.

Information provided by: http://www.sprl.umich.edu