Hydrologic Cycle

I. Global Hydrologic Cycle
    A. Pools
        1. Atmosphere
        2. Oceans
        3. Ice
        4. Groundwater and soil moisture
            i. Infiltration capacity
            ii. Water table
            iii. Vadose water or soil moisture
            iv. Groundwater

        5. Lakes
        6. Rivers

    B. Fluxes
        1. Evaporation
        2. Evapotranspiration (plants)
        3. Precipitation
        4. Runoff –
            i. often seasonal patterns
            ii. Overland flow
            iii. subsurface stormflow
            iv. stream hydrographs
                a.  baseflow
                b. discharge – volume passing through the cross-sectional area of a stream or river per unit time

                c. rising limb
                d. peak
                e. falling limb

                f. variability
                f. effects of clearcutting and landuse

            v. 10 largest rivers = 40% of world’s runoff

    C. Water in the Biosphere (Table 1-1, Wetzel)
 

Table 1-1, Wetzel, Water in the Biosphere
                                                renewal time = residence time
    D. Hydrologic Cycle diagram (see Figure 4-1, Wetzel)

II. Global distribution of water
    A. Global weather patterns
        1. Hot air rises and then cools, causing precipitation
        2. Rain at tropics
        3. Deserts at ‘horse latitudes’
        4. Rain in temperate areas

    B. Effects of mountains near the sea
        1. Moist air rises at mountain and cools
        2. Lots of precipitation on windward side of mountain
        3. Dry air on lee side of mountain
        4. Example of state of Washington precipitation patterns
        5. Global precipitation patterns – result of weather patterns and mountain ranges
    C. 3 regions on earth in terms of water balance:
        1. exorheic -- 

        2. endorheic – 

        3. arheic – 

III. Lake Water Balance
    A. Closed versus open lake systems
        1. closed –
        2. open –
            i. drainage
            ii. seepage

    B. Water balance
        1. Inputs
            a) Runoff from watershed
            b) Precipitation directly on lake surface
            c) Groundwater inputs – seepage or springs

        2. Outputs
            a) Drainage from outlet (stream)
            b) Evaporation and evapotranspiration
            c) Seepage through floor of lake

<>    C. Modeling control by rainfall and evaporation
            Simple equation balancing precipitation and evaporation on lake and catchment, and outflow from lake to yield water level change 
AL Dz = AL (PL - EL) + (AC - AL) (PC - EC) - (AL) (O)
  where:
   Dz = change in depth of the lake
   AL = Area of the lake
   AC = Area of the catchment
   PL = Precipitation on the lake
   PC = Precipitation on the catchment
   EL = Evaporation from the lake
   EC = Evaporation from the catchment
   O = outflow per unit lake area

e.g., when Dz increases, then
(PL - EL) + (AC - AL) (PC - EC) - O > 0
                       AL

This blue term is key.  If AL is greater than 1/2 AC, then the value of the blue term is less than 1,
    and the catchment watershed becomes less important

      – Importance of basin shape and watershed size

IV. Human Impacts on the Hydrologic Cycle
    A. Irrigation, industrial and domestic use of groundwater
    B. Human increases in evaporation (reservoirs, irrigation) –
            humans account for 3-10% of continental evaporation today;
            projected 50% in 100 years
    C. Discharge amount and seasonality of rivers
        1. land use
        2. water projects
    D. Aral Sea example
 

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