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THE HYDROLOGIC CYCLE 2: GROUNDWATER. The Hydrologic Cycle - Fresh Water Storage Reservoir % of Total Fresh Water Glaciers (Frozen)76% Groundwater22% Rivers & Lakes < 2% Groundwater – largest liquid fresh water storage reservoir. The Groundwater System. - PowerPoint PPT Presentation
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The Hydrologic Cycle - Fresh Water Storage
Reservoir % of Total Fresh Water
Glaciers (Frozen) 76%
Groundwater 22%
Rivers & Lakes < 2%
Groundwater – largest liquid fresh water storage reservoir.
So, To Understand Groundwater Flow We Need To Know:
1) Depth of the water table.
2) Gradient (slope) of the water table.
3) Characteristics of the rocks that the groundwater is traveling through.
Gradient = (h1- h2)/L
Where: h1and h2 are the elevations of the water
table at two locations, and L is the horizontal distance between them
h2
h1
Rock Characteristics:1) Porosity – How much water the rock will hold.2) Permeability – The ability of a rock to transmit
water. Depends on size and connectivity of pores.
PorosityA) SedimentsSoil 55% by volume
Clay 45Sand 35Gravel 20
B) RocksShale 5%Limestone 5Sandstone 15Granite 1FracturedLimestone 20
Permeability
ClayShaleSolid LimestoneSandstone Fractured LimestoneGravels & Sands
High
Low
The first person to study these characteristics of the groundwater system was a geologist named Henri Darcy.
Darcy noted that the velocity of groundwater flow was proportional to the gradient of the water table.
1) V α (h1- h2)/L but we need to account for permeability
2) V = k (h1- h2)/L where k is a permeability variable, and
3) Q (discharge) = Ak (h1- h2)/L where A is a measure
of cross sectional area
Darcy’ Law
Q (discharge) = Ak (h1- h2)/L = AV
Pore
Remember, when calculating cross – sectional area the water is only flowing through the open pores (unlike a river channel). SoA (area) = width X depth X porosity.
Problem
1) What is the gradient of the water table whose elevation is 500’ at well 1, 300’ at well 2, and the wells are 2000’ apart?
2) If the aquifer has a width of 40 feet, and a depth of 40 feet, a porosity of 5%, and a k = 0.01 ft/day, what is the groundwater velocity and discharge?
1) Gradient = (h1- h2)/L =
500’ – 300’/2000’ = 0.1
2) Velocity = k (h1- h2)/L =
0.01ft/day x 0.1 = .001ft/day
3) Q = Ak (h1- h2)/L =
40’ x 40’ x .05 x .001 = .08 ft3/day