USEFUL DEFINITIONS

• Soil Water - water contained in soil pores
• Pores - empty/void space
• Vadose Zone - zone of unsaturated conditions
• Water Table - upper limit of the saturated zone
• Aquifer - saturated zone/contains water
• Groundwater - water contained in aquifer. Flows freely into a well
• Unconfined aquifer - no confining geologic structure on the aquifer (aquifer pressure about the same as the atmosphere)
• Confined Aquifer - confining geologic formation on top of aquifer. Aquifer pressure usually greater than atmospheric
• Artesian aquifer - confined aquifer where water is so pressurized that it reaches the surface without pumping
• Perched Aquifer - geologic formation  (usually a clay lens) within vadose zone that intercepts water and creates a small, localized aquifer
• Infiltration -  movement of water through vadose zone into saturated zone
• Recharge - water entering an aquifer from precipitation
• Overland flow - precipitation is faster than infiltration and excess water runs over surface of land
• Residence Time - time a water molecule remains in a given hydrologic compartment

1. HYDRAULIC CONDUCTIVITY

• symbol - K
• units - length/time EX. (m/day)
• Ability of a particular material to allow water to pass through it

2. HYDRAULIC HEAD/FLUID POTENTIAL

• symbol - h
• units - length EX. (m)
• a measure of energy potential (essentially is a measure of elevational/gravitational potential energy)
• is the driving force for groundwater flow
• WATER ALWAYS FLOWS FROM AREA OF HIGH HEAD TO AREA OF LOW HEAD (even if this means it may go "uphill"!)
• measure head by sinking a well then measuring the level (elevation ) to which the water rises in the well in relation to a reference point which is taken as zero meters (usually sea level)
• hydraulic head determines the hydraulic gradient

3. HYDRAULIC GRADIENT

• symbol - I
• units - unitless (why? because length divided by length cancels out the units!)
• this is essentially the slope of the water table, and groundwater flow will be "down" this slope
• sink two wells and measure head.  Then find the difference between them and divide this by the flow length (distance between the two wells)
• EXAMPLE: head in well one = 100 feet.  Head in well two= 10 feet. Distance between the two wells is 10 feet. So the hydrauluic gradient is: 100 feet-10 feet/10 feet = 9

4. AREA OF FLOW

• symbol - A
• units - distance squared EX. (mē)
• Cross-sectional area of flow. (i.e. aquifer width x thickness)

5. DISCHARGE

• symbol - Q
• units - volume/time EX. (m^3/day)
• volume of water flowing through an aquifer per unit time
• FIND WITH DARCY'S LAW Q = KIA

6. FLUX

• symbol - v
• units - distance/time EX. (m/sec)
• v = Q/A = KI
• this is a velocity measure and gives the IDEAL velocity of groundwater (assumes that water molecules can flow in a straight line through the subsurface).
• this is ideal because it doesn't account for tortuosity of flow paths (this means that the water molecules actually follow a very windy path in and out of the pore spaces and so travel quite a bit slower in reality than the flux would indicate).

 
TORTUOSITY OF FLOW PATHS
Black arrow indicates overall flow direction;
red line indicates actual flow path; blue = pore space
v = how fast ; Q = how much

7. POROSITY

• symbol - n
• units - %
• percent of void space (empty space) in soil or rock.  Represents the path water molecules can follow in the subsurface
• Primary porosity - intergranular
• Secondary porosity - fractures, faults etc.

8. DARCY FLUX

• symbol - vx
• units - distance/time EX. (m/sec)
• vx = Q/An = KI/n
• This is the ACTUAL velocity of groundwater and DOES account for tortuosity of flow paths by including porosity in its calculation.

• Water table contour lines are similar to topographic lines.  They essentially represent "elevations" in the subsurface.   These elevations are called the HYDRAULIC HEAD.  And, just like a ball rolling down a hill, water in the subsurface will go from an area of high head (elevation) to an area of low head (elevation)
• Water table contour lines can be used to tell which way groundwater will flow in a given region.
• Lots of wells are drillled and hydraulic head is measured in each one
• Water table contours are drawn that join areas of equal head (these are called equipotential lines - it's like "connect-the-dots"!)
• Groundwater flow IS ALWAYS PERPENDICULAR to the water table contour lines (or flow lines)

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