quantitative elements of physical hydrology - brown€¦ · 4 we need to understand the interaction...
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Quantitative Elements of Physical Hydrology
The Hydrologic Cycle:Mass Balance and Flux in the
Water Cycle
© John F. HermanceJanuary 25, 2007
Contact information:Jack HermanceEnvironmental Geophysics/HydrologyDepartment of Geological SciencesBrown University, Providence, RI 02912-1846Tel: 401-863-3830e-mail: [email protected]
Discussion Summary
Objective: To understand the inter-relationships of the principal hydrological processes in a watershed.
These are:PrecipitationEvaporation
& transpirationDepression storageInfiltrationOverland flow
Hortonian flowSaturated flow
InterflowThroughflowGroundwater flowStreamflow generation
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Begin by defining the conventions usedin our visualization graphics.
Consider a genericwatershed . . .
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A subsection of the stream . . .
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We need to understand the interaction of these
elements . . .
Elements of the Hydrologic Cycle
Factors affecting the behavior of water and its movement through the watershed:
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Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 25, 2007
An Introduction to Mass Transport:Total flux across a boundary
Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 25, 2007
Flow across a surface elementFlow across a surface element
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How many flux lines pass through aunit area?
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How many flux lines pass through aunit area?
How many flux lines pass through aunit area?
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In this case,qnormal = qactual cos θ
The number of flux lines passing through a unit area depends on the geometrical relationship between the direction of the flux vector and the orientation of the area.
This, in turn, is abbreviated.qn = Area . qactual cos θbecomes:
qn = A . qactual
(Note that vectors are shownin bold, "areas" have direction,and this vector product is calleda "scalar", "dot" or "inner" product)
(Projection of A on to B, orB on to A.)
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Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 25, 2007
Application of these concepts tostreamflow discharge.
Application of these concepts tostreamflow discharge.
Consider a stream in plan (2D) view.Consider a stream in plan (2D) view.
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What is the total stream discharge Q?What is the total stream discharge Q?
First, . . . we need to ask how is stream discharge Q measured?
First, . . . we need to ask how is stream discharge Q measured?
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Standard (USGS) proceduresemploy rotating cups.
(Lance Ramsbey - USGS)
Standard (USGS) proceduresemploy rotating cups.
(Lance Ramsbey - USGS)
What is the total stream discharge Q?What is the total stream discharge Q?
A rotating cup (pin-wheel) flow-meterA rotating cup (pin-wheel) flow-meter
(Number of turns/minute is proportional to flow velocity)
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This is a scalar flow measurement.
This is a scalar flow measurement.
A practical application: Suppose the hydrologist has limitedaccess to the stream. Only this profile, because of depth, etc..
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But, ... the hydrologist wants to know the total discharge Q at C-D;and, for estimating erosion effects, the mean velocity of theflow at C-D.
Suppose we know the cross-sectional area A of the stream, and the average velocity of the stream V, only along the profile A-B.
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Determine Flux:Q (stream discharge) = V A cos θ A = Cross-sectional area
A Question (?):Assume that V, the average velocity of the stream, is 1 ft/s.What would be the magnitude of that component of V normal (i.e. perpendicular) to the line A-B?
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A Question (?):Assume that along A-B, the cross-sectional area of the stream is 500 ft2, and the average velocity of the stream is 1 ft/s.What is the total discharge Q across the line A-B?
A practical application: Suppose the hydrologist has limitedaccess to the stream.
Determine Total Flux:Q (stream discharge) = V A cos θ A = Cross-sectional area
V = 1.0 ft/sA = 500 sq ftcos (45 deg) = 0.7Q = 350 cfs
This is the total stream discharge.
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Next Question (?):What is the total discharge Q (cfs) across the line (through thearea?) C-D? (A = 400 ft2)
Final Question (?):What is the average velocity V?
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Question (?):What is the total discharge Q (cfs) across the line (through the area?) C-D? (A = 400 ft2)
The total stream discharge isFor A-B:V = 1.0 ft/s (measured)A(true) = 350 sq ftQ = 350 cfs
For C-D, Q is the same.
Final Question (?):What is the average velocity V?
The total stream discharge isFor A-B:V = 1.0 ft/sA(projected) = 350 sq ft (500 x cos θ)Q = 350 cfs
For C-D, Q is the same.
The average velocity at C-D isV(avg) = Q/A = 350/400 = 0.875 ft/s
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Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 25, 2007
Flow through a reference volumeFlow through a reference volume
Flux through a closed surface
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Flux through a closed surface
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Apply this concept to streamflow
Apply this concept to streamflow
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Introduce a “mathematical” volume . . .Introduce a “mathematical” volume . . .
Introduce a “mathematical” volume . . .Introduce a “mathematical” volume . . .
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(The convention is that the sense of a vector is positive (+),when it is directed out of a closed surface.)
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Flux through a closed surface
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Flux through a closed surface: Case 2
How can you have more flux leaving avolume than entering?
Flux through a closed surface: Case 2
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Next consider the special case of storagewithin the volume.
(Could be a lake, reservoir,stream reach, pond,groundwater reservoir, etc.)
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Consider "storage" in a stream.
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So what are the parameters ofa watershed to which
these concepts apply?
Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 25, 2007
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Application of the concept of residence timeto a reservoir.
Application of the concept of residence timeto a reservoir.
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A Question (?):Assuming a steady-state total volume of200,000,000 m3, and an average inflow ofstreamflow, groundwater flow and precipitationof 20,000,000 m3 / year, what is the averageresidence time of a drop of water in thereservoir?
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Another Question (?):What is the partitioning of discharge from the reservoir
a) over the spillway,b) consumptive use,c) evaporation,d) groundwater ”seepage"?
Another Question (?):What is the partitioning of discharge from the reservoir
a) over the spillway,b) consumptive use,c) evaporation,d) groundwater ”seepage"?
(This is a question for later.)(This is a question for later.)
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We next use conservation of mass to quantify the water cycle.
We next use conservation of mass to quantify the water cycle.
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Quantifying watershed processes.
© John F. HermanceJanuary 25, 2007
© John F. HermanceJanuary 25, 2007
Flow elements of the local water cyclein a watershed
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P + Qswi + Gin - (ET + Gout + Qswo) = 0© John F. Hermance
January 25, 2007
Mass Balance Relation #1
P + Qswi + Gin - (ET + Gout + Qswo) = ∆S/∆t© John F. Hermance
January 25, 2007
Mass Balance Relation #2
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(Global view of the water mass balance.)
© John F. HermanceJanuary 25, 2007
© John F. HermanceJanuary 25, 2007
Note (for example):P = I + QS
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© John F. HermanceJanuary 25, 2007
Note (for example):P = I + QS
Oftentimes, a water balance relation reflects the hydrologist’s point of view regarding a particular physical process.(What’s this “point of view”?)
© John F. HermanceJanuary 25, 2007
Note (for example):P = I + QS
orET = I - QG
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© John F. HermanceJanuary 25, 2007
Note (for example):P = I + QS
orET = I - QG
Since (from the latter)I = ET + QG . . .
© John F. HermanceJanuary 25, 2007
Note (for example):P = I + QS
orET = I - QG
Since (from the latter)I = ET + QG
Substitute for I in the 1st relation, to obtainP = ET + QS + QG
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Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 25, 2007
End of Presentation(The Hydrologic Cycle:
Mass Balance and Flux in the Water Cycle)
End of Presentation(The Hydrologic Cycle:
Mass Balance and Flux in the Water Cycle)
Contact information:Jack HermanceEnvironmental Geophysics/HydrologyDepartment of Geological SciencesBrown University, Providence, RI 02912-1846Tel: 401-863-3830e-mail: [email protected]
(An alternative view.)
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© John F. HermanceJanuary 25, 2007
The water cycle ona watershed scale
© John F. HermanceJanuary 25, 2007
The water cycle ona watershed scale