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Baseflow Separation Techniques First assumption
is that the direct precipitation and interflow
components are inconsequential (but this should
be re-evaluated in extreme situations) Overland
flow is assumed to end at some fixed time after
the storm peak
Fetter graphical separation techniques are
convenient fiction Isotope budgets and tracers
can often help improve accuracy..
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Gaining and Losing streams Gaining (effluent)
stream - baseflow entering stream - typical in
humid regions - as you move down stream, more
water in stream even though no tributaries
exist Losing (influent) stream - water table
lower than bottom of stream channel - water loss
as you go down stream - rate of loss is a
function of the depth of water and hydraulic
conductivity of the underlying alluvium - In
some cases (mountainous arid regions), you start
with a gaining stream and move into a losing
stream..
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- During baseflow recession a stream may be
gaining, but become a losing stream during
floods
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- ground-water pumping near a stream can drop the
water table locally and cause a section of stream
to be losing, while it is gaining up and
downstream
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Baseflow Recession Understanding of baseflow
recession is necessary before we can look at
hydrograph separation - baseflow of a stream
decreases during a dry period because as ground
water flows into the stream the water table
falls Baseflow recession equation is where
Q is the flow at some time t after recession has
started Q0 is the flow at the start of the
recession a is a recession constant for the
basin t is the time since recession began e is
base of natural logs -plotting Q vs t on semilog
paper should yield a straight line (with t on the
linear scale) - If more than one straight line
apparent, there may be two groundwater sources -
in most watersheds groundwater depletion
characteristics are stable since they closely
match watershed geology..
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• Determining Baseflow Storage and Ground-water
  Recharge from Baseflow
• Seasonal Recession Method
• Assumes no dams or other regulation and minimal
  snowmelt
• Need hydrographs from two or more consecutive
  years
• using approximations derived from the baseflow
  recession equation we find
• Where Vtp is the total potential volume of
  groundwater discharge (i.e. volume of water
  discharged during complete recession)
• Qo is baseflow at the start of recession
• t1 is the time for baseflow to go from q0 to
  0.1q0..

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- If the amount of baseflow in the reservoir is
calculated at the end of a recession and then the
beginning of the next recession the amount of
recharge can be obtained by the difference -
hence the amount of baseflow remaining at any
time after baseflow recession begins is -
the above assumes no consumptive use during the
time period of interest..
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• Recession Curve displacement method
• More suited to areas without strong seasonal dry
  periods
• t1 is the time for recession to cover one log
  cycle
• t1 needs to exceed D from equation
• Where D days between storm peak and
• end of overland flow
• A basin area (mi2)
• tc is a critical time which is 0.2144t1

Recharge is calculated as
T1 45 days for this chart
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• Rainfall-Runoff Relationships
• - basic goal is to predict amount of runoff that
  will occur from a given storm
• - need to design structures and neighborhoods
  based on peak discharges
• Rational Equation is simplest
• if it rains long enough, peak Q from basin will
  be the average rate of rain times
• the basin area (adjust by a coefficient to
  account for infiltration)
• time of precip has to exceed time of
  concentration for rational equation to apply
• - time of concentration is time necessary for
  water to flow from the most distant part of
  watershed to point of discharge
• - conceptually time of concentration is the
  average velocity of the longest stream channel
  times the length of the channel, plus time for
  overland flow to reach the channel..

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- rational equation assumes constant rainfall and
infiltration rate - best used for small (200
acres or less) watersheds Q is peak runoff
rate I is average rainfall intensity A is the
drainage area C is a runoff coefficient (gotten
from a table) Lower range of C is used for low
intensity storms Higher range for high intensity
storms..
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Duration Curves - often want to know how often a
stream flows at a lesser or greater discharge
than some value - duration curves usually daily
or annual flow Steps are 1. Rank flow records
(m) starting with 1 for highest flow and n for
lowest over the period of interest (if two are
equal, they each get their own rank...no ties) 2.
The probability (P) that a given flow will be
equaled or exceeded is given by 3. If
comparing multiple rivers reduce Q to discharge
per unit area of basin (e.g. m3/s/km2 4. On
probability paper (or in spreadsheet) Plot Q as
Y-axis and P as x-axis - distribution of runoff
is caused by geology of drainage basin - steeper
curves have thinner soils, lower hydraulic
conductivity, less overall baseflow..
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Connect the stream with the Basin
character 2. Thick sand deposits 3. Glacial
till with silt and clay
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Unit Hydrographs - a unit hydrograph is the
characteristic response of a given watershed to a
unit volume (depth) of effective water input
applied at a constant rate - used to forecast
response of a watershed to a given input of
water - hydrograph of direct runoff (excludes
baseflow) To develop unit hydrograph (1") 1.
Collect as much streamflow and precip data as
possible Best storms are a) individual b)
uniform temporal and spatial distribution c)
rainfall duration should be 10-30 of basin lag
time d) direct runoff should range from 0.5 to
1.75 inches 2. For each storm, separate quickflow
and baseflow 3. Calculate depth of direct runoff
(quickflow) per hour (from beginning of
quickflow)..
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4. Multiply each original hydrograph by 1 over
value in obtained in 3. 5. 6. Plot several unit
hydrographs for similar duration rains in this
way 7. Construct composite unit hydrograph take
peak as average in both x and y, and adjust until
area under curve is 1" of runoff With say a 2.5
hour unit hydrograph can then take a forecast
precipitation of say 2" and just double the
height of your 1" inch unit hydrograph to come up
with prediction of stream response to forecast
storm. Urban Hydrology - urbanization generally
increases total quickflow for a given rainfall -
faster time to peak (lower time of concentration)
and higher peak - lower rates of ground water
recharge in area of urban centers - serious in
areas where ground-water is big portion of
supply..
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