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Basic Hydrology

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Basic Hydrology. Precipitation - Runoff Relations. Watershed Morphology. Watershed morphology. Morphological properties of a watershed can affect the shape of the ... – PowerPoint PPT presentation

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Title: Basic Hydrology


1
Basic Hydrology
  • Precipitation - Runoff Relations
  • Watershed Morphology

2
Watershed morphology
  • Morphological properties of a watershed can
    affect the shape of the storm hydrograph and the
    delivery of sediment to the main channel
  • Various parameters can be calculated to describe
    the channel network and the physical
    characteristics of the watershed
  • these all affect hydrograph shape

3
Basin size
  • Delineate watershed according to the height of
    land that separates water draining to the point
    of interest from water that drains to adjacent
    basins
  • Watershed area (km2, ha)
  • smaller watersheds tend to have a more peaked
    hydrograph, more intermittent water supply
  • larger watersheds have flatter hydrographs
    because larger channel network can store more
    water

4
Watershed land slope
  • The slope of the sides of a watershed govern how
    fast water will drain to the channel
  • steep slopes - peaked hydrograph
  • gentle slopes - flat hydrograph
  • This is simply the average gradient of hillslopes
    - slope is vertical over horizontal distance,
    derived from topographic maps
  • An objective repeatable formula for land slope

where L is the total length of contours, C.I. is
the contour interval and A is the watershed area.
5
Area - elevation curve
  • Area - elevation is critical for modeling
    snowmelt
  • Can be useful in determining precipitation
    distribution from a ppt. - elevation relationship

240 Creek
median elevation
6
Matching area- and ppt- elevation relationships
can be used to compute basin average precipitation
Area - elevation relationship
Precipitation- elevation relationship
7
Indices of basin shape
  • Form factor
  • elongated - F.F. is low, flatter hydrograph
  • squatty - F.F. is high, peaked hydrograph

8
Strahlers order of streams
  • A headwater stream with no tributaries is a first
    order stream
  • When two first order streams join they form a
    second order stream
  • Two second order streams form a third order
    stream
  • etc.

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9
Bifurcation ratio
  • Bi ratio of first order to second order
    streams
  • If watershed is gt 2nd order

Plot log Nu vs. u as shown, Bi is the anti-log of
the slope of the regression line. For the
example given, Bi anti-log(0.693) 4.93
10
Effect of Bi on hydrograph shape
Assuming uniform ppt. distribution, all other
factors being equal...
Elongated basin Bi is high (13) flat hydrograph
due to even supply of water to channel
Rounder basin Bi is low ( 4.9) peaked hydrograph
because flow is concentrated
11
Channel slope and profile
  • Channel slope plays a role in the shape of the
    hydrograph
  • the steeper the slope, the more peaked the
    hydrograph

240 Creek channel profile
mean channel slope
12
Determining mean channel slope
  • Each tributary channel in a watershed has its own
    profile
  • commonly done only for the main channel
  • Calculate the slope of a line drawn such that the
    area under the line the area under the main
    channel profile
  • An index of channel slope
  • can be calculated from the
  • slopes of n equal channel
  • segments

13
Drainage density
  • Drainage density is determined by measuring the
    total length of all streams on a map and dividing
    by the watershed area
  • units of km/km2
  • for comparative purposes, you must use maps with
    the same level of detail for all basins of
    interest
  • Effect on hydrograph shape
  • high Dd - peaked hydrograph
  • low Dd - flat hydrograph

14
Valley flat
  • Area adjacent to stream or river floodplain where
    the slope is lt 8
  • Buffers the stream channel from landslides which
    may run out on the valley flat before depositing
    sediment in the channel.
  • Calculate the length of mainstem channel that has
    a valley flat, express as a proportion of the
    length of the mainstem channel.

15
Other factors
  • Lithology
  • importance can govern slope stability, bedrock
    leakage, permeability
  • Presence or absence of glaciers
  • will govern timing and mangitude of peak runoff
  • Land use...

16
Precipitation - runoff
  • Methods have been developed to predict
    characteristics of runoff as a function of
    precipitation characteristics
  • volume of runoff
  • seasonal
  • annual
  • based on seasonal or annual total precipitation
  • peak flow
  • annual peak flow - e.g., snowmelt peak
    (interior), a function of peak snow accumulation
  • storm peaks - a function of rainfall intensity

17
Runoff coefficient
  • Simplest form of ppt - runoff relation
  • ratio of total streamflow
  • over total precipitation
  • Runoff coefficient can be assessed annually,
    seasonally or monthly depending on purpose
  • Should be a characteristic quantity of a
    watershed assuming no change in land use

18
Calculating rainfall - runoff ratioExample 240
Creek, UPCWater year Sept - Aug
Since R is related to P or Q, a better way to get
the ralationship is to plot Q vs. P and fit a
regression line.
19
Runoff coefficient 240 Creek
Runoff coefficient increases with total precip.
Runoff threshold water loss to ET
20
Spring-summer runoff vs snowpack
  • This can be more meaningful than a runoff
    coefficient - e.g., 240 Creek, 1985-91

1990 - rain on snow late May
21
Predicting spring runoff in interior watersheds
  • Unlike runoff coefficient relationship,
    relationship between spring - summer runoff and
    peak snowpack passes through the origin
  • this shows that virtually all the snowpack
    contributes to spring - summer runoff
  • Slope gt 1 relationship is a very good predictor
    of snowmelt runoff but doesnt account for
    precipitation that occurs after April 1 - doesnt
    work for unusual conditions such as rain-on-snow

22
Precipitation temperature
23
Use of snow course data to predict runoff
  • For an interior watershed, snow course data
    should provide a better measure of runoff
  • Used to predict inflows to reservoirs, potential
    floods
  • For a coastal watershed, rainfall data is needed,
    but annual runoff coefficient is probably
    relatively meaningless
  • monthly runoff ratio, averaged over several years
    may be useful
  • expected to be much higher than for interior w/s

24
Effect of antecedent conditions on rainfall -
runoff relation
  • The amount of soil moisture prior to a storm will
    affect the runoff ratio for that storm, and will
    affect the shape of the hydrograph
  • wet antecedent conditions lead to more runoff per
    unit ppt., dry antecedent conditions result in
    more of the input water going to basin recharge
  • antecedent conditions are a function of ET and
    soil/groundwater drainage.
  • Not always possible to quantify these factors...

25
Antecedent Precipitation Index
  • API is a method of accounting for daily changes
    in water balance.
  • API is a decay factor - each days API is a fixed
    percentage of the previous days API (e.g., 90),
    plus daily rainfall and/or snowmelt
  • runoff coefficient will vary according to the
    API
  • the higher the API, the higher the runoff
    coefficient

26
API for Russell Creek Jan 1992
27
API for Russell Creek Jul 1992
28
Synthetic unit hydrograph
  • It has been determined empirically that the
    parameters of the unit hydrograph - lag time,
    peak and time base - can be determined from basin
    morphology
  • lag time (hours)

LC
L length of main channel Ct range 1.8 to 2.2
29
  • Time base (in days)
  • Peak flow various formulae have been advanced to
    predict peak flow
  • Rational formula Qp RIA
  • where R runoff coefficient, I rainfall
    intensity and A basin area
  • Other formulae

Cp range 0.15 to 0.19 per mm with Q in m3/s, A in
km2
30
Russell Creek 1991 - 92
Peak 0.342 (24hr) 1.17 Base R2 92
R2 83.8
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