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3. Rainfall-runoff relationships Methods of assessment

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Title: 3. Rainfall-runoff relationships Methods of assessment


1
3. Rainfall-runoff relationshipsMethods of
assessment
2
Factors that influence surface runoff
Physical-geographic factors (natural,
non-manageable)
  • Climatic (meteorological)
  • Precipitation
  • Type of precipitation (rain, snow initially less
    runoff, but ! melting season, sleet, etc.)
  • The rate (amount) and intensity
  • Duration of rainfall
  • Direction of storm movement
  • Distribution of rainfall over the drainage basin
  • Previous weather (e.g. precipitation that
    occurred earlier and resulting soil moisture)
  • Time of year/season
  • Summer - evapotranspiration rates higher,
    photosynthesis in plants - at a maximum
  • Other conditions that affect evapotranspiration -
    temperature, wind, relative humidity

3
Dangerous rainfalls
  • Summer storms (short time, high intensity)
  • are significant for smaller watersheds (outflow
    volume, peak)
  • flash floods
  • enhance erosion and transport processes in the
    watershed (bad quality of water in streams,
    smaller potential flow in channels and smaller
    volumes of reservoirs)
  • Regional rainfalls (long duration of rainfall)
  • high amount of water (area time period)
  • regional floods
  • Spring rainfalls (snow cover present)
  • sharp rise in temperature ð quick thaw ð sharp
    increase in overland flow
  • frozen ground underneath the snow ð rapid flow
    on ice ð increasing discharge

4
Physical-geographic factors(natural,
non-manageable)
  • Characteristics of watershed
  • Watershed area volume and culmination of total
    runoff
  • Shape of watershed time of concentration to the
    outlet
  • Elevation
  • Slope of the area
  • The steeper the slopes, the lower the rate of
    infiltration and faster the rate of run-off when
    the soil is saturated (saturated overland flow)
  • Strong influence on erosion and transport
    processes
  • Length of slope and length of valley lag time
    to the valley and to the outlet

5
Physical-geographic factors(natural,
non-manageable)
  • Geological and soil characteristics
  • Bedrock permeability - Run-off will occur quickly
    where impermeable rocks are exposed at the
    surface or quickly when they underlay soils
    (limited amount of infiltration).
  • Soil permeability - Soils with large amounts of
    clay do absorb moisture but only very slowly -
    therefore their permeability is low.
  • Thickness - The deeper the soil the more water
    can be absorbed.
  • Infiltration capacity - Soils which have larger
    particle sizes (e.g those derived from the
    weathering of sandstones) have larger
    infiltration capacities.
  • The infiltration capacity is among others
    dependent on the porosity of a soil which
    determines the water storage capacity and affects
    the resistance of water to flow into deeper
    layers.
  • Initial conditions (e.g. the degree of saturation
    of the soil and aquifers)

6
Anthropogenic factors (manageable)
  • Land use (e.g. agriculture, urban development,
    forestry operations)
  • Direct influence on retention capacity,
    hydrologic balance of watershed a the volume of
     direct runoff)
  • Measures
  • Increasing afforestation
  • Increasing meadow area at the expense of arable
    land
  • Limit of impervious surface
  • Prefer pervious road construction (forest and
    field)
  • Vegetation type and cover
  • Interception reducing initial surface flow
  • Evapotranspiration
  • Infiltration (the root systems)
  • Velocity of overland flow
  • Preferable vegetation cover to increasing
    retention capacity forests a meadows a
    close-seeded a grains a row crops
  • Agriculture
  • Irrigation and drainage ditches increasing the
    speed of water transfer
  • contour tillage
  • Tillage on wet land compresses the subsoil -
    creating a "plough pan" a decreasing water
    holding, infiltration and increasing
    run-off/erosion.

7
  • Human activities - development and urbanization
  • imperviousness - natural landscape is replaced by
    impervious surfaces (roads, buildings, parking
    lots) - reduce infiltration and accelerate runoff
    to ditches and streams
  • removal of vegetation and soil
  • constructing drainage networks and underground
    sewer increase runoff volumes and shorten runoff
    time into streams -gt the peak discharge, volume,
    and frequency of floods increase in nearby streams

8
  • River network
  • Routing and detention
  • Drainage density
  • This ratio is the length of river course per area
    of land. The larger the amount of streams and
    rivers per area the shorter distance water has to
    flow and the faster the rate of response.
  • River conditions
  • Surface depressions, marchlands, wetlands
  • Storage , hydrologic balance
  • Reservoirs, ponds etc.
  • Important storage volumes a retention capacity
  • Ecological balance - ecosystems
  • Prevent or delay runoff from continuing
    downstream
  • Decrease the peak discharge
  • Protection of the low-lying land downstream.
  • Water extracted for industry, irrigation, and
    domestic use, also reduce discharge.
  • ! reservoir aggradation (storage volume) -
    erosion control measures
  • Dry reservoirs polders a temporary storage
    during higher discharge (floods) a usually used
    as meadows

9
Precipitation spatial variability
  • is measured in gauges or by radar
  • Representation of precipitation depth spatial
    variability in a catchment
  • Arithmetic mean
  • Polygons (Thiessen)
  • Isohyets
  • Isohyet or isohyetal line
  • line joining points of equal precipitation on a
    map.
  • Isohyetal map

10
Precipitation temporal variability
  • A hyetograph
  • a graphical representation of the amount of
    precipitation that falls through time
  • is used in hydrology to illustrate the temporal
    variability of precipitation
  • Characteristics
  • Intensity (depth / time interval)
  • maximum
  • average
  • Cumulative intensity
  • Maximum depth
  • Time period

11
Extreme of rain events
  • Statistic analysis of maximum rainfall events
  • IDF curves
  • relations between intensities, duration and
    frequency of rain events
  • Intensity I (mm/min)
  • Duration D (min)
  • Frequency F (1/years)
  • probability of different rain event intensities
    for different durations (5, 10, 15, 30 minutes,
    24 hours)
  • an each curve represents a certain frequency of
    occurrence or a certain return period expressed
    in terms of years.
  • N value
  • the average over a number of years of observation
  • Value that is exceeded ones per N years (return
    period)
  • Rainfall depth (mm) of certain duration (e.g. 24
    hours) whose probability of appearance is 1/N
    Frequency (1/years)

N years 2 10 20 50 100
H1d,N mm 36.3 60.6 70.4 82.6 92.1
12
Hydrograph
  • hydro- water, -graph chart
  • plots the discharge of a river over time
  • a representation of how a watershed responds to
    rainfall.
  • Characteristics
  • Peak discharge Qmax (m3.s-1)
  • The highest point when there is the greatest
    amount of water in the river.
  • Time of peak (min)
  • Volume V (m3)
  • Rising limb
  • The part up to the point of peak
  • discharge.
  • Falling limb
  • The part after the peak discharge.

13
Extreme discharge
  • Extreme values - the average over a number of
    years of observation
  • Maximum (N value) QN(m3/s)
  • Value that is exceeded ones per N years (return
    period) - statistically
  • Discharge (m3/s) whose probability of appearance
    is 1/N Frequency (1/years)
  • Are required for the design of dam, spillways,
    nuclear power stations, major bridges
  • important for assessing risk for highly unusual
    events, such as 100-year floods.
  • Minimal Qm(l/s)
  • Value (discharge) that is exceeded m-days per a
    year statistically
  • Important for dry seasons, ground water storage

N (years) 1 2 5 10 20 50 100
QN (m3/s) 6 8 10.9 13.2 15.6 18.8 21.5
m day 30 60 90 120 150 180 210 240 270 300 330 335 364
Qm l/s 507 350 270 218 180 150 125 104 85 68 50 47 35
14
The surface runoff process
Rainfall excess rainfall - losses rainfall
- interception - surface retention - infiltration
Direct runoff surface runoff interflow
15
Rainfall event flood Ceský Krumlov
16
Curve Number Method (SCS-CN)
  • A method for simulating rainfall-runoff processes
  • Developed by SCS (Soil Conservation Service
    1972)
  • Widely used and efficient method
  • Determines the approximate amount of direct
    runoff from a rainfall event in a particular
    area.
  • Used for small catchments
  • CN
  • An empirical parameter for predicting direct
    runoff.
  • Developed from empirical analysis of runoff from
    small catchments and hillslope plots monitored by
    the SCS.

17
Curve number (CN) depends on
  • Soil
  • 4 classes (A D) according to infiltration rate
  • Cover and hydrologic condition of the land
    surface
  • Various types of vegetation and crops, land
    treatments and crop practices, paving and
    urbanization
  • Antecedent wetness
  • 3 classes of antecedent moisture condition (AMC)
    dry, average, wet
  • The bigger CN the higher runoff volume

18
CN catalog
AMC I AMC I AMC I AMC I AMC II AMC II AMC II AMC II AMC III AMC III AMC III AMC III
Land ID Land name SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type SCS Soil type
Land ID Land name A B C D A B C D A B C D
3 "Paved parking lots, roofs, driveways, etc. (excl. ROW)" 94 94 94 94 98 98 98 98 99 99 99 99
7 "Dirt streets" 53 66 73 76 72 82 87 89 86 92 95 96
18 "Developing urban area, newly graded (no vegetation)" 59 72 80 85 77 86 91 94 89 94 97 98
22 "Meadow - continuous grass, no grazing" 15 38 52 60 30 58 71 78 50 76 86 90
27 "Woods-grass combination - orchard - Fair" 25 45 58 66 43 65 76 82 63 82 89 92
31 "Woods - Good" 15 35 51 59 30 55 70 77 50 74 85 89
37 "Cultivated agr. - row - straight row(SR) - Good" 47 60 70 76 67 78 85 89 83 90 94 96
53 "Cultivated agr. - small grain - C - Good" 41 54 64 68 61 73 81 84 78 87 92 93
61 "Cultivated agr. - close-seeded - SR - Good" 38 53 64 70 58 72 81 85 76 86 92 94
The bigger CN the higher runoff volume
19
Volume of runoff
Oph volume of direct runoff in m3, Ho depth
of runoff in mm, F watershed area in km2, Hs
depth of rainfall in mm, A potential retention
in mm 0.2A initial abstraction in mm Hs -
0.2A effective storm rainfall in mm CN curve
number
for
20
CN method
Rainfall depth
  • Hydrologic soil
  • groups
  • Land use
  • AMC

Volume of direct runoff

Temporal distribution of runoff (hydrograph)
21
Unit Hydrograph - UH
  • UH is a hypothetical response of a catchment to
    unit rainfall excess - empirical
  • Original concept - Leroy Sherman (1932)
  • Its been developed since and applied in many
    versions.
  • The use hydrologic models known rainfall depth
    ? runoff volume ? temporal distribution

22
Unit hydrograph method
  • Response function
  • input - rainfall excess (unit volume, constant
    intensity, uniform distribution over a catchment)
  • output direct runoff
  • assumptions principle superposition or
    linearity and temporal invariance
  • Superposition
  • Output rate is dependent linearly on input rate
  • Temporal distribution is not influenced by input
    rate
  • Result output equals sum of outputs resulting
    from unit inputs
  • Temporal invariance
  • Starting time of input has no influence on rate
    or temporal distribution of output

23
Unit hydrograph 1 pulse
Q(t)Pef.u(t) u(t) ..pro Pef
1 Q(t)Pef.u(t) I. Dt.u(t) ..pro Pef ? 1
Q(t)
t h
24
Composite UH
Q(t)
t h
25
Composition of runoff hydrograph from unit
hydrographs
26
  • Thank you for your attention
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