3. Rainfall-runoff relationships Methods of assessment

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3. Rainfall-runoff relationshipsMethods of
assessment
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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

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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

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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

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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)

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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.

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• 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

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• 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

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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

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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

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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
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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.

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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
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The surface runoff process
Rainfall excess rainfall - losses rainfall
- interception - surface retention - infiltration
Direct runoff surface runoff interflow
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Rainfall event flood Ceský Krumlov
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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.

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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

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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
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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
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CN method
Rainfall depth

• Hydrologic soil
• groups
• Land use
• AMC

Volume of direct runoff

Temporal distribution of runoff (hydrograph)
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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

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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

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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
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Composite UH
Q(t)
t h
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Composition of runoff hydrograph from unit
hydrographs
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• Thank you for your attention