1. 3. Rainfall-runoff relationships Methods 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…
USGS
Diplomka
WIKI Surface runoff, Hydrograph
Water harvesting Factors affecting runoff

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 -> the peak discharge, volume, and frequency of floods increase in nearby streams

8. Surface depressions, marchlands, wetlands
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
Vodní tok s příbřežní zónou
Zvýšení vodního toku a příbřežní zóny lze dosáhnout:
úpravou trasy toku s respektováním přirozených meandrů,
důsledkem prodloužení trasy toku je i snížení podélného sklonu dna úpravou příčného profilu dle charakteru trasy toku ( nejednotný příčný profil)
odstraněním nadměrného zahloubení koryta, které snižuje hladinu podzemní vody v okolí toku
zajištěním stability koryta, omezením erozních a sedimentačních procesů, které vedou ke snižování objemů akumulačních prostorů
obnovováním vegetačních a biotechnických prvků v trati toku včetně vegetačního doprovodu
vhodným využíváním příbřežní zóny s retenčním efektem vegetačního krytu
Ke zlepšení kvality vody v toku přispívají následující opatření:
důsledná realizace protierozních opatření v povodí a provedení komplexních pozemkových úprav
regulace aplikování hnojiv a ochranných chemických prostředků
realizace ochranných pásem toků (jsou tvořena vegetačním krytem, který za normální hydrologické situace chrání toky a případně i nádrže před přímým vtokem znečištěných povrchových vod a současně také tvoří spolu s tokem kvalitní biokoridor).

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
Wiki
Vogel

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
WIKI Hyetograph

11. Extreme of rain events Statistic analysis of maximum rainfall events
I [mm/min]
D [min]
Return period [years]
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)
IDF
N [years] 2 10 20 50
100
H1d,N [mm]
36.3
60.6
70.4
82.6
92.1

12. Hydrograph Characteristics: 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
Flood frequency curve
probability distributions
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
218
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
Direct runoff
Interception + retention
Water harvesting 3.5 Rainfall-runoff relationship

15. Rainfall event – flood Český 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.
WIKI Curve number

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 The bigger CN the higher runoff volume Land name AMC I
AMC II
AMC III
Land ID
Land name
SCS Soil type A B C D 3
"Paved parking lots, roofs, driveways, etc. (excl. ROW)" 94 98 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 80 85 77 91 97 22
"Meadow - continuous grass, no grazing" 15 38 52 60 30 58 71 78 50 90 27
"Woods-grass combination - orchard - Fair" 25 45 43 65 63 31
"Woods - Good" 35 51 55 70 74 37
"Cultivated agr. - row - straight row(SR) - Good" 47 67 83
"Cultivated agr. - small grain - C - Good" 41 54 64 68 61 81 84 93
"Cultivated agr. - close-seeded - SR - Good"
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
0.2*A – initial abstraction in mm
Hs - 0.2*A – effective storm rainfall in mm
CN – curve number
for

20. distribution of runoff
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)
It’s been developed since and applied in many versions.
The use: hydrologic models – known rainfall depth → runoff volume → temporal distribution
WIKI cs Jednotkový hydrogram

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