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Basic HydrologyBasic Hydrology
Precipitation - Runoff RelationsPrecipitation - Runoff Relations
Watershed MorphologyWatershed Morphology
Watershed morphologyWatershed morphology
Morphological properties of a watershed Morphological properties of a watershed can affect the shape of the storm can affect the shape of the storm hydrograph and the delivery of sediment to hydrograph and the delivery of sediment to the main channelthe main channel
Various parameters can be calculated to Various parameters can be calculated to describe the channel network and the describe the channel network and the physical characteristics of the watershedphysical characteristics of the watershed– these all affect hydrograph shapethese all affect hydrograph shape
Basin sizeBasin size Delineate watershed according to the height of Delineate watershed according to the height of
land that separates water draining to the point of land that separates water draining to the point of interest from water that drains to adjacent basinsinterest from water that drains to adjacent basins
Watershed area (kmWatershed area (km22, ha), ha)– smaller watersheds tend to have a more peaked smaller watersheds tend to have a more peaked
hydrograph, more intermittent water supplyhydrograph, more intermittent water supply– larger watersheds have flatter hydrographs because larger watersheds have flatter hydrographs because
larger channel network can store more waterlarger channel network can store more water
Watershed land slopeWatershed land slope The slope of the sides of a watershed govern how The slope of the sides of a watershed govern how
fast water will drain to the channelfast water will drain to the channel– steep slopes - peaked hydrographsteep slopes - peaked hydrograph
– gentle slopes - flat hydrographgentle slopes - flat hydrograph
This is simply the average gradient of hillslopes - This is simply the average gradient of hillslopes - slope is vertical over horizontal distance, derived slope is vertical over horizontal distance, derived from topographic mapsfrom topographic maps
An objective repeatable formula for land slope:An objective repeatable formula for land slope:
SL C I
A
( )( . .) where L is the total length of contours, C.I. is the contour interval and A is the watershed area.
Area - elevation curveArea - elevation curve Area - elevation is critical for modeling snowmeltArea - elevation is critical for modeling snowmelt Can be useful in determining precipitation Can be useful in determining precipitation
distribution from a ppt. - elevation relationshipdistribution from a ppt. - elevation relationship
1620 1680 1740 1800 1860 1920 1980 2040Elevation (metres above sea level)
0
50
100
Per
cen
t o
f A
rea
At
Or
Ab
ove
G
iven
Ele
vati
on
240 Creek
median elevation
Matching area- and ppt- elevation Matching area- and ppt- elevation relationships can be used to compute relationships can be used to compute
basin average precipitationbasin average precipitation
1620 1680 1740 1800 1860 1920 1980 2040Elevation (metres above sea level)
0
50
100
Per
cen
t o
f A
rea
At
Or
Bel
ow
G
iven
Ele
vati
on
700
750
800
Mea
n A
nn
ual
Pp
t. (
mm
)
Precipitation-elevation relationship
Area - elevationrelationship
Indices of basin shapeIndices of basin shape
Form factorForm factor
– elongated - F.F. is low, flatter hydrographelongated - F.F. is low, flatter hydrograph– squatty - F.F. is high, peaked hydrographsquatty - F.F. is high, peaked hydrograph
F FAverage Width
Axial Length. .
.
.
Strahler’s order of streamsStrahler’s order of streams
A headwater stream with no A headwater stream with no tributaries is a first order tributaries is a first order streamstream
When two first order When two first order streams join they form a streams join they form a second order streamsecond order stream
Two second order streams Two second order streams form a third order streamform a third order stream
etc.etc.
1
12
21
1 1
2 1
123
31
Bifurcation ratioBifurcation ratio BBii = ratio of # first order to # second order streams = ratio of # first order to # second order streams
If watershed is > 2nd order:If watershed is > 2nd order:
1 2 3Stream Order u
0.5
1.0
1.5
2.0
2.5
log
(#
Str
eam
s o
f O
rder
u)
log (Nu) = 2.77 -0.693 (u)
Plot log Nu vs. u as shown, Bi is the anti-logof the slope of the regression line. For the example given, Bi = anti-log(0.693) = 4.93
Effect of BEffect of Bii on hydrograph shape on hydrograph shape
Elongated basinBi is high (=13)flat hydrograph due to even supply of water to channel
Rounder basinBi is low (= 4.9)peaked hydrograph becauseflow is concentrated
Assuming uniform ppt.distribution,all other factorsbeing equal...
Channel slope and profileChannel slope and profile Channel slope plays a role in the shape of the Channel slope plays a role in the shape of the
hydrographhydrograph– the steeper the slope, the more peaked the hydrograph the steeper the slope, the more peaked the hydrograph
0 500 1000 1500 2000 2500 3000Distance from the W eir (m )
1600
1650
1700
1750
Ele
vati
on
Ab
ove
Sea
Lev
el (
m)
240 Creek channel profile
mean channel slope
Determining mean channel slopeDetermining mean channel slope Each tributary channel in a watershed has its own Each tributary channel in a watershed has its own
profileprofile– commonly done only for the main channelcommonly done only for the main channel
Calculate the slope of a line drawn such that the area Calculate the slope of a line drawn such that the area under the line = the area under the main channel profileunder the line = the area under the main channel profile
An index of channel slope An index of channel slope
can be calculated from the can be calculated from the
slopes of n equal channel slopes of n equal channel
segments:segments: Ss
nc
ii
n
1
2
Drainage densityDrainage density Drainage density is determined by measuring the Drainage density is determined by measuring the
total length of all streams on a map and dividing total length of all streams on a map and dividing by the watershed areaby the watershed area– units of km/kmunits of km/km22
– for comparative purposes, you must use maps with the for comparative purposes, you must use maps with the same level of detail for all basins of interestsame level of detail for all basins of interest
Effect on hydrograph shape:Effect on hydrograph shape:– high Dhigh Ddd - peaked hydrograph - peaked hydrograph
– low Dlow Ddd - flat hydrograph - flat hydrograph
Valley flatValley flat
Area adjacent to stream or river floodplain Area adjacent to stream or river floodplain where the slope is < 8%where the slope is < 8%
Buffers the stream channel from landslides Buffers the stream channel from landslides which may run out on the valley flat before which may run out on the valley flat before depositing sediment in the channel.depositing sediment in the channel.
Calculate the length of mainstem channel Calculate the length of mainstem channel that has a valley flat, express as a proportion that has a valley flat, express as a proportion of the length of the mainstem channel.of the length of the mainstem channel.
Other factorsOther factors
LithologyLithology– importance: can govern slope stability, bedrock importance: can govern slope stability, bedrock
leakage, permeabilityleakage, permeability Presence or absence of glaciersPresence or absence of glaciers
– will govern timing and mangitude of peak will govern timing and mangitude of peak runoffrunoff
Land use...Land use...
Precipitation - runoffPrecipitation - runoff Methods have been developed to predict Methods have been developed to predict
characteristics of runoff as a function of precipitation characteristics of runoff as a function of precipitation characteristicscharacteristics– volume of runoffvolume of runoff
» seasonalseasonal» annualannual» based on seasonal or annual total precipitationbased on seasonal or annual total precipitation
– peak flowpeak flow» annual peak flow - e.g., snowmelt peak (interior), a function of annual peak flow - e.g., snowmelt peak (interior), a function of
peak snow accumulationpeak snow accumulation» storm peaks - a function of rainfall intensitystorm peaks - a function of rainfall intensity
Runoff coefficientRunoff coefficient
Simplest form of ppt - runoff relationSimplest form of ppt - runoff relation– ratio of total streamflowratio of total streamflow
over total precipitationover total precipitation Runoff coefficient can be assessed Runoff coefficient can be assessed
annually, seasonally or monthly depending annually, seasonally or monthly depending on purposeon purpose
Should be a characteristic quantity of a Should be a characteristic quantity of a watershed assuming no change in land usewatershed assuming no change in land use
RQ
P
Calculating rainfall - runoff ratioCalculating rainfall - runoff ratioExample: 240 Creek, UPCExample: 240 Creek, UPC
Water year Sept - AugWater year Sept - Aug
Q P R1987-88 236 640 0.371988-89 283 713 0.401989-90 522 859 0.611990-91 425 738 0.58
Since R is related to P or Q, a better way to get the ralationshipis to plot Q vs. P and fit a regression line.
Runoff coefficient 240 CreekRunoff coefficient 240 Creek
0 200 400 600 800 1000Total Annual Precipitation (m m)
0
200
400
600
To
tal A
nn
ual
Str
eam
flo
w (
mm
)
Q = 1.163 (P) - 474R squared = 83.5%
Runoff threshold: water loss to ET
Runoff coefficientincreases withtotal precip.
Spring-summer runoff vs snowpackSpring-summer runoff vs snowpack This can be more meaningful than a runoff This can be more meaningful than a runoff
coefficient - e.g., 240 Creek, 1985-91coefficient - e.g., 240 Creek, 1985-91
120 160 200 240 280 320Snowpack, April 1 (m m)
100
200
300
400
500
To
tal A
pri
l - J
uly
Str
eam
flo
w (
mm
)
Q = 1.355 (S)R squared = 99.6%
1990 - rain on snow late May
Predicting spring runoff in Predicting spring runoff in interior watershedsinterior watersheds
Unlike runoff coefficient relationship, relationship Unlike runoff coefficient relationship, relationship between spring - summer runoff and peak between spring - summer runoff and peak snowpack passes through the originsnowpack passes through the origin– this shows that virtually all the snowpack contributes to this shows that virtually all the snowpack contributes to
spring - summer runoffspring - summer runoff
Slope > 1: relationship is a very good predictor of Slope > 1: relationship is a very good predictor of snowmelt runoff but doesn’t account for snowmelt runoff but doesn’t account for precipitation that occurs after April 1 - doesn’t precipitation that occurs after April 1 - doesn’t work for unusual conditions such as rain-on-snowwork for unusual conditions such as rain-on-snow
Precipitation & temperaturePrecipitation & temperature
Jan Feb M ar Apr M ay Jun Jul Aug Sep O ct N ov D ec
0
40
80
120
To
tal M
on
thly
Pre
cip
itat
ion
(m
m)
-20
-10
0
10
20
Mea
n M
on
thly
Tem
per
atu
re (
deg
C)
Total M onthly P recip itationM ean M onthly M ax Tem peratureM ean M onthly M in Tem perature
Use of snow course data to Use of snow course data to predict runoffpredict runoff
For an interior watershed, snow course data should For an interior watershed, snow course data should provide a better measure of runoffprovide a better measure of runoff
Used to predict inflows to reservoirs, potential floodsUsed to predict inflows to reservoirs, potential floods For a coastal watershed, rainfall data is needed, but For a coastal watershed, rainfall data is needed, but
annual runoff coefficient is probably relatively annual runoff coefficient is probably relatively meaninglessmeaningless– monthly runoff ratio, averaged over several years monthly runoff ratio, averaged over several years
may be usefulmay be useful– expected to be much higher than for interior w/sexpected to be much higher than for interior w/s
Effect of antecedent conditions Effect of antecedent conditions on rainfall - runoff relationon rainfall - runoff relation
The amount of soil moisture prior to a storm will The amount of soil moisture prior to a storm will affect the runoff ratio for that storm, and will affect affect the runoff ratio for that storm, and will affect the shape of the hydrographthe shape of the hydrograph– wet antecedent conditions lead to more runoff wet antecedent conditions lead to more runoff
per unit ppt., dry antecedent conditions result in per unit ppt., dry antecedent conditions result in more of the input water going to basin rechargemore of the input water going to basin recharge
– antecedent conditions are a function of ET and antecedent conditions are a function of ET and soil/groundwater drainage.soil/groundwater drainage.
Not always possible to quantify these factors...Not always possible to quantify these factors...
Antecedent Precipitation IndexAntecedent Precipitation Index API is a method of accounting for daily API is a method of accounting for daily
changes in water balance.changes in water balance.– API is a decay factor - each days API is a fixed API is a decay factor - each days API is a fixed
percentage of the previous day’s API (e.g., percentage of the previous day’s API (e.g., 90%), plus daily rainfall and/or snowmelt90%), plus daily rainfall and/or snowmelt
– runoff coefficient will vary according to the runoff coefficient will vary according to the API:API:
» the higher the API, the higher the runoff coefficientthe higher the API, the higher the runoff coefficient
API for Russell Creek Jan 1992API for Russell Creek Jan 1992
0 10 20 30
0
40
80
Dai
ly R
ain
fall
(mm
)
0
100
200A
PI (
mm
)
API for Russell Creek Jul 1992API for Russell Creek Jul 1992
0 10 20 30
0
4
8
Dai
ly R
ain
fall
(mm
)
0
20
40A
PI (
mm
)
Synthetic unit hydrographSynthetic unit hydrograph It has been determined empirically that the It has been determined empirically that the
parameters of the unit hydrograph - lag parameters of the unit hydrograph - lag time, peak and time base - can be time, peak and time base - can be determined from basin morphologydetermined from basin morphology
lag time: (hours)lag time: (hours)
t C LLp t C0 3.
LC
L = length of mainchannelCt range 1.8 to 2.2
Time base: (in days)Time base: (in days)
Peak flow: various formulae have been advanced to Peak flow: various formulae have been advanced to predict peak flowpredict peak flow– Rational formula: QRational formula: Qpp = RIA = RIA
where R = runoff coefficient, I = rainfall intensity and A where R = runoff coefficient, I = rainfall intensity and A = basin area= basin area
– Other formulae: Other formulae:
Tt p 3
3
24
QC A
tp
p
p
Cp range 0.15 to 0.19 per mmwith Q in m3/s, A in km2
Russell Creek 1991 - 92Russell Creek 1991 - 92
0 20 40 60 80 100Max 24-hour Storm Intensity (m m)
0
10
20
30
40
Pea
k F
low
(m
3/s)
R2 = 83.8%
Peak = 0.342 (24hr) + 1.17 BaseR2 = 92 %