1. Precipitation - Runoff Relations Watershed Morphology
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. Strahler’s 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. 1 1 1 1 2 1 2 1 1 2 2 3 1 3

9. Bifurcation ratio
Bi = ratio of # first order to # second order streams
If watershed is > 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 < 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 Presence or absence of glaciers Land use...
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 ratio Example: 240 Creek, UPC Water 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
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,
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 > 1: relationship is a very good predictor of snowmelt runoff but doesn’t account for precipitation that occurs after April 1 - doesn’t 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 day’s 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. Peak flow: various formulae have been advanced to predict peak flow
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 %