Basic Hydrology

1. 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 • 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 ratioExample: 240 Creek, UPCWater 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 Runoff coefficient 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, 1985-91 1990 - 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. 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 % R2 = 83.8%