Watershed morphology n Morphological properties of a watershed can affect the shape of the storm hydrograph and the delivery of sediment to the main channel n Various parameters can be calculated to describe the channel network and the physical characteristics of the watershed –these all affect hydrograph shape
Basin size n Delineate watershed according to the height of land that separates water draining to the point of interest from water that drains to adjacent basins n Watershed area (km 2, 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
Watershed land slope n 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 n This is simply the average gradient of hillslopes - slope is vertical over horizontal distance, derived from topographic maps n 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.
Area - elevation curve n Area - elevation is critical for modeling snowmelt n Can be useful in determining precipitation distribution from a ppt. - elevation relationship 240 Creek median elevation
Matching area- and ppt- elevation relationships can be used to compute basin average precipitation Precipitation- elevation relationship Area - elevation relationship
Indices of basin shape n Form factor –elongated - F.F. is low, flatter hydrograph –squatty - F.F. is high, peaked hydrograph
Strahler’s order of streams n A headwater stream with no tributaries is a first order stream n When two first order streams join they form a second order stream n Two second order streams form a third order stream n etc. 1 1 2 2 1 11 2 1 1 2 3 3 1
Bifurcation ratio n B i = ratio of # first order to # second order streams n If watershed is > 2nd order: Plot log Nu vs. u as shown, B i is the anti-log of the slope of the regression line. For the example given, B i = anti-log(0.693) = 4.93
Effect of B i on hydrograph shape Elongated basin B i is high (=13) flat hydrograph due to even supply of water to channel Rounder basin B i is low (= 4.9) peaked hydrograph because flow is concentrated Assuming uniform ppt. distribution, all other factors being equal...
Channel slope and profile n 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
Determining mean channel slope n Each tributary channel in a watershed has its own profile –commonly done only for the main channel n Calculate the slope of a line drawn such that the area under the line = the area under the main channel profile n An index of channel slope can be calculated from the slopes of n equal channel segments:
Drainage density n Drainage density is determined by measuring the total length of all streams on a map and dividing by the watershed area –units of km/km 2 –for comparative purposes, you must use maps with the same level of detail for all basins of interest n Effect on hydrograph shape: –high D d - peaked hydrograph –low D d - flat hydrograph
Valley flat n Area adjacent to stream or river floodplain where the slope is < 8% n Buffers the stream channel from landslides which may run out on the valley flat before depositing sediment in the channel. n Calculate the length of mainstem channel that has a valley flat, express as a proportion of the length of the mainstem channel.
Other factors n Lithology –importance: can govern slope stability, bedrock leakage, permeability n Presence or absence of glaciers –will govern timing and mangitude of peak runoff n Land use...
Precipitation - runoff n 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
Runoff coefficient n Simplest form of ppt - runoff relation –ratio of total streamflow over total precipitation n Runoff coefficient can be assessed annually, seasonally or monthly depending on purpose n Should be a characteristic quantity of a watershed assuming no change in land use
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.
Runoff coefficient 240 Creek Runoff threshold: water loss to ET Runoff coefficient increases with total precip.
Spring-summer runoff vs snowpack n This can be more meaningful than a runoff coefficient - e.g., 240 Creek, 1985-91 1990 - rain on snow late May
Predicting spring runoff in interior watersheds n 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 n 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
Use of snow course data to predict runoff n For an interior watershed, snow course data should provide a better measure of runoff n Used to predict inflows to reservoirs, potential floods n 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
Effect of antecedent conditions on rainfall - runoff relation n 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. n Not always possible to quantify these factors...
Antecedent Precipitation Index n 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
Synthetic unit hydrograph n It has been determined empirically that the parameters of the unit hydrograph - lag time, peak and time base - can be determined from basin morphology n lag time: (hours) LCLC L = length of main channel C t range 1.8 to 2.2
n Time base: (in days) n Peak flow: various formulae have been advanced to predict peak flow –Rational formula: Q p = 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 m 3 /s, A in km 2
Russell Creek 1991 - 92 R 2 = 83.8% Peak = 0.342 (24hr) + 1.17 Base R 2 = 92 %