2Watershed morphologyMorphological properties of a watershed can affect the shape of the storm hydrograph and the delivery of sediment to the main channelVarious parameters can be calculated to describe the channel network and the physical characteristics of the watershedthese all affect hydrograph shape
3Basin sizeDelineate watershed according to the height of land that separates water draining to the point of interest from water that drains to adjacent basinsWatershed area (km2, ha)smaller watersheds tend to have a more peaked hydrograph, more intermittent water supplylarger watersheds have flatter hydrographs because larger channel network can store more water
4Watershed land slopeThe slope of the sides of a watershed govern how fast water will drain to the channelsteep slopes - peaked hydrographgentle slopes - flat hydrographThis is simply the average gradient of hillslopes - slope is vertical over horizontal distance, derived from topographic mapsAn objective repeatable formula for land slope:where L is the total length of contours,C.I. is the contour interval and A is thewatershed area.
5Area - elevation curveArea - elevation is critical for modeling snowmeltCan be useful in determining precipitation distribution from a ppt. - elevation relationship240 Creekmedian elevation
6Matching area- and ppt- elevation relationships can be used to compute basin average precipitation Area - elevationrelationshipPrecipitation-elevationrelationship
7Indices of basin shape Form factor elongated - F.F. is low, flatter hydrographsquatty - F.F. is high, peaked hydrograph
8Strahler’s order of streams A headwater stream with no tributaries is a first order streamWhen two first order streams join they form a second order streamTwo second order streams form a third order streametc.11112121122313
9Bifurcation ratioBi = ratio of # first order to # second order streamsIf watershed is > 2nd order:Plot log Nu vs. u as shown, Bi is the anti-logof the slope of the regression line. For theexample given, Bi = anti-log(0.693) = 4.93
10Effect of Bi on hydrograph shape Assuminguniform ppt.distribution,all other factorsbeing equal...Elongated basinBi is high (=13)flat hydrograph due to evensupply of water to channelRounder basinBi is low (= 4.9)peaked hydrograph becauseflow is concentrated
11Channel slope and profile Channel slope plays a role in the shape of the hydrographthe steeper the slope, the more peaked the hydrograph240 Creek channel profilemean channel slope
12Determining mean channel slope Each tributary channel in a watershed has its own profilecommonly done only for the main channelCalculate the slope of a line drawn such that the area under the line = the area under the main channel profileAn index of channel slopecan be calculated from theslopes of n equal channelsegments:
13Drainage densityDrainage density is determined by measuring the total length of all streams on a map and dividing by the watershed areaunits of km/km2for comparative purposes, you must use maps with the same level of detail for all basins of interestEffect on hydrograph shape:high Dd - peaked hydrographlow Dd - flat hydrograph
14Valley flatArea 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.
15Other factors Lithology Presence or absence of glaciers Land use... importance: can govern slope stability, bedrock leakage, permeabilityPresence or absence of glacierswill govern timing and mangitude of peak runoffLand use...
16Precipitation - runoff Methods have been developed to predict characteristics of runoff as a function of precipitation characteristicsvolume of runoffseasonalannualbased on seasonal or annual total precipitationpeak flowannual peak flow - e.g., snowmelt peak (interior), a function of peak snow accumulationstorm peaks - a function of rainfall intensity
17Runoff coefficient Simplest form of ppt - runoff relation ratio of total streamflowover total precipitationRunoff coefficient can be assessed annually, seasonally or monthly depending on purposeShould be a characteristic quantity of a watershed assuming no change in land use
18Calculating 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 ralationshipis to plot Q vs. P and fit a regression line.
19Runoff coefficient 240 Creek increases withtotal precip.Runoff threshold:water loss to ET
20Spring-summer runoff vs snowpack This can be more meaningful than a runoff coefficient - e.g., 240 Creek,rain on snow late May
21Predicting spring runoff in interior watersheds Unlike runoff coefficient relationship, relationship between spring - summer runoff and peak snowpack passes through the originthis shows that virtually all the snowpack contributes to spring - summer runoffSlope > 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
23Use of snow course data to predict runoff For an interior watershed, snow course data should provide a better measure of runoffUsed to predict inflows to reservoirs, potential floodsFor a coastal watershed, rainfall data is needed, but annual runoff coefficient is probably relatively meaninglessmonthly runoff ratio, averaged over several years may be usefulexpected to be much higher than for interior w/s
24Effect 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 hydrographwet antecedent conditions lead to more runoff per unit ppt., dry antecedent conditions result in more of the input water going to basin rechargeantecedent conditions are a function of ET and soil/groundwater drainage.Not always possible to quantify these factors...
25Antecedent 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 snowmeltrunoff coefficient will vary according to the API:the higher the API, the higher the runoff coefficient
28Synthetic 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 morphologylag time: (hours)LCL = length of mainchannelCt range 1.8 to 2.2
29Peak flow: various formulae have been advanced to predict peak flow Time base: (in days)Peak flow: various formulae have been advanced to predict peak flowRational formula: Qp = RIAwhere R = runoff coefficient, I = rainfall intensity and A = basin areaOther formulae:Cp range 0.15 to 0.19 per mmwith Q in m3/s, A in km2
30Russell Creek 1991 - 92 Peak = 0.342 (24hr) + 1.17 Base R2 = 92 %