Presentation on theme: "Watershed Management Water Budget, Hydrograph Analysis"— Presentation transcript:
1 Watershed Management Water Budget, Hydrograph Analysis Hydrology and Water Resources -RG744RS and GISc, Institute of Space TechnologyOctober 30, 2013
2 Watershed An area that contributes flow to a point on the landscape Basic hydrologic unit within which all measurements, calculations, and predictions are made in hydrologyPicture source:References:Bolstad (chp11): Vegetation and soils are also affected by slope and aspect in many regions of the world.The uphill area that drains to any point on a landscape is the watershed for that point. Water falling anywhere in the upstream area of a watershed will pass through that point.Slopes are used to define watershed boundaries, flow paths and directionsSlope = change in elevation (a rise) with a change in horizontal position (a run)
3 WatershedWatershed may be quite small (few square meters in an area on a ridge or high slope)May be quite large including continental areas that drains large rivers (Mississippi Rivers, Indus Basin, etc.)Any point in the main channel of large river has a large upstream watershed.
4 Drainage Network Set of cells through which surface water flows Convergence of flow direction may be used to produce streams or drainage networkDefining a Stream: any cell that has a contributing watershed larger than some locally defined thresholdReference: Bolstad’s Fundamentals of GIS
14 Basin Characteristics Drainage Basin: Area draining to a common outletDrainage Divide: separates two watersheds that drain into different outletsDrainage Area: Area encompassed by divideDrainage Density: Stream length/unit areaStream Order System: ….Stream gradient: Drop of elevation/unit lengthStream Frequency: Number of channels/unit areaBasin Relief: Highest elevation – lowest elevationTime of concentration: Time of travel from the farthest point in the catchment area to the gauging stationBasin Area: Varies in size from few acres to thousands of square miles
15 Stream Order1st through 12th Order by Strahler's (1952)1-3 also called ‘Headwaters Streams’ that serve as a critical hydrologic link between the surrounding landscape and the larger, connecting stream outflows.Anything larger than 4th up to 12th order is considered a river.Stream Order (Su): Stream ordering is the first step of quantitative analysis of the watershed. The stream ordering systems has first advocated by Horton (1945), but Strahler(1952) has proposed this ordering system with some modifications.
16 Source: WMO, Guide to Hydrological Practices: ftp://ftp. wmo Source: WMO, Guide to Hydrological Practices: ftp://ftp.wmo.int/Documents/MediaPublic/Publications/Guide_to_Hydrological_Practices/WMOENG.pdf
17 Source: WMO, Guide to Hydrological Practices: ftp://ftp. wmo Source: WMO, Guide to Hydrological Practices: ftp://ftp.wmo.int/Documents/MediaPublic/Publications/Guide_to_Hydrological_Practices/WMOENG.pdf
18 Inflow = Outflow ± Change in Storage Water Budget/BalanceBasin Hydrologic Mass BalanceInflow = Outflow ± Change in StorageInflowsPrecipitationSurface water InflowGroundwater InflowArtificial Import (Pipes)OutflowsEvaporation (surface water, land areas)RunoffGroundwater OutflowArtificial Export (withdrawal from surface and groundwater)StorageSurface water in streams, lakes and pondsSoil moistureIce and snow on the surfaceTemporary depression storageIntercepted water on plant surfacesGroundwaterAn inventory of all sources, sinks, and storages is a Water Budget. (source: Source:
28 Rainfall and Runoff Analysis in Watersheds To determine surface runoff from a watershed due to a particular stormRainfall and Runoff RelationshipCatchment may be treated as ‘black box’ having processes that control the rainfall to runoff transformationCatchmentInputOutput
29 Rainfall Runoff Modeling Source: Water Resources Engineering By Larry W. Mays
30 Surface Runoff Includes: Overland flow Precipitation falling directly on stream channelsDepends on:Basin CharacteristicsSize, shape, slope, land use/cover, soil type, antecedent conditionsStorm CharacteristicsStorm intensity, storm duration, spatial variation, movementHydrographSize, shape, condition of flow conveyance systemsSource:condition of flow conveyance systems (sometimes called the degree of development of the flow system or drainage density).
31 Hydrograph AnalysisHydrologic response of rainfall at the outlet of an areaHydrograph: Graph of discharge (cubic feet per second) in a channel vs. timeArea under curve yields the volume of runoffStream flow = Direct Runoff + Base flowDirect Runoff (DRO) = Rainfall Excess or (rainfall – losses)Source:Losses = interception, infiltration, depression storage, etc. (sometimes called basin recharge)The term recession curve is sometimes reserved for any descending limb of the hydrograph..Source: Prof. Ke-Sheng Cheng
32 Hydrograph Component Direct Runoff Surface runoffInterflow or Quick Interflow: is runoff that infiltrates the top layers of soil and exits to stream prior to reaching zone of saturationBaseflow: Entry of groundwater into streamDelayed interflow: component of interflow which contributes to baseflowGroundwater runoff: flow component contributed to the channel by groundwater (extremely slow)Surface Runoff: overland flow (sheet flow), shallow concentrated flow and open channel flow.Surface runoff includes all overland flow as well as all precipitation falling directly onto stream channels. Surface runoff is the main contributor to the peak discharge.Rising limb (mostly surface runoff), crest, recession limb (corresponds to water release from storage, lower part of it to groundwater flow contribution).Interflow is a rapid phenomenon when compare to baseflow but slower than surface runoff.Quick Interflow: which contributes to direct runoff
33 Parts of Hydrograph Rising Limb Crest or Peak: Maximum rate of flow for the eventFalling limb or Recession Curve
34 Factors Affecting Hydrograph Shape Climatic CharacteristicsRainfall intensity: higher intensity storm produces rapid rise in hydrograph and higher peakRainfall duration: important when duration is more than time of concentrationTemporal distribution: in summer greater losses lesser peak, in winter vice versa (also in winter soil moisture is high producing more runoff)Spatial distribution: ???Catchment CharacteristicsSizeShapeElevationSlopeDrainage density and topologySoil Type and land useSource 1: Engineering Hydrology by GhummanSource 2:
35 Catchment Characteristics Size of the CatchmentVolume of runoff for a given rainfall input is proportional to the size of catchmentBut the response characteristics of a large catchment is different from a small catchmentrainfall -runoff response for a smaller impervious catchments is different from a larger vegetative watersheds for a given rainfallDifferent response characteristics due to the relative importance of the different phases of runoff (overland flow, interflow, base flow, etc.)
36 Catchment Characteristics: Shape Catchment with same area but with different shapeNarrow ends towards outletSlow rising hydrograph with lower peakSame amount of water: as area and effective rainfall are assumed to be the same for both
37 Catchment Characteristics Shape of CatchmentPear shaped catchment with narrow ends towards upstream and broader end near outletFast rising Hydrograph with high peakWater passing through outlets of both catchment is sameSource:
38 Catchment Characteristics ElevationVariation in temperature and precipitation at different elevationTemperature reduces with the increase in elevation and at very high altitude precipitation falls as snowThe floods from snow melt are usually low peak and broader base
39 Catchment Characteristics SlopeLarger slopes generate more velocity than smaller slopes causing fast runoffSame rainfall input to 2 catchments of equal area but different slopes, the one with steeper slope generates a hydrograph with steeper rising and falling limbsFlatter slope: slow rising moderated hydrographFigure source: Water Resources Engineering By Larry W. Mays.
40 Catchment Characteristics Surface RoughnessOther factors are natural and channel storage, stream length, channel density, antecedent moisture conditions, vegetation etc.Figure source: Water Resources Engineering By Larry W. Mays
41 Catchment Characteristics & Hydrograph Shape Source: Prof. Ke-Sheng Cheng, National Taiwan UniversityAlso need to consider the storm duration and time of concentration.Source: Prof Cheng, Taiwan
42 Baseflow SeparationWhat is observed flowing in the stream is the total dischargeThe combined hydrograph can be split up into two parts: the base flow and the overland flow added to interflowProcess of separating the direct runoff from the base flow is called Baseflow SeparationMethods of Baseflow Separation:Straight line methodEmpirical MethodInflection Point MethodAll methods are arbitrary and somewhat inaccurateSource: Prof. Ke-Sheng Cheng
43 Splitting up of a complete stream flow hydrograph into its components requires the knowledge of the geology of the area and watershed characteristics.
45 Base Flow Separation: Straight Line Method The simplest one consists in arbitrarily selecting the beginning of the rising limb as the value of the baseflow and connecting this point with a horizontal line to a point in the recession limb of the hydrograph.join points X and Z as shown in Figurenot very accurate methodThis method is not very accurate
46 Base Flow Separation: Empirical Method N (Days)1. Extend base flow graph (Figure) along its general trend before the rise of the hydrograph up to a point P directly below the runoff hydrograph peak.2. From P, a straight line PQ is drawn to meet the hydrograph at point Q, separated from P in N days (N calculated by an empirical formula).extending a line from the beginning of the recession to a point directly beneath the peak discharge and then connecting this point to the beginning of the rising limb.N (in days) = A0.2 A = area of the drainage basin in square kilometers (N= A0.2 , A is in miles) N is from the point of peak discharge to the point where flow is completely dominated by base flow
47 Base Flow Separation: Inflection Point Method tp = Time to peak discharge from start of rainfallInflection point on the falling limb is often assumed to be pointwhere direct runoff endsSource: usfAlso called concave method.Concave: spoon upright
48 Net Storm Rainfall and Hydrograph Example 2-1 (Bedient)Discrete step method for calculating area under hydrograph