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Watershed Management Water Budget, Hydrograph Analysis

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Presentation on theme: "Watershed Management Water Budget, Hydrograph Analysis"— Presentation transcript:

1 Watershed Management Water Budget, Hydrograph Analysis
Hydrology and Water Resources -RG744 RS and GISc, Institute of Space Technology October 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 hydrology Picture 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 directions Slope = change in elevation (a rise) with a change in horizontal position (a run)

3 Watershed Watershed 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 network Defining a Stream: any cell that has a contributing watershed larger than some locally defined threshold Reference: Bolstad’s Fundamentals of GIS

5 Watershed Delineation
Source: Q1 = Qa+Qb

6 Watershed Delineation
Source: USEPA:


8 Source: Watershed Delineation: http://www. geo. brown

9 Source: Watershed Delineation: http://www. geo. brown

10 As a general rule, water flows downhill perpendicular to contour lines
Source: How to delineate watershed boundaries:

11 Source: How to delineate watershed boundaries: http://www. uri
When contour lines cross a stream, they form a “V” that always points uphill.

12 Source: How to delineate watershed boundaries: http://www. uri

13 Source: Bolstad

14 Basin Characteristics
Drainage Basin: Area draining to a common outlet Drainage Divide: separates two watersheds that drain into different outlets Drainage Area: Area encompassed by divide Drainage Density: Stream length/unit area Stream Order System: …. Stream gradient: Drop of elevation/unit length Stream Frequency: Number of channels/unit area Basin Relief: Highest elevation – lowest elevation Time of concentration: Time of travel from the farthest point in the catchment area to the gauging station Basin Area: Varies in size from few acres to thousands of square miles

15 Stream Order 1st 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:

17 Source: WMO, Guide to Hydrological Practices: ftp://ftp. wmo
Source: WMO, Guide to Hydrological Practices:

18 Inflow = Outflow ± Change in Storage
Water Budget/Balance Basin Hydrologic Mass Balance Inflow = Outflow ± Change in Storage Inflows Precipitation Surface water Inflow Groundwater Inflow Artificial Import (Pipes) Outflows Evaporation (surface water, land areas) Runoff Groundwater Outflow Artificial Export (withdrawal from surface and groundwater) Storage Surface water in streams, lakes and ponds Soil moisture Ice and snow on the surface Temporary depression storage Intercepted water on plant surfaces Groundwater An inventory of all sources, sinks, and storages is a Water Budget. (source: Source:

19 Water Budget Source:

20 Examples ( ) – 19 = 0






26 Rainfall Runoff Response
Flow Measured from USGS Gage 403 Inside Harris Gully RF Q Source: Hydrology and Floodplain Analysis: Philip B. Bedient Rainfall Measured from USGS Gage 400 at Harris Gully Outlet February 12, 1997 on Harris Gully


28 Rainfall and Runoff Analysis in Watersheds
To determine surface runoff from a watershed due to a particular storm Rainfall and Runoff Relationship Catchment may be treated as ‘black box’ having processes that control the rainfall to runoff transformation Catchment Input Output

29 Rainfall Runoff Modeling
Source: Water Resources Engineering By Larry W. Mays

30 Surface Runoff Includes: Overland flow
Precipitation falling directly on stream channels Depends on: Basin Characteristics Size, shape, slope, land use/cover, soil type, antecedent conditions Storm Characteristics Storm intensity, storm duration, spatial variation, movement Hydrograph Size, shape, condition of flow conveyance systems Source: condition of flow conveyance systems (sometimes called the degree of development of the flow system or drainage density).

31 Hydrograph Analysis Hydrologic response of rainfall at the outlet of an area Hydrograph: Graph of discharge (cubic feet per second) in a channel vs. time Area under curve yields the volume of runoff Stream flow = Direct Runoff + Base flow Direct 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 runoff Interflow or Quick Interflow: is runoff that infiltrates the top layers of soil and exits to stream prior to reaching zone of saturation Baseflow: Entry of groundwater into stream Delayed interflow: component of interflow which contributes to baseflow Groundwater 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 event Falling limb or Recession Curve

34 Factors Affecting Hydrograph Shape
Climatic Characteristics Rainfall intensity: higher intensity storm produces rapid rise in hydrograph and higher peak Rainfall duration: important when duration is more than time of concentration Temporal distribution: in summer greater losses lesser peak, in winter vice versa (also in winter soil moisture is high producing more runoff) Spatial distribution: ??? Catchment Characteristics Size Shape Elevation Slope Drainage density and topology Soil Type and land use Source 1: Engineering Hydrology by Ghumman Source 2:

35 Catchment Characteristics
Size of the Catchment Volume of runoff for a given rainfall input is proportional to the size of catchment But the response characteristics of a large catchment is different from a small catchment rainfall -runoff response for a smaller impervious catchments is different from a larger vegetative watersheds for a given rainfall Different 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 shape Narrow ends towards outlet Slow rising hydrograph with lower peak Same amount of water: as area and effective rainfall are assumed to be the same for both

37 Catchment Characteristics
Shape of Catchment Pear shaped catchment with narrow ends towards upstream and broader end near outlet Fast rising Hydrograph with high peak Water passing through outlets of both catchment is same Source:

38 Catchment Characteristics
Elevation Variation in temperature and precipitation at different elevation Temperature reduces with the increase in elevation and at very high altitude precipitation falls as snow The floods from snow melt are usually low peak and broader base

39 Catchment Characteristics
Slope Larger slopes generate more velocity than smaller slopes causing fast runoff Same rainfall input to 2 catchments of equal area but different slopes, the one with steeper slope generates a hydrograph with steeper rising and falling limbs Flatter slope: slow rising moderated hydrograph Figure source: Water Resources Engineering By Larry W. Mays.

40 Catchment Characteristics
Surface Roughness Other 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 University Also need to consider the storm duration and time of concentration. Source: Prof Cheng, Taiwan

42 Baseflow Separation What is observed flowing in the stream is the total discharge The combined hydrograph can be split up into two parts: the base flow and the overland flow added to interflow Process of separating the direct runoff from the base flow is called Baseflow Separation Methods of Baseflow Separation: Straight line method Empirical Method Inflection Point Method All methods are arbitrary and somewhat inaccurate Source: 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.

44 Base Flow Separation

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 Figure not very accurate method This 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 rainfall Inflection point on the falling limb is often assumed to be point where direct runoff ends Source: usf Also called concave method. Concave: spoon upright

48 Net Storm Rainfall and Hydrograph
Example 2-1 (Bedient) Discrete step method for calculating area under hydrograph

49 Predicting Rainfall-Runoff

50 Regression Analysis works best for long time periods


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