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
Ridge

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: 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/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
usf

50. Regression Analysis
works best for long time periods