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Watershed ManagementWater Budget, Hydrograph Analysis
HYDROLOGY AND WATER RESOURCES -RG744
RS AND GISc, INSTITUTE OF SPACE TECHNOLOGY
OCTOBER 30, 2013
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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
• 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)
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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.)
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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
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Watershed Delineation
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Watershed Delineation
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Ridge
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As a general rule, water flows downhill perpendicular to contour lines
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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
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Stream Order
1st through 12th Order by Strahler's (1952)
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Water Budget/Balance
• 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
• Basin Hydrologic Mass Balance
Inflow = Outflow ± Change in Storage
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Water Budget
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Examples
(10 + 5 + 4) – 19 = 0
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0
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6:00 0
8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 0 Time
0.00
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Rainfall Runoff Response
Flow Measuredfrom USGS Gage 403Inside Harris Gully
Flow Measuredfrom USGS Gage 403Inside Harris Gully
February 12, 1997 on Harris Gully
Rainfall Measuredfrom USGS Gage 400at Harris Gully Outlet
Rainfall Measuredfrom USGS Gage 400at Harris Gully Outlet
QRF
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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
CatchmentInput Output
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Rainfall Runoff Modeling
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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
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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: Prof. Ke-Sheng Cheng
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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)
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Parts of Hydrograph
Rising Limb
Crest or Peak: Maximum rate of flow for the event
Falling limb or Recession Curve
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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
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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
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Catchment Characteristics: Shape
Catchment with same area but with different shape
Narrow ends towards outlet
Slow rising hydrograph with lower peak
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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
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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
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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
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Catchment Characteristics
Surface Roughness
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Catchment Characteristics & Hydrograph Shape
Also need to consider the storm duration and time of concentration.Source: Prof Cheng, Taiwan
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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
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Base Flow Separation
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Base Flow Separation: Straight Line Method
This method is not very accurate
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Base Flow Separation: Empirical Method
N (in days) = 0.862 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
N (Days)
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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
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Net Storm Rainfall and Hydrograph
Example 2-1 (Bedient)
Discrete step method for calculating area under hydrograph
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Predicting Rainfall-Runoff
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Regression Analysis
works best for long time periods
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