Watershed ManagementWater Budget, Hydrograph Analysis

HYDROLOGY AND WATER RESOURCES -RG744RS AND GISc, INSTITUTE OF SPACE TECHNOLOGY

OCTOBER 30, 2013

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)

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.)

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

Watershed Delineation

Watershed Delineation

Ridge

As a general rule, water flows downhill perpendicular to contour lines

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

Stream Order

1st through 12th Order by Strahler's (1952)

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 BalanceInflow = Outflow ± Change in Storage

Water Budget

Examples

(10 + 5 + 4) – 19 = 0

0 200 400 600 800

1000 1200 1400 1600 1800 2000

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

0.10

0.20

0.30

0.40

0.50

0.60

Rainfall Runoff Response

Flow Measuredfrom USGS Gage 403Inside Harris Gully

February 12, 1997 on Harris Gully

Rainfall Measuredfrom USGS Gage 400at Harris Gully Outlet

QRF

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

Rainfall Runoff Modeling

Surface Runoff

Includes: Overland flow Precipitation falling directly on stream channelsDepends 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

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 runoffStream flow = Direct Runoff + Base flowDirect Runoff (DRO) = Rainfall Excess or (rainfall – losses)

Source: Prof. Ke-Sheng Cheng

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)

Parts of Hydrograph

Rising Limb Crest or Peak: Maximum rate of flow for the event Falling limb or Recession Curve

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

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

Catchment Characteristics: Shape

Catchment with same area but with different shape Narrow ends towards outlet Slow rising hydrograph with lower

peak

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

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

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

Catchment Characteristics

Surface Roughness

Catchment Characteristics & Hydrograph Shape

Also need to consider the storm duration and time of concentration.Source: Prof Cheng, Taiwan

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

Base Flow Separation

Base Flow Separation: Straight Line Method

This method is not very accurate

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)

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 ends

Net Storm Rainfall and Hydrograph

Example 2-1 (Bedient)

Discrete step method for calculating area under hydrograph

Predicting Rainfall-Runoff

Regression Analysis

works best for long time periods