Lake Tana Flood Zone Mapping Using GIS Tana Flood zone mapping.pdf · Write the output name as...

Regional Training Course on Integrated Flood Management (IFM)

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Lake Tana Flood Zone Mapping Using GIS

Regional Training Course in Integrated Flood Management

By Abeyou Wale

Bahir Dar University, Bahir Dar

Ethiopia

June 07-11/2010


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Part �.......................................................................................................................................................... 3

Generating Bathometric map of Lake Tana and establishing Volume-Elevation and Volume-Area curves..................................................................................................................................................................... 3

1.1. Data format preparation (*.xls to *.csv)....................................................................................... 3 1.2. Importing bathometric data to ILWIS Environment:................................................................... 3 1.3. Table to point map........................................................................................................................ 6

1.4. Generating the surface of the lake bed / Interpolation ................................................................. 7 1.5. Opening the digital elevation mode and making profiles ............................................................ 9 1.6. Merging Bathometric data to the SRTM DEM.......................................................................... 14 1.7. Volume-Area and Volume-Elevation Relationship of Lake Tana basin.................................... 19

1.8. Fitting Volume-Elevation and Volume-Area relationship for the live storage of Lake Tana.... 25

Part �........................................................................................................................................................ 26

Areal Rainfall Estimation of Lake Tana Basin ......................................................................................... 26

2.1. Station location data format preparation “*.xls to *.csv) file format......................................... 27 2.2. Importing Rainfall station location data to ILWIS Environment:.............................................. 27 2.3. Converting attribute table to a point map................................................................................... 28 2.4. Nearest point interpolation of Metrological station................................................................... 29 2.5. Calculating the weights of the stations....................................................................................... 32 2.6. Areal rainfall estimation............................................................................................................. 32

Part �........................................................................................................................................................ 34

Lake Tana Open water Evaporation ......................................................................................................... 34

Part �........................................................................................................................................................ 36

Runoff from rivers /inflow from rivers/.................................................................................................... 36

4.1. Runoff from gauged catchment ..................................................................................................... 36

4.2. Runoff from ungauged rivers......................................................................................................... 37

Part �........................................................................................................................................................ 38

River outflow and lake level ..................................................................................................................... 38

Part �........................................................................................................................................................ 39

Water Balance Model / Lake level simulation.......................................................................................... 39

Part �........................................................................................................................................................ 41

Flood Extent by GIS ................................................................................................................................. 41

7.1. Observing the lowest point of the basin..................................................................................... 41 7.2. Zoning dead storage of the lake ................................................................................................. 43 7.3. Observing the extent of historical maximum flood level........................................................... 44 7.4. Flood zoning of Take Tana ........................................................................................................ 45


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Part ��

Generating Bathometric map of Lake Tana and establishing Volume-Elevation and Volume-Area curves

1.1. Data format preparation (*.xls to *.csv)

o Open Bathometric data from the working directory (C:\IFM\Day5) o Click on File > Save As .. choose CSV (comma delimited in the save as type) then click on save

button followed by Yes o Keep the file name as “Bathometric data”

1.2. Importing bathometric data to ILWIS Environment:

o Opening ILWIS : go to Desktop and double click on ILWIS icon o Click on the Navigator tab under operation tree as shown below, and navigate to the working

folder (C:\IFM\Day5)


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o Importing the CSV bathometric data: click on File menu > Import > Table / select “Bathometric

data.csv” then click Next

o Click Next accepting the default Use ILWIS import o Click next accepting the default comma delimited type of table o You can see the attributes of the “Bathometric data” CSV file, observe that there is a label at the

top of the table and it has 4 attributes ID, E, N and Lake floor altitude; click on Next button


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o The next step is Editing the table column and assigning the type of domain for the attribute

� Double click on “Column1”, “Column2”,”Column3” and “Column4” and edit them as shown below

� Assign the domain type for “E”, “N” and “ALT” value by double clicking on the assigned domain type

� Name “Column1” as “ID” and assign a domain ID and number of line to skip will be one which is the header file finally click on Next button followed by finish

o Now after some time you will be able to see a table called “Bathometric data” under your

working folder of ILWIS catalog


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o Double click on the imported attribute table “Bathymetric data”; there are a total of 4660 sample points collected in and around Lake Tana.

1.3. Table to point map

o Go to Operation menu of ILWIS main Window > Table operation > Table to point map � Select “Bathymetric data” attribute on the table list box � X column will be “E” and “N” for Y column; Coordinate system will be “UTM_m” � Write “Bath_Point” for the output point map and click on show button


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� The output map will show as the pattern of data collection

1.4. Generating the surface of the lake bed / Interpolation

o Go to Operation tree of ILWIS main window .> Interpolation > Point interpolation > o Moving average

� On the point map list box choose “Alt” on the bathymetric attribute after clicking on the plus sign

� Put 2 for weighting exponent and 8000 limiting distance � Write the output name as “Bath_Raster” and use “Submap_gilgelabay” as a

GeoReference from the list box (see below) � Accept the default value range and precision � Finally click on show button; after some time you will be able to see the result of the

interpolation

Inverse distance weighted (IDW) interpolation determines cell values using a linearly weighted combination of a set of sample points. The weight is a function of inverse distance. By defining the higher {power} option, more emphasis can be put on the nearest points. Thus, nearby data will have the most influence, and the surface will have more detail (be less smooth). As the power increases, the interpolated values begin to approach the value of the nearest sample point. Specifying a lower value for power will provide a bit more influence to surrounding points a little farther away.


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� Accept the default and click ok

� Click on Add layer toolbar from the opened interpolated map “Bath_Raster” choose “Lake” segment map: choose Black single color and line width of 3 in the display option dialog box then click on ok button. The result is shown below.


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� You can also Add the “Bath_Point” sample points collected, by clicking on Add layer toolbar and selecting “Bath_point” map

� When the Display option appears: click on symbol button and make the size to 3 to make it smaller

1.5. Opening the digital elevation mode and making profiles

o Open the digital elevation model: double click on “TanaBasinDEM”; accept the default display option and click ok, click on add layer and select “Bath_Raster” with 50% Transparent under display option, add layer “Tana” segment map with Black single color and line width of 3 in the display option dialog box finally add layer “Cross section” segment map with Black single color and line width 3 in the display option dialog box

o Go to File menu > open pixel information, move the pixel information dialogue box to the upper write corner of you window then, move your curser inside the lake boundary segment map and at outside the lake segment map boundary. Observe the difference from the pixel information dialogue box.

o Observe; inside the lake boundary “TanaBasinDEM” measures 1786 except on the island area which is the lake level and the “Bath_Raster” reads elevation below 1786 which is the reduced level of the lake bed. What do you think the difference between those two maps “TanaBasinDEM” and “Bath_Raster” inside the lake boundary.


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o Making a profile on the interpolated bathymetric map “Bath_Raster” and SRTM DEM

“TanaBasinDEM” for the given “cross-section” segment map from NW to SE � Converting the line segment to a point map by distance method: go to

Operation menu > vectorize > segment to point / choose the cross section segment map on segment map list box

� Use the method distance put 300 m and output file name as cross section

� Click on show button, accept the default display option and click on ok

� You can zoom in by tool bar to see the point map in detail o Extracting the elevation values of those points from the SRTM DEM “TanaBasinDEM”

and interpolated Bathymetric map “Bath_Raster” � Opening the point map “cross section” as a table : right click on “cross section”

point map from ILWIS catalog and click on open as table

� Activating the command line of the attribute: go to View menu of the attribute table and click on command line, now the command line will be visible

� See that there are around 225 points in the “cross section” point map and the first


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attribute is Coordinate which is the X,Y location of points

� Extracting elevation value of points from SRTM DEM: write this small script on the command line of the attribute table

Profile_DEM=mapvalue(TanaBasinDEM,coordinate) � Click on ok button, now a new attribute table is created with attribute name

Profile_DEM: which is the Z value of the cross section points over SRTM DEM � Extracting elevation value of cross-section points on the Bath_Raster (lake bed level)

model: write this small script on the command line of the attribute table Profile_BATH=mapvalue(Bath_Raster,coordinate) � Click on ok button accepting the default, now a new attribute table is created with

attribute name Profile_BATH: which is the Z value of the cross section points map across the lake bed level

� After extracting the elevation values of the point map over the two raster maps the result is shown below:

MAPVALUE( ) function Returns the value, class, or ID of a map Map at a certain (X,Y) Coordinate. Syntax MAPVALUE(Map, Coordinate) Input Map is a raster, polygon, segment or point map Coordinate is an (X,Y) coordinate Domain type: a coordinate system Output MAPVALUE returns: a value, a class name or an ID

Domain: same as input Map


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� The next procedure will be plotting and observing the difference: go to Edit menu of the attribute table > select all

� Go to Edit menu > copy � Open MS Excel: go to Start > Program files > Microsoft Excel > Microsoft

Office Excel then go to edit menu and paste the coped ILWIS file � Delete the first three columns which are not important to plot the profile and

add a new column on the left side for the cumulative distance with 300 m interval starting from 0 distances.

� Plotting the profile: go to insert menu chart of (office 2003) XY scatter chart type


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� Click on Next button and click on serious tab followed by clicking add button

� For Serious1 give a name DEM, X value will be Distance (from 0 distance column) and Y values be Pofile_DEM column

� Now you will be able to see the preview

� Click on Add button to add the second serious Distance VS Profile_BATH � For Serious2 give a name BATH, X value will be Distance (from 0) and Y

values be Pofile_BATH � Click on next and enter the following information


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o Finally don’t forget to save the excel file on the working directory D:\ IFM \ day5 as “NW to SE

section”. Later we will use it.

1.6. Merging Bathometric data to the SRTM DEM

This procedure is used to merge the bathometry data into the digital elevation model at water surface (elevation of 1786)

Substituting the Bath_raster data into the DEM at the water surface


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o Go to Operation menu > Raster operation > Map calculation : Write the following small script on the expression text box

iff(TanaBasinDEM=1786,Bath_Raster, TanaBasinDEM) This script will substitute 1786 value of the DEM by the respective interpolated bathymetric data, for the values different from 1786 the digital elevation model will be kept the same o Write the output raster name as “BathDEM” and make the domain Value from the list box,

see figure below:

o Accept the default for the value range and precision and click ok o A new map is calculated by combining the digital elevation model and the Bathometry

data o Add the layer lake segment map and observe that there is no 1786 value inside the lake

boundary which was the lake level, now 1786 is substituted by the lake bed level, in other words “BathDEM” is a without water map.

IFF( ) function IFF(a, b, c) Input a Is the test condition: a boolean expression containing at least one map name or one column name. b, c expression containing at least one map name or one column name, or simply a value, class name, ID, etc. Output IFF returns: If a=true, b is returned;

if a=false, c is returned; if a=undefined, undefined is returned.


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Making the profile for the new “BathDEM” raster map o Opening the “cross section” point map as table: right click on Cross section point map

and click on open as table � Extracting elevation value of cross-section points on the “BathDEM” (lake bed

level + DEM data model): write this small script on the command line of the attribute table

BathDEM=mapvalue(BathDEM,coordinate) � Click enter and accept the default values of column properties dialogue box

� Click on the new attribute “BathDEM”, make a right click and copy its contents � Open the saved previous excel file name “NW to SE Section” � Paste the copied column and plot the BathDEM against the cumulative distance as a

third series as shown below. Profile for SRTM DEM, interpolated bath and for the merged one


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Shade view of Lake Tana basin


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1.7. Volume-Area and Volume-Elevation Relationship of Lake Tana basin

Volume-Area and Volume-Elevation relation are used for simulation of lake level, computation of Volume-Area and Volume-Elevation will be computed in ArcGIS after exporting *.asc file format o Exporting “BathDEM” ILWIS raster file to Arc/Info ASCII (.ASC) file format

� Right click on “BathDEM” raster file > Slect Export .. choose Arc/Info ASCII (.ASC) form the format list box, direct the output to the working directory

� Finally click on ok button

o Importing the exported “BathDEM” Asc file in to ArcGIS environment

� Opening ArcGIS: go to Start > program files > ArcGIS > ArcMap finally click on ok to open a new empty map

� Click on add data button to add the exported ASC file format: go to the working directory select “BathDEM” and click on Add button, click on ok; ok button

� Double click on “BathDEM” at the table of contents: layer properties dialogue box will appear then click on Symbology followed by classified 30 classes and click on ok button


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Volume-Area and Volume-Elevation relationships can be determined using ArcGIS 3D Analyst > Surface Analyst > Area Volume Statistics Tool, the interpolated bathymetric model “BathDEM” will be sliced at 40 cm interval from the bottom of the lake 1776.2 and the respective volume, surface area and elevation will be calculated. Finally Volume-Area and Volume-Elevation relationships will be fitted by a polynomial trend to be used in the lake level simulation model.

o Slicing the “BathDEM” at 40 cm interval and calculating Area, Volume and Elevation � Adding 3D Analyst toolbar: right click on any toolbar > select 3D analyst. 3D

analyst toolbar will be visible on the working area � Click on the down arrow next to 3D analyst button > Surface analyst > area and

volume, as shown below:


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� Observe that minimum and maximum values of the “BathDEM” are 1772.6 and

4107.0 respectively � Click on calculate statistics below plane radio button and put a check mark on

save/append statistics to text file; finally show the directory for the output text file and write “Area Volume” as the file name in the working directory

� Start the calculation from the lowest point 1772.6 m height of plane and click on

Calculate statistics � The result is shown below


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� Continue the calculation by entering height of the plane for 1773.0, 1773.4, 1773.8,

1774.2, 1774.6, 1775 etc up to 1791 followed by clicking Calculate statistics button; actually the historical maximum water level of the Lake Tana is 1788.02 m observed 21-Sep-1998

� Finally after calculating at 40cm interval from 1772.6 to 1791.0 click on Done button

� Go to window explorer and open the calculation statistics “Area Volume” as text file or as excel file

The area and volume are calculated between the reference plane and the surface. The {reference_plane} argument determines whether these calculations are performed above or below the plane. Use ABOVE or BELOW keywords to specify which option to use. When using ABOVE, the projected area and surface area for the portion of the surface above the given {base_z} are determined. The volume represents the cubic area between the plane and the underside of the surface. When using BELOW, the areas for the portion of the surface below the given {base_z} are determined. Volume is the cubic area between the plane and the top of the surface. The default {reference_plane} is ABOVE.


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� After arranging the excel file the next result will be achieved

Elevation Area km2

Volume Mm3 Elevation

Area km2

Volume Mm3 Elevation

Area km2

Volume Mm3

1772.6 0.00 0.00 1779.0 2258.49 7834.90 1785.4 3020.47 25354.33 1773.0 9.05 0.27 1779.4 2336.52 8754.00 1785.8 3031.37 26563.41 1773.4 92.01 18.67 1779.8 2411.97 9704.14 1786.2 3038.03 27775.99 1773.8 283.91 97.08 1780.2 2481.45 10683.01 1786.6 3043.02 28990.75 1774.2 584.78 270.17 1780.6 2551.81 11688.79 1787.0 3050.24 30208.02 1774.6 802.39 540.18 1781.0 2619.54 12722.58 1787.4 3063.99 31430.98 1775.0 1057.93 904.96 1781.4 2679.81 13782.16 1787.8 3079.57 32657.52 1775.4 1228.34 1362.60 1781.8 2736.44 14865.27 1788.2 3116.92 33898.05 1775.8 1389.52 1891.18 1782.2 2789.55 15970.42 1788.6 3146.22 35145.93 1776.2 1536.12 2477.45 1782.6 2840.00 17095.29 1789.0 3192.79 36409.50 1776.6 1650.92 3112.87 1783.0 2884.95 18239.57 1789.4 3231.20 37697.00 1777.0 1757.97 3792.82 1783.4 2920.03 19399.96 1789.8 3265.67 38992.50 1777.4 1863.53 4517.29 1783.8 2950.14 20573.43 1790.2 3323.23 40316.84 1777.8 1973.88 5285.66 1784.2 2974.55 21757.47 1790.6 3360.74 41647.27 1778.2 2082.12 6097.87 1784.6 2993.75 22950.03 1791.0 3414.32 42997.56 1778.6 2174.31 6948.85 1785.0 3008.02 24149.03


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� Potting Volume-Area and Volume-Elevation by XY scatter chart type � To create secondary axis, click on elevation Volume-Elevation relation and choose

format data serious > Axis > click on secondary axis radio button � To reverse elevation reading double click on secondary axis (elevation) > scale tab

> put a check mark on values in reverse order

0.00

500.00

1000.00

1500.00

2000.00

2500.00

3000.00

3500.00

4000.00

0.00 10000.00 20000.00 30000.00 40000.00Volume (Mm3)

Are

a (k

m2)

1770.0

1775.0

1780.0

1785.0

1790.0

1795.0

Ele

vati

on

(m

am

sl)

Area km2 Elevation

Polynomial fitted Volume-Area and Volume-Elevation

A = 1E-09V3 - 3E-05V2 + 0.450V + 369.9R² = 0.975

E= 3E-13V3 - 2E-08V2 + 0.000V + 1773.R² = 0.995

1770.0

1772.0

1774.0

1776.0

1778.0

1780.0

1782.0

1784.0

1786.0

1788.0

1790.0

1792.00.00

500.00

1000.00

1500.00

2000.00

2500.00

3000.00

3500.00

0.00 7500.00 15000.00 22500.00 30000.00 37500.00

Are

a (k

m2 )

Volume (Mm3)

Vol-Elev Log. (Vol-Area) Poly. (Vol-Elev)


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Lake Tana is totally controlled as long as the water level of the lake remains lower than the elevation of the spillway (1987 m amsl), the minimum operating level of the weir is 1784 m amsl. The lake level above 1784 is the live storage , Therefore we can fit this part of the lake level from 1784 up to 1789 m amsl where 1788.2 was the maximum historical lake level

1.8. Fitting Volume-Elevation and Volume-Area relationship for the live storage of Lake Tana

o Right click on Volume-Area relationship > click on add trend > select polynomial degree 3 go to option tab and put a check mark on display equation on chart and display R-squared value on chart, repeat this procedure for the Volume-Elevation relation

A = 1E-10*V3 - 8E-06*V2 + 0.2236*V + 853.68 R2 = 0.9984 E = -1E-09*V2 + 0.0004*V + 1776.2 R2 = 1

E = -1E-09*V2 + 0.0004*V + 1776.2

R2 = 1

A = 1E-10*V3 - 8E-06*V2 + 0.2236*V + 853.68

R2 = 0.9984

2500

2600

2700

2800

2900

3000

3100

3200

3300

10000 15000 20000 25000 30000 35000 40000

Volume Mm3

1780

1781

1782

1783

1784

1785

1786

1787

1788

1789

1790

Vol-Area Vol-Elev


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Part ��

Areal Rainfall Estimation of Lake Tana Basin Under the working folder, there is excel file with name “rainfall data plus station location”; open and observe that there are two worksheets with name “station location” and “daily rainfall”. The nearby metrological stations around Lake Tana are Delgi, Addis Zemen, Bahir Dar, Deke Estifanos, Chawhit and Zege, there X,Y location and daily rainfall data from 1995 to 2000 is given Station location

ID Stat Name X Y 1 Delgi 288661.93 1351767.76 2 Addis Zemen 356387.52 1346685.92 3 Bahir Dar 325816.75 1282862.45 4 Deke Estifanos 311574.51 1315964.26 5 Chawhit 306437.34 1363564.65 6 Zege 316875.96 1291596.44

Daily rainfall data

Objective of the procedure

� Importing the XY location of the stations in ILWIS environment as a table � Converting the table to point map � Interpolating the point map by Nearest point method /Thiesson polygon method � Masking the Thiesson polygon by Lake Tana Raster map and calculating the weights of the

stations across the Lake Tana Areal rainfall.


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2.1. Station location data format preparation “*.xls to *.csv) file format

o Open “Rainfall data plus Station Location.xls” from the working directory, activate the station location worksheet

o Click on File > Save As .. choose CSV (comma delimited in the save as type) and write the file name as “rainfall station location” then click on save button followed by Yes. Finally don’t forget to close the excel file, this is because file sharing is not allowed.

2.2. Importing Rainfall station location data to ILWIS E nvironment:

o Opening ILWIS by double clicking on desktop ILWIS icon o Click on the navigation tab under operation tree and navigate to the working folder o Importing the CSV rainfall station location data: click on File menu > Import > Table /

select “rainfall station location.csv” click Next

o Click on next button, accepting the default use ILWIS import and click next accept also

Comma delimited and click next see the prieview and click next o Edit the columns properties as shown below then click on next button and finally accept

the default out put table name and click Finsh; as shown below


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o Now station location table is imported: double click on “rainfall station location” attribute

table: the result is shown below

2.3. Converting attribute table to a point map

o Converting the attribute table to a point map: right click on “rainfall station location” attribute table > table operation > table to point map

o X column will be X and Y column will be Y and use UTM_m coordinate system for the coordinate system from the list box, write the output point map as “Rainfall station”; finally click ok show button, see figure below:


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o Open the “lake” segment map and add layer “Rainfallstation” and observe the distribution of rainfall station around the lake.

2.4. Nearest point interpolation of Metrological station

o Right click on “rainfallstation” point map > interpolation > nearest point o Make submap_gilgilabay as a georeference from the list box see below. o Finally click on show button


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o Accept the default and click ok. Add “Rainfallstation” point map and “lake” segment map. Observe that all of those station will have a weight by nearest point interpolation method

o Masking “rainfallstation” thiesson polygon raster map by “Tana” raster map: go to

operation main window > raster operation > map calculation o Write this small script under expression text box of map calculation dialogue box

Ifnotundef(lake,rainfallstation)


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o The result is shown below

IFNOTUNDEF( ) function IFNOTUNDEF(a, b) : If a is not undefined, then return b, else return undefined. IFNOTUNDEF(a, b, c): If a is not undefined, then return b, else return c.


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2.5. Calculating the weights of the stations

o Go to operation menu > statistics > histogram o Under map list box select “TanaThiess” and click on show button

o Result of the histogram

ID Name No of Pixel Area m2 % Area

1 Delgi 54094 438161400 14.3 2 Addis Zemen 62517 506387700 16.53 3 Bahir Dar 5262 42622200 1.39 4 Deke Estifanos 188329 1525464900 49.8 5 Chawhit 40369 326988900 10.68 6 Zege 27579 223389900 7.29

2.6. Areal rainfall estimation

Daily rainfall data for five station from Jan-1, 1992 up to Dec-31, 2000 is given o Open “Rainfall data plus Station Location.xls” from the working directory and

click on “daily rainfall” worksheet o Add anew column to calculate Areal rainfall

o Areal rainfall equation for lake tana is shown below Tana Areal rainfall =0.0139*BDR + 0.143*Delgi + 0.498*Deke Estifanos + 0.1653*Addis Zemen + 0.1068*Chewhit + 0.0729 * Zege

o Write the following script under Areal rainfall column in excel worksheet =0.0139*C2+0.143*D2+0.498*E2+0.1653*F2+0.1068*G2+0.0729*H2


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Lake Tana daily areal rainfall


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Part ��

Lake Tana Open water Evaporation Evaporation is the process whereby liquid water is converted to water vapour (vaporization) and removed from the evaporating surface (vapour removal). Solar radiation, air temperature, air humidity and wind speed are climatological parameters to consider when assessing the evaporation process. For this specific study Modified penman method is used to estimate the water balance component.

o To estimate the daily open water evaporation the nearby station Bahir Dar station is used which has a daily e maximum and minimum Temperature, average relative humidity, wind speed etc on daily basis

o Albedo for Lake Tana can be estimated from a number of satellite images, lake area albedo ranges from 0.05 to 0.062 an average of 0.058 is used to estimate the lake evaporation (Abeyou W. 2007)

o Open the excel file under working folder with file name “Open water Evaporation” o The spreadsheet has a Modified penman model used to estimate the daily open

water evaporation for Bahir Dar station

The result of daily evaporation is located at AD column from 1992 to the end 2000 The result of open water evaporation shows long-term average of 1690.71 mm from 1992 to 2000


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Year Total Annual Evap

mm 1992 1680.76 1993 1677.89 1994 1689.72 1995 1691.37 1996 1696.95 1997 1709.15 1998 1696.43 1999 1686.01 2000 1688.09

Average 1690.71


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Part ��

Runoff from rivers /inflow from rivers/

4.1. Runoff from gauged catchment

o For this study rivers that have a continuous daily flow data recorded, only four rivers / Gigel Abay, Gumara, Ribb and Megech are used directly as an input for the water balance model to simulate lake level.

o The data is located in the working folder with file name “Runoff from gauged and ungauged “ under “gauged river” worksheet

o Total river inflow from gauged rivers is computed under Total inflow column

0

50

100

150

200

250

300

350

400

450

500

7-M

ar-9

4

23-S

ep-9

4

11-A

pr-9

5

28-O

ct-95

15-M

ay-9

6

1-Dec

-96

19-Ju

n-97

5-Ja

n-98

24-Ju

l-98

Gilgel Abay Gumara Ribb Megech

Runoff data from gauged catchments


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4.2. Runoff from ungauged rivers

An ungauged catchment is the one with inadequate records (in terms of both data quantity and quality) of hydrological observations to enable computation of hydrological variables of interest (both water quantity or quality) at the appropriate spatial and temporal scales, and to the accuracy acceptable for practical applications (PUB: Predictions in Ungauged Basins). These ungauged catchments refer to catchments having topographic and climatic properties that are available without observed discharge data.

In this study runoff from ungauged catchment is adopted from Abeyou W. 2007 study, where ungauged runoff is estimated by transferring calibrated model parameters of gauged catchments based on catchment characteristics referred as regionalization.

o Runoff from ungauged catchment is adopted from previous study and the result is available with file name “runoff from gauged and ungauged catchments” under the “total ungauged runoff” spreadsheet.

Runoff from ungauged

0

100

200

300

400

500

600

700

15-Jun-94 28-Oct-95 11-Mar-97 24-Jul-98 6-Dec-99 19-Apr-01 1-Sep-02


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Part ��

River outflow and lake level The outflow and lake level data are observed around Bahir Dar town. Outflow from Lake Tana is by the Blue Nile River which starts at Chara Chara near the city Bahir Dar. Observed lake level and out flow data are located under the working directory with file name “Lake level and outflow”

o Open the excel file in the working directory with file name “Lake level and outflow”, contains the daily lake level and outflow data from 1995 to the end 2000.

o Figure below shows the relation between outflow by Blue Nile and the observed Lake

Level

Lake Level VS Outflow

0

100

200

300

400

500

600

700

800

06/15/94 10/28/95 03/11/97 07/24/98 12/06/99 04/19/01 09/01/02

Date

Ou

tflo

w (m

3/s)

1785

1785.5

1786

1786.5

1787

1787.5

1788

1788.5

Lak

e le

vel (

m a

msl

)

Outflow BN Lake level


39

Part ��

Water Balance Model / Lake level simulation

After estimation of the lake water balance components (inflows such as lake areal rainfall, runoff from rivers and outflows by Blue Nile River and open water evaporation) a spreadsheet water balance model is developed to simulated lake level by volume-area and volume-elevation relationships. The initial volume and area of the lake is simply defined by fixing the initial value to an observed lake level. In the model both evaporation and rainfall are defined as a function of the lake surface area that is updated in response to the inflows and outflows.

o Opening the spreadsheet water balance model: go to the working directory and open a spreadsheet with file name “lake Tana water balance model”

o The next step is to copy and paste the results of the water balance components from the previous excel files /Areal rainfall, open water evaporation, runoff from gauged and ungauged rivers, outflow and finally lake level to compare the result with the simulated one.

Spreadsheet model


40

Comparison of observed and simulated lake level

1785.5

1786.5

1787.5

1788.5

12/7/94 12/7/95 12/6/96 12/6/97 12/6/98 12/6/99 12/5/00

Date

Lak

e L

evel

(m a

msl

)

Simulated Observed


41

Part ��

Flood Extent by GIS Zoneing the “BathDEM” digital elevation model:

7.1. Observing the lowest point of the basin

� Right click on the “BathDEM” digital elevation model and choose > statistics > histogram click on Show button.

� The minimum value is 1772.6 m amsl, to see the location of lowest point across lake Tana basin

write the following script on the command line Lowest = iff(BathDEM<1773, “Below 1773”,”above 1773”) � This script will assign “Below 1773” for elevation value less then 1773 m amsl which is the lost

point on lake Tana basin and assigns “above 1773” for elevations greater than or equal to 1773 m amsl

� Paste the above script on the command line and click enter �

� Raster map definition dialogue box will appear, click on create new domain button to create the

possible values of the map which are “Below 1773” and ”above 1773”


42

� Creating domain dialogue box will appear, write “above and below 1773” as the domain name

and put a check mark on group check box. Finally click on ok button

� When you click ok Domain group dialogue box will appear, close this one for the software to create the domains automatically. Click on show button followed by yes and yes to add those two domains

� Now you can see the deepest point of lake Tana, the red spots at the center of the lake which are


43

around 13 m deep measured from 1786 m amsl

7.2. Zoning dead storage of the lake

o The minimum operating level of the weir is 1784 m amsl, let as see the flood extent for 1784 m

amsl � Writing the following script on the command line Dead_Storage =iff (BathDEM< 1784, “Dead Storage”,”above 1784”)


44

Flood zone for 1784 lake level

7.3. Observing the extent of historical maximum flood level

o Extracting flood zone for the historical maximum lake level 1788.2, write the following script on the command line and click enter


45

Historical_max=iff(BathDEM<1788.2, “Water”,”Land”)

Historical maximum flood

7.4. Flood zoning of Take Tana

The next procedure is slicing the digital elevation model from the minimum 1772.6 to 1800 m amsl

o Go to File menu of ILWIS main window > Create > Domain…Create new domain dialogue box will appear


46

Elev Upprer Bound Name Code Elev

Upprer Bound Name Code

1773 1773 B1773 A 1787 1787 B1787 O 1774 1774 B1774 B 1788 1788 B1788 P 1775 1775 B1775 C 1789 1789 B1789 Q 1776 1776 B1776 D 1790 1790 B1790 R 1777 1777 B1777 E 1791 1791 B1791 S 1778 1778 B1778 F 1792 1792 B1792 T 1779 1779 B1779 G 1793 1793 B1793 U 1780 1780 B1780 H 1794 1794 B1794 V 1781 1781 B1781 I 1795 1795 B1795 W 1782 1782 B1782 J 1796 1796 B1796 X 1783 1783 B1783 K 1797 1797 B1797 Y 1784 1784 B1784 L 1798 1798 B1798 Z 1785 1785 B1785 M 1799 1799 B1799 AA 1786 1786 B1786 N 1800 1800 B1800 AB

o Click on ok button, Domain Group dialogue box will appear then click on Add Item button shown below;


47

o Then enter the upper bound, name and code starting from 1773 name B1773 and code A

finally click ok

o Add also the remaining slicing zones from code B to AB by clicking on Add item button on

the Domain Group dialogue box, finally the result will look like:

o Attaching the “zoning” domain to the “BathDEM” digital elevation model � Right click on the “BathDEM” digital elevation model > image processing > slicing… � Write “Flood_Zones” on the Output raster map dialogue box and choose “Zoning”

Domain list box and finally click on show button.


48


49


50

The END

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