HAND ON SESSION 2
DEM Processing II
-Hydrological ApplicationInstructors: Dr. Kyoshi Honda and Mr. Sarawut Ninsawat
Introduction
In this session, you will be able to use DEM derived data and remote sensing data in
rainfall-runoff modeling, which is necessary in flood studies. Simple rainfall-runoff
models developed specifically for this training will be used in this session. The
models are called loose-coupled with the GIS, meaning, data has to be derived from
GIS and be used to the rainfall-runoff models. They do not share the same database.
Objective and Expected Outputs
After this session, you are expected to use DEM and remote sensing data in your
hydrological applications.
Data
Data derived from project1.apr
aster_4class.asc (land cover classification data)
ndviaster.asc (NDVI data in the study area)
Software/Program:
ArcViewGIS (ESRI)
Custom made computer programs specifically developed for this training by
the Honda Lab Group:
CN – for runoff volume calculation
CNfTime – for excess (direct runoff ) hyetograph derivation
Unithydro – for direct runoff calculation using a unit hydrograph of a
watershed.
Method
A. Model descriptions
A.1 CN
The CN model is used for runoff volume calculation. It requires the CN of each land
use class in the watershed. There are two ways to supply CN values to the model: (i)
by using tabular values wherein CN for each land use class are specified (the
classified remote sensing data is provided here), and (ii) by using NDVI values – CN
was assumed to have a linear relationship with NDVI.
Example input file using tabulated CN (inputTable.txt)
5,2.56,4000.,2
700.,75.
900.,82.
900.,71.
700.,87.
800.,75.
The first line contains data on (i) no. of land use class inside the watershed, (ii)
average rainfall in the watershed (mm) – this can be derived using Thiessen polygon
or inverse distance method using several rainfall stations, (iii) total area of the
watershed (ha), (iv) antecedent moisture condition (AMC ) before the rainfall occur.
The second to last line contain the data of (i) area of that particular land use class in
the watershed (ii) CN2 value from the table.
Example input file using NDVI-CN relationship (we assume it linear) – inputVI.txt
5,2.56,4000.,2,100.,70.,-0.52,0.65
700.,-0.52
900.,-0.23
900.,0.06
700.,0.36
800.,0.65
The first line contains data on (i) no. of NDVI class inside the watershed, (ii) average
rainfall in the watershed (mm), (iii) total area of the watershed (ha), (iv) antecedent
moisture condition before the rainfall occur, (v) maximum CN value, (vi) minimum
CN value for the soil group in the watershed, (vii) minimum NDVI value and (viii)
maximum NDVI value in the watershed during the rainfall event. The second to the
last line contain data on (i) area of that particular NDVI class in the watershed and (ii)
mean NDVI value of this NDVI class.
Example of an output file of CN (output.txt)
nclass, rainfall(mm), totalArea(ha), amc
5
2.56
4000.00
2
=========================
subarea, CN2
=========================
700.0
100.0
900.0
92.6
900.0
85.1
700.0
77.4
800.0
70.0
=========================
S=
44.7 mm
Ia=
2.6 mm
Pe=
0.0 mm
Fa=
2.6 mm
Q= 0.00E+00 m^3
CN=
85.0
A.2 CNfTime
The CNfTime calculates the hyetograph of a excess rainfall using the Time
Distribution of SCS-CN Abstraction calculation. The average CN value for the
watershed is needed in the calculation.
Example of input file (input.txt).
7,80,2
0.,5.08,22.86,32.258,58.674,118.11,134.366,136.144
The first line contains data on (i) no. time interval, (ii) CN from the previous CN
estimation, and (iii) Antecedent soil moisture condition ( AMC ). The second line
contains (iv) the cumulative rainfall at every time interval in mm – NOTE: always
start with 0.0 hour, so in this case, the second line contains an n+1 data, where n is the
no. of time interval.
Example of an output file from CNfT (output.txt)
T(h), CumP,
Ia,
Fa,
Pe,
0
0.00
0.00
0.00
0.00
1
5.08
5.08
0.00
0.00
2
22.86
12.70
8.76
1.40
3
32.26
12.70
14.95
4.61
4
58.67
12.70
26.67
19.31
5 118.11
12.70
39.63
65.78
6 134.37
12.70
41.72
79.94
7 136.14
12.70
41.93
81.51
Hye
0.00
1.40
Do not forget to read the
readme files included with
programs, they provide
valuable information about
how to use the program.
3.20
14.70
46.48
14.16
1.57
0.00
A.3 unithydro
This program calculates the direct runoff and stream flow (specify baseflow) in a
watershed using a discrete time convolution of a linear hydrologic system. It requires
the ordinates of the unit hydrograph (UH) of a watershed. UH actually can be
determined by deconvolution of the discrete time convolution equation, and it requires
a measured (long term) storm hydrograph from the watershed.
Example of input file (input.txt).
6,9,2409.30
0.98,2.76,13.58,44.78,13.84,1.54
0.00445,0.119,0.258,0.276,0.161,0.0498,0.0421,0.0303,0.0190
1.36
Line one contains data on (i) no. of pulse, (ii) no. of elements of the unit hydrograph
and the (iii) area of the watershed in ha. Line two contains the data on (iv) the amount
of rainfall pulse (excess) for every time interval (mm). Line three contains the (v)
ordinates of the unit hydrograph (mm/hr-mm). Line four has the data of (vi) baseflow
from the watershed (m/s).
Example of an output file from unithydro (output.txt)
(m^3/s)
0.0
0.1
0.4
1.3
0.4
0.0
0.0
0.0
0.0
0.0
0.8
2.2
10.8
35.7
11.0
1.2
0.0
0.0
0.0
0.0
1.7
4.8
23.4
77.3
23.9
2.7
0.0
0.0
0.0
0.0
1.8
5.1
25.1
82.7
25.6
2.8
0.0
0.0
0.0
0.0
1.1
3.0
14.6
48.3
14.9
0.0
0.0
0.0
0.0
0.0
0.3
0.9
4.5
14.9
0.0
0.0
0.0
0.0
0.0
0.0
0.3
0.8
3.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.2
0.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
Direct, Stream
Runoff, flow
0.0
9.1
0.9
10.0
4.3
13.4
18.7
27.8
65.7
74.8
116.8
125.9
123.7
132.8
82.0
91.1
37.2
46.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.7
0.0
0.0
0.0
0.0
4.6
0.5
0.0
0.0
0.0
12.6
3.9
0.4
0.0
0.0
2.8
9.1
2.8
0.3
0.0
0.4
1.7
5.7
1.8
0.2
22.0
15.2
8.9
2.1
0.2
31.1
24.3
18.0
11.2
9.3
B. Deriving hydrological parameters/data from DEM and remote sensing data using
GIS
Calculating runoff volume using CN model
B.1 CN values from Table
1. Open ArcViewGIS.
2. Open project1.apr from d:\training\dmr, start View1 by double clicking it.
3. Now, we are only interested in watershed 4, so we should create a grid
theme that contains only watershed 4. Go to Theme, select Table, then
select value 4 in the data field. Close table. Go back to Theme, select
Convert to Grid, assign grid name as Watershed4 in d:\training\dmr.
Then click YES to add it as a theme in the View window. Check the
“lonely” watershed by clicking the check field of the theme.
4. Now, import the ASCII raster data of land classification and add it as a
theme. Go to File, Import Data Source, select ASCII raster, click OK, go
to d:\training\dmr, click aster_4class.asc, click OK. Then assign a name of
the new grid theme, go back to d:\training\dmr, give a name of landclass.
Check your new grid theme. Class1 – bareland, Class 2 - built up areas,
Class 3 – water feature and Class 4 – Vegetation (includes forest and
upland agriculture).
5. We should summarize the land use class of the watershed to determine the
area for each particular land use. Activate landclass in the View window
by clicking it. Go to Analysis, Summarize Zones, choose Watershed4 in
the dialog window. Click OK. Then select AREA to chart, click OK.
6. Check the RESULT of Step 5. Take note of the no. of grids in a class.
Also note that one grid has an area of 225 m2 (15 m x 15 m).
7. Check for CN values for each specific land use class in the given CN
Table (to be provided). Use Soil Group C. CN for Bare soil, check Dirt
under Streat and Roads (CN=87). CN for Built up areas, check
Commercial and business areas, and Residential – average lot size 1/3 acre
30% impervious (CN=(94+81)/2=87.5), only estimate of what built up
areas are. CN for water feature is equal to 100. CN for vegetation, check
for Forest, poor cover (CN=77). Why not good cover? (Check NDVI later).
Then decide.
8. Prepare the input file (inputTable.txt) for CN model.
9. Assuming a rainfall event of 136.14 mm, calculate the volume of runoff
from the watershed under normal rainy season (AMC=2). Run CN.exe
10. Calculate the runoff volume if the soil was dry before the rainfall event
(AMC=1). Compare the result with Step 9.
11. How about the soil was wet (AMC=3)? Compare the result with Steps 9
and 10. Explain.
12. SCENARIO ANALYSIS, what happens if the forest is reduced to 1800
ha because the 293.56 ha were converted to built up areas (mixed of
commercial and residential)? Repeat Step 9-11.
Lonely watershed
Landclass
Summarize landclass inside the watershed4.
B.2. CN values from NDVI
1. Back in ArcViewGIS, we should import our NDVI data from remote
sensing.
2. Go to File, Import Data Source, choose ASCII Raster, click OK, go to
d:\dmr\training, select ndviaster.asc, assign a name, go back to
d:\training\dmr, name it as NDVI, click NO to cell values as integer (it
should be float), then add as a theme, click OK.
3. Now we have to we have to reclassify our NDVI to create a table attribute.
Activate NDVI, go to Analysis, Reclassify, choose value in the
classification field, nine class is ok (9 is default in ArcView), click OK.
4. Activate Reclass of NDVI , go to Analysis, Summarize Zones, select
Watershed4 in the dialog window, select Area to chart, click OK. Note of
the NDVI class under Class 3 to 9 because these are contained in the
watershed. The most important is, note the Mean NDVI value of an NDVI
class. Example Class 3 (Mean NDVI = -0.195), NDVImax=0.65,
NDVImin=-0.52, CNmax =100 (water bodies), CNmin = 70 (See Table).
5. Prepare the input file (inputVI.txt) for CN model.
6. Calculate the volume of runoff as in B1. Steps 9-11.
7. Compare the results.
Summarize NDVI inside the watershed4.
Calculating Hyetograph using CNfTime model
B.3 Hyetograph using CNfTime model.
1. Prepare input data (input.txt).
2. Assume a cumulative rainfall (mm) of
5.08,22.86,32.258,58.674,118.11,134.366,136.144 for time
1hr to
7 hr.
3. Calculate the Hyetograph using CNfTime using the CN values from (i)
Table and (ii) NDVI. Do this under AMC=1,2 and 3. Check the results.
Calculating the Direct Runoff and Streamflow using unithydro model
B.4 Unit hydrograph application.
1. Assume a unit hydrograph ordinate (mm/hr-mm) in watershed4 as
0.00445,0.119,0.258,0.276,0.161,0.0498,0.0421,0.0303,0.0190
2. Calculate the direct runoff and stream flow (baseflow = 1.36 m/s) using
the hyetograph calculated for the 134.14 mm rainfall from B.3.
3. Prepare the input file (input.txt) for unithydro. Run unithydro.exe.
4. Make a graph of the pulses, unit hydrograph, and direct runoff.
300
180
250
Rainfall, mm
160
140
200
120
100
150
80
100
60
40
50
20
0
UH, m 3hr-1mm -1 or Q, m 3 s-1
200
0
1
2
3
4
5
6
7
8
9
10
11
Time (every 0.5 hr)
Example of a unit hydrograph, superimposed with pulses and direct runoff
hydrograph.
End of this session. Thank you.