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Effect of Topography in Satellite Rainfall Estimation Errors: Observational Evidence across Contrasting Elevation in the Blue Nile Basin.
Gebrehiwot Niguse Tesfay1, Menberu Bitew2, Mekonnen Gebremichael2
1Ethiopian Institute of Water Resources, Addis Ababa University, Ethiopia.2Department of Civil and Environmental Engineering, University of Connecticut, USA.
Introduction
Accurate observation of precipitation is essential to hydrological applications and water resources planning and management aspects. In complex terrain region of the Blue Nile, the effect of topography on rainfall variability was not clearly estabilished. Accurate measurement of rainfall using dense rain gauges was very essential for the assessment spatial rainfall variability. We deployed 70 recording rain gauges across the Blue Nile over elevation ranging from 600 m.a.s.l to 3300 m.a.s.l with a focus on capturing spatial rainfall variability within fine resolution ground rainfall grid. The spatial rainfall variability is assessed based on the relatively dense rain gauge networks.
The main goal of this study is to evaluate the effect of topography on rainfall variability in two contrasting elevated gridded areas using newly deployed dense rain gauge networks in the Blue Nile basin. Understanding and quantifying rainfall variability at the contrasting elevation locations will provide important information in quantifying the errors associated to the representation of arial rainfall from ground data.
Methodology
Results
Conclusions
Acknowledgment
Contact Information
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Figure 2. DEM of Study site, gauge location and the vertical difference between the two sites
Table 2. Precipitation correlations in the highland area
Figure 3. Relationship of distance and Rainfall between stations in lowland site
Figure 4. Relationship between distance and Rainfall between stations in highland site
It has been observed that, elevation and horizontal distance difference have significant effect on total amount and the frequency of rainfall. In this experimental investigation, maximum rainfall event in the study time is obtained in the lowland but the accumulated is less than the other.
This project is made possible by the generous support of the American people through the United State Agency for International Development (USAID) and Higher Education for Development (HED) office.
Gebrehiwot Niguse Tesfay, E-mail: [email protected], www.eiwr.org
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1. Field Work and Analysis of Data
To achieve the stated objective, the following three procedures has been used:
2. Data Used
For the purpose of this study, the following precipitation data has been collected:
Precipitation(Total)
Distance and Elevation
Data Unit Millimeter (mm) Meter, mData Range July 01 – Sep 30,
2012 ------------
Time series 15 minutes 1mGrid Resolution 0.25o X 0.25o
3. Study AreaThe correlation shows quick response to change in horizontal distance between stations in
the lowland as compared to other site.
Rainfall variability Statistical indicator analysis
G1-G2 G2-G3 G3-G4 G4-G5 G5-G6 G6-G8 G8-G90.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
110.00
120.00
Gi stands for Station location in low land area
Horizontal Distances between stations, km
Elevation difference between consecutive stations, m
G1
-G2
G2
-G3
G3
-G4
G4
-G5
G5
-G6
G6
-G8
G8
-G9
0
0.5
1
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Coerelation ()R^2) between consecutive stationsHorizontal Distances between stations, Km
Station Code
Elevation, a m.s.l
Elevation difference between consecutive station, km
Distance between station, km
Correlation between station
N06 2067
27 5.5 0.56
N07 2094
90 3.2 0.63
N09 2184
170 9.0 0.36
N10 1924
12 3.4 0.6
N12 1936
49 3.4 0.62
N15 1985
62 6.55 0.48
N20 2047
566 20.5 0.14
N27 2898
In site with smaller elevation( averagely 600m a.s.l) 12.6km, 8km and 2.73km are correlated respectively by 0.0006, 0.38 and 0.76. The three hourly average rainfalls is 1.125 mm with maximum event value of 103.38mm. This is an indicator that, the average rainfall value is not a representative of all point rainfall data.
Similarly, in the higher elevation (averagely 2500m a.s.l) area; for rain gauges departed by 20.5km, 9km, 6.5km and 3.4 are related respectively by 0.14, 0.37 and 0.48 and 0.6. The three hourly average rainfalls is 1.386 mm with maximum value of 63.25mm.
40°0'0"E
40°0'0"E
34°0'0"E 38°0'0"E
38°0'0"E
36°0'0"E
36°0'0"E
13°0
'0"N
12°0
'0"N
11°0
'0"N
10°0
'0"N
9°0'
0"N
8°0'
0"N
8°0'
0"N
Figure 1. DEM of Blue Nile Basin
Legend
Raingauge Locations
Raingauge Locations
Lowland site
Value
High : 1697
Low : 566
Highland site
Value
High : 3291
Low : 1869
Legend
Blue Nile DEM_30m
Value
High : 4235
Low : 435
³
0 0.10.20.30.40.05Decimal Degrees
Table 1. Precipitation correlation in the low land area
Station Code
Elevation, a m.s.l in m
Elevation difference between station, m
Distance between station, km
Correlation between consecutive station
G1 721.6
12.4 4.6 0.42
G2 734
57 7.9 0.4
G3 677
26 8.0 0.38
G4 700
23 7.3 0.81
G5 725
14 2.7 0.76
G6 712
98 5.7 0.71
G8 614
93 12.60 0.001
G9 707
N06
-N07
N07
-N09
N07
-N10
N10
-N12
N12
-N15
N15
-N20
N20
-N27
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
5
10
15
20
25
Coerelation (R^2) between consecutive station
Horizontal distances between consecutive stations, in Km
37°45'0"E
37°45'0"E
35°0'0"E 37°15'0"E
37°15'0"E
36°45'0"E
36°45'0"E
36°15'0"E
36°15'0"E
35°45'0"E
35°45'0"E35°15'0"E11°30'0"N
11°15'0"N 11°15'0"N
11°0'0"N 11°0'0"N
10°45'0"N 10°45'0"N
Deploying rain gauges within Blue Nile Basin
Studying the rainfall variability Standard statistical analysis was used. is done based on
correlation (R2) between consecutive rainfall stations with in lower elevation and higher elevation grid sites.
Collect primary rainfall data over 2012 summer monsoon July 1, 12 to Sep 30, 12 from the
deployed rain gauges
N06
-N07
N07
-N09
N07
-N10
N10
-N12
N12
-N15
N15
-N20
N20
-N27
0
5
10
15
20
25
0
100
200
300
400
500
600
Ni stands for station locations in the Highland
Horizontal distances between consecutive stations, in Km
Elevation difference between stations
Low land
High land