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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: gebrehiwot.tesfaye@uconn.edu, 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