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Monitoring Annual Flooding in the Nile Basin Using Synthetic Aperture Radar (SAR).

The content for this case study is from the Nile River Awareness Kit (RAK) Interactive CD-ROM which was produced in 2005. The case study was developed by Hatfield Consultants Ltd. (Vancouver, Canada) with funding from the Canadian Space Agency. Funding for the project was provided by the Canadian Space Agency under the Tiger Program, designed to promote sustainable management of water resources in Africa.

The Nile RAK CD is a collaborative partnership between the Nile Transboundary Environmental Action Project (NTEAP) of the Nile Basin Initiative and a project team led by Hatfield Consultants. The primary objective of the RAK is to use multimedia and interactive tools to support the sustainable management and use of the environment and resources within the Nile Basin. The Nile RAK project supports the objectives of NTEAP for addressing high-priority transboundary environmental issues through an improved understanding of the relationship between water resources and the environment.

Introduction:
The climate of the Nile basin varies substantially, with low rainfall in Egypt to high rainfall in Ethiopia and the countries of the Equatorial Lakes Plateau and nearby mountains. During the months of June to September, precipitation levels increase, particularly in the highlands of Ethiopia where seasonal heavy monsoon rainfall brings increasing flows to the rivers of lower regions of the basin. Flooding occurs in many parts of the basin, affecting the daily activities of local inhabitants. Understanding the dynamics of the local hydrological regime is of significant importance to local, national and regional authorities. Space-borne satellite technology can be a powerful tool in achieving this understanding; particularly radar based data sets. The unique ability of radar to acquire images in all weather conditions and its ability to discriminate land/water boundaries make it a highly effective tool for the mapping and monitoring of flood conditions.

The table below shows the distribution of mean annual rainfall in the Nile River basin. Note the high mean annual rainfall of the countries in the upper catchments (D.R. Congo and Rwanda) in relation to the low mean annual rainfall of the lower reaches of the river (Sudan and Egypt).

Country

Average Rainfall (mm/year)

Minimum

Maximum

Mean

Burundi

895

1,570

1,110

D.R. Congo

875

1,915

1,245

Egypt

0

120

15

Eritrea

240

665

520

Ethiopia

205

2,010

1,125

Kenya

505

1,790

1,260

Rwanda

840

1,935

1,105

Sudan

0

1,610

500

Tanzania

625

1,630

1,015

Uganda

395

2,060

1,140

Distribution of mean annual rainfall in the Nile River basin
Source: UNEP Spatial Characterisation Tool (1997)


The Synthetic Aperture Radar (SAR) sensor on-board the RADARSAT-1 satellite is extremely sensitive to, among other parameters, land surface moisture and has been used to map flooding events in other parts of continental Africa, such as Mozambique, Namibia, Ethiopia and Kenya (Hatfield Consultants Ltd., 2005; UNOSAT, 2005).

Under the scope of the Nile River Awareness Kit project, a study was conducted in two regions of the Nile basin. The main objective of the study was to determine and map flooded areas during the monsoon season of 2005 for Khartoum in Sudan and Lake Tana in Ethiopia. A further aim was to demonstrate the effectiveness of RADARSAT-1 for flood monitoring and mapping to the Flood Preparedness and Early Warning (FPEW) sub-project of the Eastern Nile Subsidiary Action Project (ENSAP). This report is a summary of the results for the Lake Tana study. For further information and complete results, please refer to the full project report.

Study Areas:
Flood plain areas are typically shaped by two main processes: 1) the lateral migration of the river channel over time due to in-stream erosion and deposition, and 2) periodic flooding that leads to reshaping of the landscape by floodwaters and the formation of new landforms resulting from the deposition of river sediments. Differences in elevation over the flood plain areas are generally low and entire areas can appear flat. Nevertheless small differences in height occur between areas of the flood plain that can affect flooding patterns, habitat features and vegetation types.


The Nile basin and location of study areas. Click here to enlarge the image.
Image credit: Hatfield Consultants

The highlands of Ethiopia and Eritrea are within the Intertropical Convergence Zone (ITCZ) and experience monsoons and heavy rainfall during the summer months, causing localized flooding and the subsequent rise of the Blue Nile. Both the Atbara River in the northern part of the Ethiopian Plateau and the Blue Nile draw their floodwater from these rains and flood at the same time. Relatively flat areas along the banks of Lake Tana, the largest lake in Ethiopia, are also prone to flooding. During this period, the Blue Nile contributes to 85% of all water in the Nile, while the White Nile maintains a steady flow throughout the year. The heavy rainfall and subsequent increase in flow cause both rivers to accumulate and flood the low-lying plains surrounding Khartoum where the White and Blue Nile rivers meet. The flood waters utilized by the Sennar Dam and the Jebel Aulia Dam are essential to the agricultural activities of the Gizera plain, south of Khartoum.

Approach:
To meet the objective of the study the RADARSAT-1 SAR sensor was used to map flooded areas. Landsat-7 ETM+ and elevation data from the Shuttle Radar Topography Mission (SRTM) provided the basis for the geometric registration of all of the RADARSAT-1 imagery. To simplify the analysis, areas outside the Nile basin and non-flood plain areas were removed using a mask.

Prior to analysis, it was necessary to perform spatial filtering of the image to improve the definition of flooded areas and reduce speckle noise present in most SAR images. An adaptive 3x3 speckle filter (gamma filter), was applied to the images. The extraction of flooded areas was accomplished using a thresholding (density slicing) technique whereby low or dark radar returns indicative of flooded areas, are isolated and classified (Hatfield 2001). The determination of flooded and non-flooded areas required the application of additional auxiliary data sets. Local land cover maps, field data collected in Khartoum and hydrographic data obtained from the FPEW sub-project of the ENSAP and the Food and Agricultural Organization (FAO) of the United Nations were used in conjunction with expert judgement to understand the landscape and define the threshold values. Seasonally coherent archive Landsat ETM+ imagery and GIS data layers were also used to provide ancillary information during the analysis.


RADARSAT-1 wide beam image acquired July 27, 2005 and flooded areas extracted from thresholding, Lake Tana, Ethiopia. Manual editing and a majority filter were applied to the classified image to generalize the data and remove small features. These features are usually the result of the classification of image speckle not removed by the gamma filter.
Image credit: Canadian Space Agency (2005) and Hatfield Consultants

Results:
A total of 4 wide beam mode RADARSAT-1 images were acquired for the 2005 monsoon season. The peak flooding or heaviest rainfall period in the Lake Tana region typically occurs between late July and early August. Without local water gauging stations to indicate the highest water levels for 2005, it was not possible to validate peak flooding conditions or estimate the greatest area inundated.

Although peak conditions were not detected, a clear progression of flooding along the shores of Lake Tana was clearly identified. A comparison of all of the 2005 images to a baseline dry season image, acquired March 14 2002, indicate that on July 27 2005, an area of 89 km2 was inundated. Furthermore subsequent dates saw a reduction in inundated areas. The following table and figure provides a summary of inundated areas for three image dates. As illustrated by the figure inundated areas where consistent with areas identified by the local authorities as flood prone.

Table 1. Inundated areas or seasonal water bodies for three images acquired during the monsoon season of 2005.

Image Date

Inundated Area (sq. km)

March 14 2002

4

July 20 2005

53

July 27 2005

89




Flood progression and derived extents during the monsoon season of 2005. Click here to enlarge the image.
Image credit: Canadian Space Agency (2005) and Hatfield Consultants

Inundated areas extracted from the RADARSAT-1 images also supported and enhanced the GIS data layers provided by the ENSAP FPEW sub-project. As a result, flood prone datasets can be edited and updated in the GIS database to reflect actual scenarios based on 2005 flood data.

In addition, a multi-temporal analysis was conducted to illustrate the effectiveness of time series RADARSAT-1 imagery in determining changes in the surrounding environment. Please refer to the full report for additional detail.

Conclusion:
The primary remote sensing task in this study was to determine and map inundated areas. The image analysis performed using the multi-date imagery acquired for the project effectively identified these areas. Summer flooding during July 2005 and August 2005 in two regions of the Nile Basin was successfully mapped using RADARSAT-1 imagery. Images were segmented, or classified, into two distinct categories, open water and non-flooded areas. Although this simple but efficient methodology is able to accurately classify inundated areas, transition zones or areas of inundated vegetation are not easily captured. Furthermore, the image products developed in the project proved useful in updating vital information used in the planning of flood preparedness strategies.

As this was a desktop study, with no ground validation, it was only possible to create a generalized data set. However, efforts were made to validate the results of the study using existing data collected during the 2005 flood season.

Contact information
For more information about the NTEAP please contact: Nile Transboundary Environmental Action Project Gedion Asfaw, Regional Project Manager P.O. Box 2891 Khartoum, Sudan Web site: www.nileteap.org

or

Grant S. Bruce, Vice-President and Partner, Hatfield Consultants Partnership, Email: gbruce@hatfieldgroup.com