Zambezi floods

By Jochen Kerkmann (EUMETSAT)

According to Wikipedia, the 2009 Angola, Namibia and Zambia floods were a natural disaster which began in early March 2009 and resulted in the deaths of at least 131 people and otherwise affected around 445,000 people. The floods affected seven regions of Namibia, three provinces of Zambia, two regions of Angola and part of Botswana.

The floodwaters damaged buildings and infrastructure and displaced at least 300,000 people. A state of emergency was declared in northern Namibia and there were fears that a disease epidemic could ensue. The Red Cross agencies and governments of the two countries responded to the disaster, and aid was distributed by the World Health Organization.

IRIN, a project of the UN Office for the Coordination of Humanitarian Affairs, reported that in Botswana the authorities took precautionary measures as rising waters in the Okavango Delta, an inland river system which terminates in the arid Kalahari, threatened an outbreak of waterborne diseases. Water levels in the delta had risen to their highest levels since 1939, according to the District Commissioner for Ngamiland, Beneddette Malala.

As reported on the NASA Earth Observatory web page, water levels on the Zambezi River remained high in April 2009 (see Terra MODIS image from 14 April 2009, source: NASA). The swollen river had destroyed crops and forced thousands of evacuations in Namibia's Caprivi region. Water, black in this false-color image combining visible and infrared channels, stretched across kilometers of land that were dry in January (see Terra MODIS image from 4 January 2009, source: NASA), before seasonal rains started. Plant-covered land is green, and bare earth is tan. The flooded Zambezi spills over into the Chobe River, the Bukalo Channel, and Lake Liambezi in the 14 April image.

The satellite images shown below were recorded a few days later, on 19 April. The Metosat-9 RGB product shows the flooded areas of Eastern Namibia (Caprivi) in black colour. Also the rising levels of water in the Okavango Delta are visible by the relatively dark green colour. The MODIS NDVI product (shown for comparison) does not show the flooded areas, as the difference between the two visible channels VIS0.6 and VIS0.8 is small for water surfaces and, thus, the colour of flooded areas is the same as that of very dry (no vegetation) land.

It should be noted that the Zambezi River routinely floods during the annual rainy season, but in 2009, the floods were more extensive than normal. On 12 April, the United Nations Office for the Coordination of Humanitarian Affairs expected the floods to last through mid-May.

This estimate turned out to be a bit too optimistic, as demonstrated by a long-term animation of daily 9:00 UTC Meteosat-9 RGB images for the period 12 March to 8 June (i.e. from the end of the rain season well into the dry season).

In the Caprivi strip, the water started to receed by mid-April 2009, but on 8 June parts of the Chobe River and Lake Liambezi were still flooded. The same Meteosat-9 long-term animation also shows the drying of the Makgadikgadi Pan and the southward propagation of water in the Okavango Delta.

A similar three-month animation has been prepared for the area of the Etosha Pan in Northern Namibia (see animation), and a shorter one-month animation for the Bulozi floodplain in Western Zambia (see animation). The Etosha Pan loop shows the general reduction of the vegetation density during the period and the drying of the affluent rivers on the north side of the Etosha Pan.

Looking at the Etosha Pan itself, the western part, initially flooded, quickly dries up shown by the colour change from cyan to brown-red. In the eastern parts, that remain partly flooded during the entire period, the water flowing in from the northern affluents (mainly Ekuma river) seems to turn east (the pan is slightly tilted to the east, with the eastern section about 5m below that of the west), thus, creating a kind of clockwise circulation of water in the Etosha Pan.

Note: although not planned as an initial application of altimetry, altimeters like the one carried by Jason-2 are capable of monitoring river and lake levels with a precision of about 2cm. The high interest in such application has quickly been demonstrated to the extent that CNES has extablished a project to process altimetry data for coastal and hydrology applications (called 'Pistach' project) giving access to surface elevation data beneath the Jason-2 tracks every 10 days. An example of such data for the 2009 Zambezi floods is given on the AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data) web page, where several plots showing the rise of the water levels of the Zambezi (source: AVISO) and Okavango (source: AVISO) have been published for the period 21 December 2008 to 1 April 2009.