Sudan Seasonal Monitor

Sudan Meteorological Authority

Federal Ministry of Agriculture and Forestry

Issue 3 June 2008

Summary

· Rainfall by early May returned southwards to its average position after having progressed far north of the usual in April. This unusual April movement led to light widespread rainfall across Sudan from the West (Darfur) to the East (Gedaref/Kassala). See Page 1.

· During April, significant rainfall was confined to the border regions of southern West Equatoria and Central Equatoria, while fairly low rainfall predominated in East Equatoria and central-southern Jonglei. See Page 2.

· Early May was very dry across all of Sudan and the situation will need to improve in East Equatoria – Jonglei (and elsewhere) in order for planting of late maturing sorghum to take place with success. See Page 3.

· Vegetation levels are increasing northwards with higher greenness in West Equatoria, along the border with Uganda and the highlands in the East. In some areas of East Equatoria and southeastern Jonglei below average vegetation levels are noticeable. See Pages 3 and 4.

· La Niña conditions are decaying to neutral which will hold through early 2009. See Page 4.

· Forecasts issued in May, for July-August-September rainfall lead to marked expectations of a wetter than average period as the most likely scenario. See Pages 5.

Seasonal Progress


Fig 1a – Position of the ITF over Africa in June Dekad1 2008 (red) compared to average position (black) and previous position (orange) (Source : CPC).	Fig 1b – Position of the ITCZ over Sudan along the current season compared to a 15 year average. (Source: CPC).

Rainfall in Sudan mostly results from northwards movement of humid air masses from March to August and their southwards retreat from September to November. At their northernmost reach, these humid air masses meet with drier and warmer air to form the InterTropical Front (ITF). Since the rains follow south of the ITF, the seasonal progress of the rainy season and of its quality can be evaluated by tracking the ITF. Fig 1(a) shows a map with the latest ITF position, the previous position and the usual position, while Fig 1(b) shows a plot with the average position over Sudan from the beginning of the season.

The current ITF position is on the average across Sudan and has stayed in same position since early May .

During May, significant rainfall was confined to Southwestern Sudan, in particular in Western Equatoria and also West Bahr-el-Ghazal in late May (Fig 2a). In general, across Southern Sudan only moderate amounts were registered from late April until the third week of May. Elsewhere in the country, amounts were low, though extending well in to northeastern regions of Kassala and Gedaref.

The regions of South Kordofan – Upper Nile and Blue Nile, and also Unity and Jonglei the rainfall amounts correspond to a much drier than average May with monthly amounts mostly below 30mm, broadly between 20-40% of the average (Fig 2b).

However, early June brought an improvement in conditions (Fig 2c) : rainfall was good and widespread across most regions; in particular in South Kordofan, Upper Nile and Blue Nile, good rainfall amounts occurred and the very dry conditions that had been predominant during May came to end. This was also the case in southeastern Sudan. Also first significant rainfalls in South Darfur, White Nile, Sennar and Gadarif were observed .

The total rainfall from the beginning of the season (Fig 2d) shows significant improvement in Jonglei and East Equatoria due to better rainfall in late May and early June, but levels are still below average. Elsewhere in Southern Sudan conditions are above average. The rest of the country shows well above average conditions along the East (Blue Nile to Kassala) due to very good early rainfalls. The May dryness in the South Kordofan region brought cumulative rainfall levels down to values close or below average.

A more in-depth evaluation of the effect of rainfall can be made by comparing it to the water demand imposed by the environmental conditions (i.e. temperature, humidity, solar insolation and wind levels). This comparison between rainfall and environmental water demand is expressed by a ratio between the two, the Moisture Index (MI). So higher (lower) MI values correspond to wetter (drier) conditions.

The MI is useful because you can use it to estimate if conditions suitable for crop planting have occurred and if crops have their water requirements satisfied.

The Moisture Index (MI) in Sudan remained at low levels until early June after which high values spread north leading to the onset of the growing season throughout much of Southern Sudan, Blue Nile, South Kordofan and parts of Sennar, Gadarif, South and West Darfour.

Values above 35% (yellows and greens) indicate suitable conditions for planting and early crop development.

Note the northwards spread in high moisture conditions in early June

In these regions by early June, MI values reached levels high enough to satisfy the water requirements of developing crops (Fig 3c) and to facilitate the onset of the growing season, provided moist conditions remain in place during mid and late June.

Eastern parts of East Equatoria was the exception, with dry conditions still remaining in place.

To detect the onset of the growing season (i.e. occurrence of suitable conditions for planting and early crop development), we identify areas where the MI exceeds a value of 35% (which corresponds to the water needs of a crop in the first stages of development) for at least two 10 day periods. Fig 4 highlights the dates on which the growing season started across Sudan :

In southernmost Blue Nile and its areas near the Ethiopia, growing season onset took place in late May, approximately in step with usual planting dates of the typical long maturing sorghum of this region.

Early June saw very marked progress northwards in the onset of the growing season which extended into West Darfur, South Darfur, all of South Kordofan and Blue Nile and over into eastern Sennar and southern Gedaref. It is likely that some of this progress is reversed in early June as such wet conditions April and mid may not last over such wide areas at this time of the season. Note that widespread planting April not occur yet – it is far too early for the mechanized farms of Blue Nile, Sennar and Gedaref; in traditional agriculture, millet planting in eastern of South Darfur, millet and sorghum planting in western and southern South Kordofan respectively and sorghum planting in central-south Blue Nile is likely to take place. conditions are suitable but need to remain in place to enable agricultural activities to start.

Vegetation condition and development are assessed by means of the NDVI (Normalized Difference Vegetation Index) – this is a satellite derived parameter which responds (almost) uniquely to vegetation and is available on a global scale every ten days.

Compared to average – above average across most of S Sudan (specially Jonglei, Unity) as consequence of the strong April rainfall allowing the May dryness in some areas not to have major impact. Vegetation much above average in Jonglei, possibly due to maintenance of good rainfalls in the mountains along the border and flow from higher ground in Ethiopia (see Beg of season map in Fig 4)

Southern Sudan - However most of this is judged to be due to natural vegetation growth. Agricultural activities are still at a relatively early stage, as they start off mostly in May. Actually May dryness might have led to localised delays in planting of the dominant sorghum crop.

Northern Sudan – still early for significant development, though the effect of early rainfalls can be seen in above average vegetation in some areas of Darfur, South Kordofan, Blue Nile and Sennar. Most of this will be natural vegetation not related to agricultural crop development.

Fig 5a – NDVI in earlyJune 2008. Greener (darker) tones correspond to higher NDVI values.

Fig 5b – NDVI difference from average in early June 2008. Yellows and reds represent below average vegetation development, greens and blues represent above average vegetation development. Note above average development across most of Southern Sudan, specially in Jonglei and Unity.

Current La Niña conditions are decaying to neutral and are expected to remain so from mid 2008 to until early 2009.

July-August-September (JAS) is the crucial period for most crops in Sudan, in particular for the northern regions. Forecasts for JAS rainfall have been prepared in May by a variety of sources, including SMA.

SMA prepared its own forecast for June-September rainfall (Fig 6a). Wetter than average season is predicted in most parts of the country, except in the Red Sea coastal area and Greater Bahar Al Gazal where the probability of having average rainfall amounts is more likely.

Fig 6b displays a forecast for JAS 2008 rainfall from IRI (International Research Institute).and Fig 7 a forecast from a third source (ECMWF, European Centre for Medium-range Weather Forecasts). These forecasts fom reputed international institutions are all in agreement with the SMA forecast even though there maybe some differences in the detail – the indications for above average rainfall in Darfur are a bit stronger and for Southern Sudan average rainfall is forecast instead. Also it should be noted that other forecasts which went in the opposite direction (as mentioned in previous issues) have now been updated and are now conforming to the same scenario as described above.

In summary, a wetter then average season is expected probably more so in the West of the country, less so in the South. The scenario is favourable for agricultural production ,specially in the traditional sector and in the more marginal areas. On the other hand, when seasons are wetter, more extreme precipitation events become more likely and a repeat of the large rainfalls and localised flooding events of the last season becomes a possibility. Since the expected scenario for the Ethiopian Highlands is also of above average rainfall, high levels along the Blue Nile, Atbara and Gash rivers can also be expected.

However, these are forecasts for long periods of time and wide areas and it is not possible to provide an idea of when or precisely where high rainfall or flood events will occur. We can only say they are more likely to occur.

Fig 6a – Probabilistic forecast prepared in May, for July-August-September (JAS) 2008 rainfall for Africa (source: IRI). Boxes indicate likelihood of above (top), on (middle) and below (bottom) average conditions. Green to blue indicate areas of increasingly more likely above average conditions

Fig 6b – Forecast of JAS 2008 rainfall tendency. Green to blue indicate areas of above average tendency, yellows to reds indicate areas of below average tendency