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 northern limit, these humid air masses meet with drier, warmer air forming the InterTropical Front (ITF). Since the rains follow south of the ITF, the seasonal progress of the rainy season and 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.

We can see that the rainfall has been moving north along its average position (Fig 1), after an early season period (April-May) when it was well north of the average and some south of the average positions in July.

Rainfall will reach its northernmost position by end of August and after that it will retreat southwards. Areas where the rainy season is starting late are likely to suffer a very short rainy season. 

July and Early August Rainfall in Sudan     

July rainfall, in general was on or below average over most of Sudan, excepted for some areas in the southeast of the country. Highest rainfall amounts were registered as usual in the southwestern areas of West Bahr-el-Ghazal state with late July being very wet in the Wau region. Elsewhere in South Sudan, Eastern Equatoria and areas of southern Jonglei enjoyed good rainfall (reaching twice the average for July), see Fig 2b; on the other hand, in Upper Nile, Western Equatoria and parts of Central Equatoria, rainfall was low for the time of the year (60%-80% of the average).

In northern Sudan, the current panorama is looking worrying, in particular over the marginal agricultural regions of North Darfur, the western regions of North Kordofan (En Nahoud locality), Gezira, northern areas of Gedaref and Kassala state, where markedly low rainfall is causing significant delays to the start of the cropping season (see next section for more detail). July rainfall in these regions was 50% or less of the average.

In early August (1^st-10^th) the situation worsened, with this period being generally very dry across most of Sudan – this month is key to the success of the agricultural season in the northern marginal regions.

Fig 2c) shows clearly low rainfall amounts in early August across the whole of Kordofan, eastern North Darfur, White Nile, Gezira and Kassala. At this stage of the season there is very little time to improve agricultural season perspectives. Conditions were also unfavourable in West and Central Equatoria in South Sudan.

Recent rainfall reports for mid August indicate significant improvement in the rainfall across most of the country. However, western North Kordofan (En Nahoud and Ghebeish localities) do not show any improvement and crop development perspectives for this region are looking very pessimistic. Western South Kordofan (Al Salam and northern Abyei) also shows little improvement with dry conditions spreading to Kadugli.

A notable exception is the Great Bahr-el-Ghazal – Warab region where persistent high rainfall has been registered since the last few days of July. Localized rainfall amounts in Wau region may have exceeded 300mm in the first 3 weeks of August. Results are possible damage to crops, and rise of River Jur levels feeding into the Bahr-el-Ghazal river across Warab and northern Unity states.

Conditions for Crop Development

This section provides a more crop specific analysis by looking at a balance between rainfall and the water demand imposed by the environmental conditions (i.e. temperature, humidity, solar insolation and wind levels). This balance is expressed by the ratio between the two, usually known as Moisture Index (MI). Higher (lower) MI values correspond to wetter (drier) conditions. The MI can be used to estimate if conditions suitable for crop planting have occurred and if crops have their water requirements satisfied.

Timing of the 2008 Growing Season

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 3 shows the dates on which the growing season is estimated to have started across Sudan :

South Sudan : There were earlier than usual starts of the season, in particular in the northern and western parts of this region due to the earlier than usual northwards progress of the rainfall (see Fig 1b) in the initial stages of the season (March-April). But in the southeastern regions of southern Jonglei and East Equatoria, low early season rainfall caused later starts and in the easternmost areas no suitable conditions have been detected yet, though recent rainfall will improcve prospects for pastoral livelihoods.

North Sudan : Good early starts in South Darfur, South Kordofan across to Blue Nile and parts of Sennar, though later moisture conditions were not so favourable. The key feature this season is how late growing season conditions are happening in North Darfur, North Kordofan, White Nile, Gezira, Gedaref, and parts of Sennar – Fig 3 shows that across many of these areas no growing season conditions have been verified.

This corresponds to delays of up to 6 weeks compared to the 2007 season and 3-4 weeks compared to usual starts. These strong delays are worrying since to complete crop cycles, traditional subsistence farmers now require good, regular rainfall until about the end of September which is unlikely to happen.

 

Preliminary Crop Perspectives

Based on the previous indicators, the following regions face poor to very poor agricultural production and/or pasture prospects :

·       North Darfur – the more marginal areas of Mellit, Malha and Kutum as well as the eastern producing areas of Um Kedada.

·       North Kordofan – the western areas in the localities of En Nahoud and Gebeish and the more northern areas of the state.

·       White Nile – the more northern areas of Ed Dueim and Getaina, dominated by traditional agriculture and livestock

·       Gezira – the eastern rainfed and pastoral half of the state

·       Kassala – nearly all rainfed agricultural areas of the state, both traditional and mechanized

In all cases, the situation is due to a severe delay in the arrival of the rains. Data for the period of mid August (11^th -20^th) reveals that consistent rains have finally arrived across many of these regions. With planting so late and crop cycles lasting 60-90 days depending on variety, significant improvement in the production perspectives requires at least a period of 5-6 weeks of reasonably well distributed rainfall in suitable amounts. This implies that the rainfall which has started in mid August needs to hold until late September.

If this is not the case, severe shortfalls in production will result across most of these regions.

The worst situation is now in western North Kordofan where even mid August rainfall has failed. Most households here depend on millet and groundnut planted on sandy soils, which cannot store much water, and are particularly sensitive to end of season dryness. Only very short cycle varieties are now likely to yield some production, but these are not really typical of these regions. Local farmers may well face a complete failure of the cropping season.

Vegetation Status

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.

Vegetation development in Southern Sudan has now reached the seasonal peak. Across most of the region vegetation is at average to moderately below average levels, except for areas of Upper Nile and in East Equatoria as a result of the above average July rainfall (see Fig 2b), indicating an improvement in pasture conditions. Elsewhere, there is a mixed pattern of below average conditions, becoming better defined in Jonglei state and Central Equatoria; the patterns in West Bahr-el-Ghazal are attributed to cloud interference. Overall, considering the pattern of rainfall and vegetation behaviour, agriculture and pasture performance in this region is likely to be unsatisfactory, below the levels of the past three years.

In northern Sudan current vegetation conditions are now reflecting the delayed rainfall patterns. Below average conditions have developed and intensified in the more northern areas as vegetation development is also delayed relative to the average. In particular, the data identifies areas in western North Kordofan – North Darfur, White Nile and in northern Gedaref-southern Kassala as the worst affected. It also shows poor conditions  in the mechanized farming areas of (western) Sennar, South Kordofan and White Nile.

Combined with the rainfall and growing season indicators, all evidence points to serious problems for the agricultural season in northern Sudan in particular for traditional farmers in sandy soils. Areas where livelihoods are almost exclusively based on agriculture (e.g. western North Kordofan) will be hardest hit. For areas such as southwestern North Kordofan it maybe too late.

Mechanized farming areas have a better outlook as deep clay soils can hold moisture for longer periods. Rainfall patterns in some of the mechanized farming areas (Sennar, South Kordofan, southern White Nile) would indicate possibility of timely planting, so there maybe other factors such as availability of finance at work to enhance the problems.

Improvements are now crucially dependent on a continuation of the mid August rainfall throughout September, but in all likelihood cereal production in Sudan this season will be below average and well below the levels of the past three years. Market prices will need close monitoring from now on.

Seasonal Perspectives

Rainfall Outlook for September-October-November 2008

The seasonal forecasts presented here throughout 2008 (from April) had consistently indicated above average rainfall as the most likely scenario (the one with higher probabilities). The previous issue already pointed out that these seasonal forecasts for 2008 had to be viewed with some reservations given the progressive moderation of the predicted probabilities and the way the season was evolving.

Note that these forecasts are probabilistic and hence they will fail some of the time : in simple terms, a forecast of 80% probability of above average rainfall is expected to fail 20% of the occasions.

The current available forecasts cover the later part of the rainfall season – September to November, and are shown in Figs 5a (from IRI, International Research Institute) and 5b (ECMWF, European Centre for Medium-range Weather Forecasts). Both point to a wetter than average period.

The obvious question is : given the poor predictions of the previous forecasts, will these be any better? We do not have enough elements to answer. The intuitive reaction is to moderate their indications to a closer to the average scenario.