Farmers in the summer rainfall areas of South Africa can look forward to positive weather and rainfall conditions in 2018. However, the south-western winter rainfall regions are unlikely to see relief from the crippling drought this summer and will have to wait until winter 2018 to learn their fate.

Past conditions in context

“Following the 2015/16 drought across most of South Africa, above-normal rainfall during 2016/17 brought welcome relief for the summer rainfall region. The current summer therefore begins on a much more positive note than in 2016/17, which started with reservoirs over the summer rainfall region at low levels and extreme drought conditions over the Lowveld,” says Dr Johan Malherbe, senior researcher at CSIR.

Over the summer rainfall region, dam levels are on average up 12% from the levels for the same time last year (Figure 1). Only dams in the Western Cape are on lower levels than a year ago.

Figure 1: Dam levels per province, expressed as a percentage of the full capacity, for November 2016 (black bars) and November 2017 (red bars). (Data courtesy of the Department of Water Affairs)

The difference in water availability is strongly related to the rainfall during the previous summer. The Standardised Precipitation Index (SPI) for the twelve-month period until November 2017 shows a clear wet bias compared to the same twelve-month index until November 2016 (Figure 2).

Figure 2: Twelve-month Standardised Precipitation Index until November 2016 (left) and November 2017 (right).

 

“At the twelve-month time scale, rainfall conditions have improved markedly over the summer rainfall region compared to the situation last year the same time. Some parts around the Highveld are shown to be extremely wet by November 2017,” says Dr Malherbe.

Until both November 2016 and November 2017, the drought focussed strongly over the southern parts of the country. While severe drought was visible at the twelve-month time scale until November 2016 over the southern parts of the country, focussing on the western parts of the Eastern Cape, the focus has shifted to the winter rainfall region during 2017.

Relatively dry conditions at the twelve-month time scale until November 2016, followed by another year of severe to extreme drought over the same region, resulted in the low levels of reservoirs noticed currently over the Western Cape. The production of wheat over the region is expected to be markedly less than last year and the long-term norm, due to low rainfall, especially over the Swartland, during the 2017 winter. Low dam levels will also have a negative effect on irrigated crops in this region.

Neel Rust, chief operating officer at Laeveld Agrochem, says the Crop Estimates Committee’s fifth production forecast (December 2017) indicates a marked drop in the Western Cape’s wheat crop for 2017 compared to 2016. Click here to read the latest crop estimate.

According to Rust, wheat farmers in the Western Cape cannot produce wheat profitably at these return per hectare levels, which places immense pressure not only on the farmers but also on the financiers, especially considering the coming planting season of April and May 2018.

The current drought conditions in the south-west over the winter rainfall region and the relatively wet conditions over much of the summer rainfall region are reflected in the anomaly of the cumulative vegetation activity, as represented by the Percentage of Average Seasonal Greenness (PASG) for the period June to November 2017.

The PASG is calculated by expressing the cumulative vegetation activity, derived from Earth Observation data, as a percentage of the long-term mean for the same period. Figure 3 shows the PASG for the period June to November 2017.

Figure 3: Percentage of Average Seasonal Greenness for the period June to November 2017.

Vegetation activity clearly experienced stress during the last few months over the south-western parts of the country, including the entire winter rainfall region.

In contrast, cumulative vegetation activity is estimated to be above normal, judging by the PASG values exceeding 100% over much of the northern and eastern parts of the country. Much of the eastern maize production region, where the planting window ended during November or earlier, is located within the areas of anomalously high vegetation activity. Moreover, these areas received above normal rainfall during October and November 2017, with sufficient rainfall for planting.

Positive outlook for the current summer rainfall season

“For the time being, it seems as if the positive climate conditions over the summer rainfall region is set to continue in broad terms. Coupled Global Climate Models, used for seasonal prediction, have recently trended more favourably in their outlook for summer rain over Southern Africa,” says Dr Malherbe.

This recent positive outlook for a reasonable summer over much of the interior is based on the reaction of climate models to the changing Sea Surface Temperature Anomalies (SSTAs), mostly in the equatorial Pacific Ocean.

Over the central to eastern Pacific, the negative SSTAs have recently crept downward across the so-called La Niña threshold. Persistently stronger than normal easterly trade winds across large sections of the equatorial Pacific during winter 2017 – and continuing to this day – have resulted in a very late developing La Niña, a global ocean-atmosphere state that is associated with above normal rainfall over the summer rainfall region.

“If these anomalies persist for another two months, this year will be counted as a La Niña summer.”

According to Rust, it is encouraging that the forecast points to a continuation of good rains in the second half of the summer rainfall area.

“This is good news to especially producers in the Western parts of the Free State and the north-west area, seeing that plantings, which have been the case the past four years, happen later. The last few seasons’ good rains – which are essential before planting can commence – moved on by about six weeks.

However, late plantings carry a greater risk as the summer row-crops are still in their growth stage when early frost occurs. Under normal conditions, producers would welcome early frost as it helps in reducing the summer row-crops’ moisture content, but with late plantings there is the risk that the plant may be damaged, as it is still in its growth phase.”

Dr Malherbe says that anomalous SSTs over the equatorial Pacific Ocean will not necessarily force weather anomalies across South Africa, even though there exists a strong statistical correlation between the SST anomalies and rainfall over South Africa.

This correlation may also simply mean that both rainfall over the subcontinent and the SSTs across the equatorial Pacific Ocean are influenced by the same factor, namely the strength of easterly winds at a global scale. That is, the strength of the easterly winds across large parts of the world may drive both the SSTAs and rainfall across the summer rainfall area.

“As the trade winds across much of the equatorial Pacific are stronger from the east than normal, it may be an indication that the forcing associated with the La Niña event during the coming summer may be sufficient to result in wet conditions over much of the summer rainfall region,” continues Dr Malherbe.

“Over the winter rainfall region, it is not impossible, or at least highly unlikely, that drought conditions will be broken during the coming summer. This area will have to wait at least until winter 2018 to know if the water crisis will be alleviated.” – Press release

Dr Johan Malherbe, in conjunction with AgriSeker and supported by the Land Bank, writes a weekly summary on the current weather conditions titled CUMULUS, which is published on the AgriSeker website: www.agriseker.co.za.