The R59 (tar road) at Viljoenskroon in the Free State has been thoroughly flooded by the adjacent salt pan (Witpan) since December 2022. In the 9 months since it flooded in December, the water level has decreased only by 30 cm in vertical depth as measured from the visibility of a road sign that is partially under water. Based on the visibility levels of the roadside infrastructure, the road surface at the deepest point is at least 1.5 meter below the water level. This body of water is an incredible sight to behold now, with abundant birdlife enjoying the new expanded habitat. However, it is obviously quite an inconvenience for travellers needing to get from the Northwest and Gauteng Provinces to the Bothaville area and beyond, that now has to zig-zag through Viljoenskroon town or the industrial complex. According to the locals, the water level of the pan has never been this high in the history of the town, which leads me to two questions: 1) Why did this happen at the end of December 2022 and 2) Why is the water level dropping so incredibly slowly.
1) What made the conditions so much different in December 2022 in relation to previous high rainfall events to cause the flooding? Local rainfall records show that, even though rainfall was fairly high during November 2022 and the beginning of December 2022, the rain generally fell in lower amounts, for example 8 to 10 mm of rainfall at a time, at regular intervals. The water level in the pan was low and stayed low. This was because the rainfall in the areas feeding the pan was absorbed by the soil with relatively low run-off and because evaporation was matching, or exceeding the rate at which run-off reached the pan. However, the rainfall was adequate, and the soil became gradually saturated. Then, during the second and third week of December 2022 the rainfall increased drastically. Over the course of 7 to 10 days there were various storms, with rainfall measuring between 50 and 200 mm every time. Before the water could dissipate, the next storm would come. This resulted in torrents of run-off coming from more than 10 km away in certain places, just gathering speed and volume as it approached the pan. It opened up new waterways and pushed through small dam walls and just kept accumulating as it reached the pan. The water level increased in less than a week from well below the average level to the highest level ever recorded. People from the local community are also confident that changes in farming methods and the application of drainage ditches to keep rainwater from flooding crops exaggerated the effect. In addition, the pan has no outlet and water can only dissipate through evaporation, which brings me to my second point…
2) Why is the water level staying so high? The main reason for this is that the pan has no outlet. The water has to evaporate or be pumped out. The flooding took place in December and the rainy season continued very late into summer and the beginning of winter. This means that evaporation has only had the winter months to have any kind of effect and evaporation is much lower in winter than in summer due to lower temperatures and less sun exposure. This is also the main geological reason why a salt pan formed in this area. Salt pans generally form in areas that are flat, so water remains fairly shallow, and have high evaporation rates. In winter, it typically stays full of water and leaches minerals out of the soil, and in summer, the water can only evaporate, as it has no outlet, and the salt percentage in the water increases.
It will be interesting to see how the pan responds during the next rainy season, which is due to start within a month. Will evaporation win the race, or has the picture of the landscape in this area been altered for the long-term? If no water is pumped out of the pan, based on the current and expected evaporation rate for the area, the water will take approximately 3 years to evaporate to a point where the road will be accessible again. However, many different factors may have an influence on this and I am excited to see how it “pans” out.
Dr. Hélen Prinsloo
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