Six questions and answers about persistent dehydration

From a meteorological perspective, we speak of drought when there has been less precipitation than normal for an extended period of time and a structural precipitation deficit can build up. Many factors affect the severity of dehydration. We are particularly prone to drought in the summer, when temperatures rise and more soil moisture can evaporate due to the sun. The existing soil moisture is then converted into water vapor that ends up in the atmosphere, causing the soil to dry out even more. Most of the water evaporates at high temperatures with a dry type of air and lots of wind. Soil type also plays a role. Sandy soil dries out faster because it loses more water due to its large grain and more pores. Clay soils retain more water due to their smaller grain size and are therefore less sensitive (quickly) to drought.

The duration of drought is also an important factor in estimating its severity, especially for effects on groundwater levels. Groundwater levels generally respond somewhat more slowly to fluctuations in rainfall. But when it dries up for a longer period of time, the groundwater level can drop as well. Groundwater levels in summer are lower than in winter anyway, due to more rainfall in the winter months and less evaporation. In a normal year, the summer precipitation deficit can usually recover in the winter, but when there is a period of severe dryness and the precipitation is also less in winter, the precipitation deficit may not fully recover. When a drought occurs again the following year, the effects of the drought will be greater and groundwater supplies may continue to decline.

At the moment we can talk about a very dry situation in our country, even if we look at the forecast of precipitation for the coming days. The lack of precipitation accumulated gradually during the spring. From January to April, there were about 45.6 mm less than usual in Uccle. March in particular was exceptionally dry at just 2.2 mm (compared to a normal 59.3 mm). In addition, the month was also very sunny with low humidity. The second half of April and the first half of May were also very dry, which resulted in the accumulation of precipitation deficits. We can gradually talk about an exceptional drought, which may be more dangerous than the record 1976.

The current drought is caused by the continuous supply of areas of high pressure that ensure turbulence with rainfall at a distance and/or severely weakened. March 2022 was primarily marked by a static “Omega blockade”, a typical jet stream pattern in which there is a constant region of high pressure sandwiched between two depressions.

A prolonged drought can have many negative consequences, particularly on nature and the agricultural sector. At first, superficially rooted plants will be particularly affected, after a longer period of drought and lower groundwater levels, deep-rooted plants and trees can gradually dry out. The same applies to agricultural crops that are also affected by drought. A prolonged drought can lead to significant crop losses.

Dry soil with plenty of sun and wind increases the fire risk in our nature reserves. In many nature reserves in Belgium, there is already increased vigilance and strict rules regarding smoking are enforced† Given the dry condition of the soil and nature, one cigarette butt can lead to a massive wildfire† Dry wilderness areas such as Kalmthoutse Heide and parts of the High Fens are particularly sensitive to such wildfires.

In addition to the direct effects of drought, there are also indirect effects. For example, a long dry period in the run-up to summer can also contribute to higher temperature extremes, because the top layer of soil has largely dried out. Part of the solar radiation is used to evaporate moisture in the soil. This process requires heat, which creates a cooling effect during evaporation, so that not all of the sun’s heat is used to warm the soil. However, when the top layer of soil dries out, a much larger portion of the solar radiation can be used directly to heat the soil (and little or rarely to evaporate), which can lead to higher temperatures. The combination of an intense heat wave with the supply of (semi) tropical air, along with drying out of the soil, can lead to temperature extremes. A similar situation happened in 2019. We first recorded over 40 degrees in the shade in the summer. This summer was preceded by a very dry spring. The effect of evaporation mentioned above may have played an important role in this.

In addition, the dry top layer also leads to acceleration of rainwater run-off. Dry soils are sometimes more compact (clay soils often show cracks) and have what we call a “hydrophilic” character in formal terms: a kind of waterproof layer forms on the soil. Compare it to the dry soil in your plant pot, where the water you pour in it stays for a while before it seeps into the soil. According to this mechanism, rainwater that falls on very dry soil will flow more quickly into sewers or streams, so that the water can seep into the soil less quickly. When very heavy rain falls on dry land, flooding and mud disturbance are more likely to occur.

Dry spells have become more frequent in recent years, such as in 2018, 2019 and 2020. Just like this year, the spring of 2020 was very dry. April and May were the driest months since UCL measurements began. When we look at drought statistics over a longer time scale, we also effectively note that there is a linearly increasing trend in the occurrence of dry spells in spring.

Climate warming initially causes temperatures to rise, which may have an effect on the evaporation of soil moisture. On the other hand, global warming may also cause changes in circulation patterns in our atmosphere. Some climate projections for the future show an increasing effect of high pressure areas in spring and summer, a trend we’ve seen a lot in recent years.

In addition, there are also many scientific studies linking global warming to the weakening of the jet stream. The jet stream is what drives our weather and determines whether we have to deal with areas of high or low pressure. This high altitude atmospheric river is powered by the horizontal variation in atmospheric temperatures (between the polar regions and the equator). Because of global warming, the polar regions are currently showing faster warming than anywhere else in the world. These areas heat up two to three times faster! As a result, the temperature contrast between the pole and the equator gradually decreases, resulting in a weaker jet stream. Weaker jet streams travel slower and create more wobble, which may contribute to more frequent occurrences of blocked weather systems. For example, it can ensure the stability of the high pressure area and can stay in the same area for a long time. This increases the chance of a prolonged dry period.

Periods of drought can seriously disrupt our water supply. In the event of severe and persistent drought during the summer period, this can contribute to water scarcity with lower groundwater levels and increased water consumption. When it dries up, we automatically use more water, which in turn depletes water resources. So it is also important to use water economically and to make ourselves resilient in the face of severe droughts.

Due to its densely populated character and high degree of hardening, Flanders is a region very prone to drought and potential water scarcity. However, we can change that by making our space more climate robust. The Blue Deal is the appropriate Flemish policy framework for this.

You and I can do our part by focusing as much as possible on the reuse of rainwater. We can collect the rain – when it falls – to reuse the water later. This can be done, for example, by installing a rainwater tank (which in many cases is already mandatory for new construction), or a rain barrel. In addition, we can also create more space for rainwater intrusion into the soil, by designing our gardens as green as possible and softening them. Pebbles and other forms of front yard paving are very popular today, but they disrupt the rainwater balance, as water that falls on them can no longer naturally penetrate the ground. By focusing on softening and greening, we can solve this problem and ensure more infiltration. In this way we contribute literally and figuratively, or rather we break out.