The fingerprint of last weeks’ heavy precipitation on soil moisture in Flemish gardens

Last week, the southeast of Belgium had to cope with extreme precipitation, resulting in hallucinatory images of floodings. These large amounts of precipitation also leave clear traces in the soil moisture measurements of the CurieuzeNeuzen microclimate network.

As you can see on the map below, gardens in the province of Limburg, Antwerp and Flemish Brabant show an absolute peak in soil moisture of up to more than 20% in some places compared to the reference level last weekend.

Difference in soil moisture in Flemish lawns between the peak level on July 15 (after the days of heavy precipitation in the center and east of the country) and the average soil moisture on July 11, as a reference.

Lawns as sponges

Such soil moisture peaks clearly demonstrate the importance of our lawns, gardens and nature as a sponge during heavy rains: all the water that can be absorbed by our garden soils is at least temporarily trapped, and lowers the pressure on our sewers and rivers, thus reducing the risk of flooding. The observed increases in soil moisture even occurred in garden soils that were already very wet, after a very wet first half of July (the average soil moisture percentage on July 11 in Flemish lawns was 38%).

However, at times of extreme precipitation such as this, much of the precipitation does not get absorbed into the soil: there is a maximum amount of precipitation that soils can take at one time before they are completely saturated. The excess water will have to run off above ground, causing flooding. That maximum depends among others on soil type, precipitation history (very wet, but also very dry soils can absorb less water) and soil health (soils with a high diversity of soil life can absorb more water). If a large part of the soil is also covered with concrete or asphalt, the capacity of the soil as a water buffer rapidly decreases. The result: more flooding.

Also, the data from the lawn clouds clearly show the consequences of the long duration of this unusually stationary rainstorm. On 14/7, when the heavy rainfall in Flanders was still concentrated in the east of the region, the increases in soil moisture in the lawns of the CuriousNeuzen network in Limburg were still limited to 10 to 15%.

Difference in soil moisture in Flemish lawns between the peak level on July 14 (after the day with heavy precipitation only in the eastern part of the country) and the average soil moisture on July 11, as a reference.

More extreme weather

We also expect more of these extreme precipitation events in the future. Even if the total amount of precipitation in Belgium remains the same, it will be more difficult for plants to get water if that precipitation falls in fewer, but larger showers, just because the soil becomes saturated and has to lose much more water.

This summer, unlike previous years so far, Flanders was on the ‘wet side’ of persistent weather events in Europe, resulting in a lot of precipitation. This precipitation did allow the soil water stocks to fill up again. Such a wet start also reduces the chance of heat waves in our gardens: the summer sun will need a lot of energy to evaporate all that water, leaving less energy for heating up. A wet soil as we have now is the best air conditioner against heatwaves one can have. With the data from this summer, CurieuzeNeuzen will dive deeper into the role of this soil moisture in keeping our gardens cool.

The patterns on the maps above also clearly show that there can be large regional and local differences in the impact of precipitation on soil moisture. Our scientists will analyze these patterns to see if and how much garden location and management can affect the impact of precipitation on soil moisture, and how much we ourselves can manipulate the infiltration potential of our gardens.

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