The wettest summer in two centuries is an unexpected windfall for our citizen science project ‘CurieuzeNeuzen in de Tuin’ (CNidT). Our garden sensors fill a black hole in our knowledge: how can gardens act like sponges and buffer extreme rainfall?

Check out the original article from De Standaard, in Dutch, by Ine Renson!

An unexpectedly wet spring, which cumulated in the extreme precipitation of mid-July with a hopeless string of crappy days in August: for a large-scale citizen science project on heat and drought in gardens and parks, it seems like a worst-case scenario. But the climate researchers of the University of Antwerp who are responsible for the project show no sign of panic. On the contrary.

‘This is better than we could dream of,’ says Jonas Lembrechts, who is the scientific coordinator of the project. ‘With CNidT we are investigating how gardens can arm themselves against extreme weather conditions. Given the trend of recent summers, the focus was on heat and drought. But our soil sensor, which records moisture and temperature, can tell us as much about wet and cold. Because of this summer’s exceptional situation, we are therefore happily switching the focus to extreme precipitation. It’s just the other side of the same coin.

For scientists, this is a golden opportunity. This is the wettest summer in two centuries, and just now we have 5,000 sensors in the soil all over Flanders. We were unexpectedly handed a unique dataset,’ says Lembrechts. ‘With unprecedented accuracy, we can map out how our gardens are able to accommodate large amounts of precipitation.’

“Snertzomer”

Just how extreme this summer was can be seen from the graphs drawn by Stijn Van de Vondel. As a test case, the researcher had already installed a TOMST TMS-sensor in his Kempish garden last summer. This allows us to visually compare the wettest and one of the driest summers of the past century. ‘Except for a few June days, this summer was much wetter than the previous one,’ he says. According to the data from the Royal Meteorological Institute (RMI), June and July had exactly the same number of rainy days as the previous year. But when it did rain, a lot more fell from the sky. This is also evident from my soil sensor: the humidity level at the end of June shot up to 30 percent and has not dropped below that since, with peaks in July between 40 and 50 percent. Imagine a sandbox with half sand and half water in it: that’s a mud puddle. We were at the limit of what a soil can swallow in July.’

But contrary to popular belief, this summer was no cooler than the last. The temperature curves of 2020 and 2021 above are playing leapfrog. This pattern is also confirmed by the figures from the RMI. ‘Because of the rain, we have the idea that it is a thoroughly crappy summer,’ says Van de Vondel. But as far as the temperature is concerned, that’s not true: this drizzly summer was not colder than average. That too is relevant in the light of climate change: we won’t get really cold summers very often anymore.’

Pavement

Concerning that extreme wetness, CNidT brings fascinating insights. ‘The RMI data tell us where exactly how much precipitation has fallen,’ says Van de Vondel. ‘The Vlaamse Milieumaatschappij has accurate data on groundwater levels. But there is a blind spot in between: how much of the precipitation that has fallen is actually stored in the soil and transported to the deeper layers of the ground, e.g. via our gardens? And what proportion flows away via sewers, canals and rivers?’

For the first time, we are going to be able to fill in the missing link in detail and on a large scale,’ nods Lembrechts. We can indicate which gardens store a lot of water and which do not. And, importantly, why. Because there are often large differences between two gardens in the same municipality. Insights into these dynamics is essential if we are to pursue a good water policy.

From previous ecological research we know that the same factors that protect our lawns from extreme drought are also decisive in the fight against flooding. One hypothesis is that the degree of hardening in a garden or its surroundings plays a crucial role. ‘The degree of paving seems essential to determine how much a garden can buffer, and thus to what extent you can avoid the risk of flooding,’ says Lembrechts. ‘It determines how hard a garden has to work to handle the falling water. For example, in a highly sealed region with small gardens, each garden will have to swallow more. We’re trying to put that into numbers now.’

Whereas in Wallonia it was mainly the topography that determined the consequences of the flooding (extreme precipitation running down the slopes and converging at the lowest point in the valley), in Flanders it is mainly the large and spread-out paved surfaces that are the Achilles’ heel, in addition to the fact that we continue to build in flood plains.

But vegetation also explains how well a garden can buffer water. Lembrechts: ‘Rain sticks to the leaves of trees and plants, it always takes a while for the drops to fall through. This causes a delay in the peak of the water flow, which can make a crucial difference during a cloudburst. Trees and plants therefore form an initial buffer. Moreover, through their roots they suck some of the water out of the soil, and evaporate it through their leaves.’

4,100 Olympic swimming pools

De-watering, greening: the measures to be taken against drought and heat will also protect us against the consequences of extreme precipitation. Next month, the researchers hope to come up with accurate figures and insights into the sponge effect of our green spaces. ‘But already it is clear how important they are as a lever in the fight against climate change,’ says Van de Vondel. ‘During the mid-July flood, the garden complex, along with the parks and natural areas, buffered 4,100 Olympic-sized swimming pools of water, according to an initial estimate. That’s huge. That volume would otherwise have run off and put extra pressure on our sewers and roads. Every gallon of water sucked up by a garden results in less inconvenience during extreme rain events.’

The defense against the consequences of severe weather or rising sea levels is therefore not only in ‘gray infrastructure’ such as dikes, dams and sewers, but increasingly also in ‘green infrastructure’ such as gardens, wetlands, wadis and flood plains. The realization is dawning that we no longer need to defend ourselves against water, but that we must learn to live with water and the opportunities it offers. The data from CuriousNoses provides insight into how we can do that more efficiently.

This is actually a story of collective responsibility’, says Lembrechts. The question is not so much which Flemish garden functions best as a sponge or air-conditioning system, but how we can use the garden complex as a whole to optimally store water or cool cities. Some gardens buffer more than others, but each contributes its own bit. We don’t always realize it, nor do you see it from the street. But together we have a big impact. If we were to pave over our gardens and green spaces en masse, the news would be even more dramatic in the next extreme rain event. If we do the opposite, we can mitigate its impact. The decisions you make in your garden thus make a huge difference in economic and ecological cost, and most importantly in the human suffering involved.