The group of scientists, who included Hartogensis’ Meteorology & Air Quality colleague professor Jordi Vilà and six young Wageningen researchers who were collecting data for their PhD projects, wanted to get a better understanding of the role the atmosphere plays in the water cycle. How does the atmosphere behave in various circumstances and what does this do to the water cycle? And how does the presence or absence of greenery influence the circulation of water in the atmosphere? The models that are used at present for weather forecasts give a rough picture at best of these small-scale changes in the atmosphere; the research project’s aim was to improve these models.

The researchers carried out their research in the summer of 2021. “The summer is the time of year when the contrast between wet and dry areas is greatest”, explains Hartogensis. “We knew that the irrigated land affects the local weather, but how that process works exactly was not yet clear.” The researchers performed measurements on land and in the air. The data they collected ranged from information about soil moisture and the opening of the plant’s stomata to surface temperature, radiation, heat exchange, water vapour, CO₂ and vertical weather profiles extending to an altitude of 10 kilometres. The scientists also studied how the atmosphere warmed up and what role sea breezes and turbulence play in this process. The leaves of plants are a particularly important factor.

“The plants in the wet area have a lot of moisture in the soil”, explains Vilà. “It is very hot there: temperatures rise to 35 degrees and higher. So you get a kind of competition between the plant, the water in the soil and the dry air. The plant transports water from the soil to the leaves and opens the stomata to take in CO₂ for photosynthesis. The price the plant pays for this is losing water to the atmosphere through evaporation. To model this process accurately, we measured the uptake of CO₂ and loss of water for individual leaves using special instruments attached to the leaves. The moisture that evaporates from the crops ends up in the atmosphere, where it mixes with the less humid air above. That process continues up to an altitude of around one kilometre, which is the atmospheric boundary layer. At the same time, we see that the boundary layer is much higher up above dry land as the soil contains less moisture to evaporate, so there is more energy available for heating the air. That sets circulation in motion, comparable to an onshore breeze”, explains Hartogensis. The data was collected by remote sensing on land and satellites, airplanes, drones, tethered balloons, radiosondes and other advanced instruments in the air, some of which the WUR meteorology group had developed itself.

Thanks to all these measurements, the researchers now have a better understanding of the role played by photosynthesis in atmospheric heating and cooling, and how that affects the weather. The radiosonde, for example, was sent up once an hour to get a vertical profile of the atmosphere, which showed the researchers how the temperature changed. That let the researchers see how the circulation between moist and dry areas works.

And that is precisely the aim of the LIAISE project. Large amounts of data are being collected about the influence of irrigation on the local weather in a dry area, which gives scientists a better picture of the processes at play. This will let adjustments and improvements be made to the weather models. “There’s a silent revolution going on in meteorology. People often say forecasts are getting better because the computers are better”, says Professor Vilà. “Sure, more powerful computers can do more calculations faster, but you still need scientific understanding to improve the weather model.” And that is the revolutionary aspect, according to Vilà. Hartogensis agrees: “Look at the past 50 years and you can see how much better we have got at predicting the weather and the benefits this has brought society, not only economically, but also for people’s holidays or walking the dog without getting caught in a downpour.”

The weather also shows how the climate is changing. Extreme weather, such as drought, is one of the consequences of climate change. “The Catalonian farmers are noticing it too”, says Vilà. “I talked to them every day and they can see the landscape changing: certain trees are disappearing and the soil life is different now. In the past, people just said the local climate was affected by the irrigated area. Our measurements and model studies are helping to separate out local effects due to irrigation and the influence of climate change on the weather. We are increasingly having to deal with extreme weather, and you need to collaborate to figure out how it all works.” Experiments this big only turn up once every few years and are even rarer for this particular application, but both scientists are convinced such experiments will be organised more frequently in future to extend our scientific knowledge. “To understand the complexity of the processes, you need experiments like this with people from various disciplines coming together and carrying out measurements on a wide range of timescales and physical scales”, says the project manager, Hartogensis. “You need the eco-physiologist who studies the leaves, micrometeorologists such as us who examine the exchanges on the scale of a couple of kilometres, and the satellites that collect data from space.”