Fertile soil washed away by heavy rains

We may find barren Mediterranean landscapes like those in Greece aesthetically pleasing, but they are the product of a soil erosion process that began in ancient times after the forests were cut down. This historical fact can serve as a warning, because in the EU hundreds of millions of hectares are considered to be endangered by erosion, two-thirds of which is caused by water. In a project funded by the Austrian Science Fund FWF, principal investigator Andreas Klik and his team from the University of Natural Resources and Applied Life Sciences Vienna have now examined this latter phenomenon in more detail, since there are still a number of unanswered questions about the connections between soil erosion and rainfall.

“The aspect most crucial for erosion is the kinetic energy of the raindrops,” explains Klik. The drops stir up soil particles, which are then transported away. The strength of the effect depends on the size of the drops and their fall velocity. As Klik specifies, raindrops behave very differently depending on their size: “A raindrop does not look the way you might imagine – like a teardrop or a little sphere. It is flattened out by air pressure as it falls, then becomes convex and finally takes on the shape of a small parachute and bursts into several smaller droplets. This happens from a size of about six millimetres.”

Measuring device errors

“Since the 1990s, there have been devices to determine the distribution of droplet size and fall velocities,” notes Klik. These measuring devices, which are called distrometers, can measure passing raindrops with the help of a laser. Although they have become affordable in the past ten years, they are still not in frequent use. “We bought a distrometer specifically to study the splash erosion of soil particles as a function of the droplets' kinetic energy.”

Actually, the aim was to test already existing erosion models and to compare measurement data from distrometers in Austria, the Czech Republic and New Zealand. But Klik encountered a problem: “We had a number of partners. The Austrian Federal Office for Water Management has the same device we have, a team in the Czech Republic has a different device, and in New Zealand there was a third type. We took all these distrometers to a place in Petzenkirchen near Wieselburg in Lower Austria and saw that they produced completely different results.” The difference was significant, amounting to as much as 30 percent in some cases. In some devices, Klik noticed a large number of very fine droplets, which would normally be too small for a raindrop, and suspects that the error source is there.

Estimating of the energy of raindrops

Hence, Klik and his colleagues set about calculating the energy of the drops directly on the basis of rain intensity. Measurements in Wieselburg and at a long-established measuring station in Mistelbach showed that the energy of the drops can be estimated quite well by determining the precipitation intensity every five minutes. The measurements revealed pronounced regional differences, as Klik notes: “Mistelbach has 550 millimetres of precipitation per year, which is less than Wieselburg, where we measured 900 millimetres. However, the average droplet diameter was 1.1 millimetres in Mistelbach and 0.8 millimetres in Wieselburg. This means that precipitation in Mistelbach is actually more erosive.” Despite the smaller droplet size, however, the erosion in Wieselburg is stronger overall because of the higher rainfall.

Dry soils at risk

Klik's team then investigated precisely how raindrops interact with the soil in a field study. The team had intended to supplement results of the measuring stations in Austria with measurements in Prague and Christchurch in New Zealand, but New Zealand provided an insufficient amount of data owing to low rainfall. “We looked at very small areas of soil without vegetation, about 20 square centimetres in size. After each rainfall we went out and investigated how much soil had been loosened from the measurement zone and splashed into the surrounding area,” Klik explains. This showed that there is a high risk of erosion, especially in dry soils, while moist soil is more stable: “Moist soil is more cohesive and less likely to be splashed about. Dry soil is at risk because larger chunks are broken up into individual particles that are more easily removed with the surface flow.”

The jeopardy of extreme weather

The results of this basic research project provide scientific evidence for phenomena that have been observed for a long time and they confirm that the problem is threatening to become exacerbated by increasing weather extremes. “We have been measuring in Mistelbach for 25 years, and we have documented 150 events. Among that number there have been four or five particularly heavy rainfalls that are responsible for more than 80 percent of the soil erosion,” says Klik.

Klik shares his findings regularly with agricultural colleges, but he emphasises that healthy soil is not only vital for agriculture: “We get 100 percent of our drinking water in Austria from groundwater. Every drop of precipitation comes into contact with soil, which acts as a filter. The longer this filtering distance and the cleaner a soil is, the better the groundwater.”

The knowledge gained from this project, which included project partners from the Czech Republic and New Zealand and was co-financed by the Czech Science Fund GACR, is designed to facilitate assessment of the erosive effect of rainfall in the future and the introduction of targeted measures.

Personal details

Andreas Klik is a soil physicist and hydrologist. Recently retired, he formerly held a position at the Institute for Soil Physics and Rural Water Management at the University of Natural Resources and Life Sciences, Vienna. His focus lies on soil protection and soil health. The Austrian-Czech project, which ran for three and a half years, received EUR 341,000 in funding from the Austrian Science Fund FWF and was completed in 2021.

Publications

Johannsen L.L., Zambon N., Strauss P., Dostal T., Neumann M., Zumr D., Cochrane T.A., Blöschl G., Klik A.: Comparison of three types of laser optical disdrometers under natural rainfall conditions, in: Hydrological Sciences Journal, Vol. 65, 2020

Johannsen L.L., Zambon N., Strauss P., Dostal T., Neumann M., Zumr D., Cochrane T.A., Blöschl G., Klik A.: Impact of Disdrometer Types on Rainfall Erosivity Estimation, in: Water Vol. 12, 2020

Zambon N., Johannsen L.L., Strauss P., Dostal T., Zumr D., Neumann M., Cochrane T.A., Klik A.: Rainfall parameters affecting splash erosion under natural conditions, in: Applied Sciences 2020

Zumr D., Mützenberg D.V., Neumann M., Jerabek J., Laburda T., Kavka P. et al.: Experimental Setup for Splash Erosion Monitoring – Study of Silty Loam Splash Characteristics, in: Sustainability Vol. 12, 2020