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A 3D-model reveals nutrient transport pathways to nearby watercourses

30 November 2011 Aalto University

Nutrient loads from agriculture are higher than from any other source in Finland. Aalto University School of Engineering researcher Lassi Warsta built a computer model, which can be used to investigate carrier media of nutrients, i.e. water and suspended sediment in agricultural fields.

Heavy autumn rainfall can wash nutrients from fields into nearby waterways.  In Finnish waterways, for example, 67 percent of phosphorus load is derived from agriculture, and only 5 percent from the municipal wastewater.

− Nutrients are transported from the fields either dissolved in water or adsorbed on sediment particles. Therefore, it is important to understand both water flow and soil erosion mechanisms in agricultural fields, says Lassi Warsta.

A three-dimensional model of a typical Finnish clayey, subsurface drained field is running on Lassi Warsta’s workstation. After six years of work, the FLUSH computer model is packed with wisdom. Mathematical formulas describing water flow were developed already in the beginning of 20th century, but their application in practice is still difficult. Warsta’s 3D-model combines numerous mathematical formulas into one handy tool. It can be used to investigate how rainwater and suspended sediment travel through soil layers to the subsurface drains of a clay field. When runoff and sediment load are known, the result can be used to estimate the amount of nutrients lost from the field. 

Aalto water researchers have focused on computational modelling of water flow since 1990’s. Data for the Warsta’s model was collected in the 90's at the University’s own three-hectare research field. Located in Kirkkonummi, Southern Finland. This high-tech field had its own weather station, and real-time instruments continuously monitoring groundwater elevation, water flow in subsurface drains as well as the suspended solid and nutrient content of waters.

− An agricultural field is difficult thing to model: on undulating, clay fields flowing water collects into rills, increasing erosion, says Warsta.

What new information has the FLUSH model revealed? At the very early stage of research, Warsta is reluctant to generalize the results. The simulations have at least confirmed suspicions that the subsurface drains have a major role in transporting nutrients into watercourses.

− On clay fields, water and suspended sediment are transported quickly to subsurface drains through large macropores, ie. worm holes and shrinkage cracks,Warsta says.

It means that the nutrients adsorbed on sediment particles and dissolved in water have very little time to react with the soil before they pass to the drains. Another major factor affecting nutrient loads is the autumn tillage, which can notably increase soil erodibility.

These preliminary findings are now investigated using the new tool. FLUSH model may be used for example to test the effectiveness of protection measures set by EU legislation that strive to prevent fertilizers from ending up in waterways.

− The model suggests that actions should be targeted on undulating fields near water courses, Warsta says.

The current version of the FLUSH model covers only the summer and autumn seasons, but the researchers are already building processes descriptions for wintertime. In the future, the model also covers runoff from the melting snow.

Warsta shows on the computer screen how a heavy rain shower covers the research field with water. If the fertilization had been done just before the rain, the benefits might have been small. Modern information technology can be a huge benefit to farmers.

− Savings are significant, when nutrients remain in the field, and are not lost with runoff, reminds Warsta.

http://lib.tkk.fi/Diss/2011/isbn9789526042893/