Hazard and risk assessment of wind erosion and dust emissions in Denmark - a simulation and modelling approach

Wind erosion is considered to be one of the major global environmental problems. For example dust storms can cause serious damages and the protection of civil, industrial and agricultural areas represents a major environmental and economic challenge. Global ‘anthropogenic’ dust emissions have been estimated up to 50% of the total atmospheric dust (IPCC 2001), but recent studies indicated that less than 10-25% of global dust emissions originate from agricultural soils.
Wind erosion is an important land surface process in Europe, which has caused the emission of the finest and most valuable soil particles and nutrients. More than 70% of the soil types in Denmark have a sandy texture. Denmark is also subject to strong offshore and onshore winds, therefore, Danish soils are considered especially vulnerable to wind erosion.
On such poorly aggregated soils, which are treated with conventional farming, tillage ridges are the only roughness element that are able to protect soils against wind erosion in the absence of plant cover. Historical evidence demonstrates that wind erosion has had significant effects on Danish agricultural lands. Various actions have been implemented to control wind erosion in Denmark such as wind break establishment and implementation of protective cultivation techniques. However, there are still some concerns among farmers and researchers regarding local wind erosion, particularly during early spring, when highest wind erosivity coincides with mostly bare fields.
The primary motivation for this study was the occurrence of wind erosion in one of the four study sites (field C) in central Jutland, North of Viborg in Denmark, although this field was managed and maintained similarly to the other test sites. The urge to find the main reason for this event propelled us to accomplish this investigation. The main aim of this study was to assess the effect of tillage direction on hazard and risk of soil, dust, and nutrient losses by wind erosion from agricultural land in Denmark. The study was based on scenario analysis of erosive winds, ridge height, soil moisture, and field orientation. Indeed, the principal originality of this dissertation is the use of erosive wind probability distributions during dry periods and two main tillage direction scenarios (parallel and perpendicular to the wind) to calculate the hazard and risk assessment of soil, dust and nutrient loss for a single wind erosion event. Furthermore, due to the lack of quantitative information in the study area about wind erosion rate and dust emissions, testing the effects of wind break establishment around agricultural fields on wind erosion rates was another aim of the present PhD-project.
In this study, the amounts of soil and nutrients losses were examined using a wind tunnel under different surface conditions: flat surface, parallel tillage, and perpendicular tillage direction in relation to the dominant wind direction. Four different types of soils from four different study sites were chosen for the simulations and modelling: three soils with loamy sand texture (D50 of 178µm, 194µm, and 214µm) and about 1.5% of carbon content. The fourth soil was an organic soil rich in organic matter (SOC) (12%) with slightly less sand (D50 of 69µm). The results of the wind tunnel tests were also used to correlate the nutrient and dust (PM2.5 and PM10) enrichment in wind erosion sediments for different tillage directions.
Since some of the erosive winds occur simultaneously with precipitation or when the lands are wet after a rainfall event, this research employed a practical approach to use erosive winds during dry periods to improve the quality of predictions. In order to determine the hazard and risk of wind erosion on total soil, dust, and nutrient losses by erosive winds during dry periods, a single-event wind erosion evaluation program (SWEEP) was applied. 32 different scenario simulations on theoretical ploughed agricultural fields were performed. These included: wind speed, ridge height, ridge orientation, and soil moisture content. In addition, all of these scenarios were calculated for unsheltered and sheltered conditions by a single row wind break network.
In order to test the model performance, the results of the predicted total soil loss were evaluated against the observed results from wind tunnel experiments using three common criteria coefficient: Root Mean Square Error (RMSE), coefficient of determination (R^2), and index of agreement (d). Finally, a relative sensitivity analysis was performed to find the most important input parameters for the scenarios, in order to evaluate which parameter controls or accelerates the wind erosion process on poorly aggregated sandy soils. All of these scenarios were assumed in the absence of crop cover or residue or stone cover.
Results showed that the parallel tillage operation experienced the greatest erosion rates for all soil types. However, due to a greater enrichment ratio of dust size particles from perpendicularly tilled surfaces, the scenarios with perpendicular tillage experienced the most significant nutrient enrichment. The main reason for this phenomenon is most probably the trapping of larger particles by the perpendicular furrows. This indicates that the highest rate of soil protection does not necessarily coincide with lowest soil nutrient loss and dust emissions. Therefore, for the evaluation of protection measures on these soil types in Denmark, it is important to differentiate between their effectivity to reduce total soil erosion, dust emission, and nutrient loss.
Results from wind data analysis regarding the general trend of wind direction demonstrated that the prevailing wind direction is predominantly from westerly direction. Temporal analysis of erosive wind velocities indicate that the most sensitive time for the occurrence of an erosive wind erosion event is March in the time between 12:00 to 15:00.
The results from the model performance evaluation for loamy sands class 2 and 3 proved a remarkable similarity between the SWEEP model results and observed values from wind tunnel simulations, but for loamy sand class 1, the SWEEP model under-estimated total soil loss. Regardless of the different scales of a wind tunnel simulation and the field scale model, it seems that SWEEP was not able to predict accurately soil loss for very fine sandy soils. The relative sensitivity analysis confirmed that ridge orientation and wind direction were the most sensitive factors which accounted together for 51 percent of total sensitivity (equally 25.5% for each parameter). This implies that ridge orientation in relation to the wind direction in ploughed lands, without vegetation cover, can accelerate the total soil loss by wind erosion.
Results showed that all of the scenario numbers, which were performed for the average amount of soil surface water content (0.15 Mg/Mg), have not shown any hazard and risk values for soil loss regardless of soil type. Except of one scenario with a 10 cm ridge height perpendicular to the wind (SN12), there were no predicted hazard and risk values for the perpendicular ploughed soil surface with a 10 cm ridge height under all conditions of soil moisture and wind speed. Therefore, there were only 9 scenarios among all 32 possible scenarios with a minimum amount of total soil loss and PM10.
Since in the current condition of the four study sites, fields A and B are ploughed perpendicularly to the wind and parallel tillage to the wind direction was done for fields C and D, the scenario analysis for current conditions showed that field C with loamy sand class 3 observed the highest potential risk to wind erosion with 6 active scenarios in contrast to 3 active scenarios for farms A and B. Results demonstrated that a 5 cm increase of ridge height in unsheltered area during highest erosive winds led to a minimum of seven times reduction of total soil loss hazard (55.80 versus 7.70 t/ha) and risk values (10.04 versus 1.39).The results also showed that the highest risk of nutrient losses were related to TOC and CaCO3 with 137.09 (761.89 kg/ha of hazard) and 29.99 (166.66 kg/ha of hazard) values respectively in scenario number 31 as the worst case scenario. Using a single row wind break could reduce these risk values by up to 4 times. On field D with organic soil, like in field C, the land is currently ploughed parallel to the erosive wind direction. However, because of this soil type inherent resistance to wind erosion, there was only one scenario with the minimum amount of hazard and risk value (SN32). Other potential scenarios for this field did not show any values for the total soil, PM10, and nutrient losses. In addition, the establishment of a wind break could decrease the risk of total soil loss, dust emission (PM10), and all nutrient loss risks in the worst case scenario to more than 70 %. Results illustrated that unlike in hazard assessments, which represented SN32 as the worst case scenario to total soil loss, dust emission, and nutrient mobilization, among all potential risk scenarios, the highest risks have occurred in SN31 due to a higher probability of accordance for erosive winds in dry periods compared to SN32 (18% versus 2.5%).
By using appropriate land management techniques to control the destructive effect of wind erosion in the study sites, especially by establishing a wide network of shelterbelts around the farms, the effect of wind erosion could be considerably reduced. However, the results of this study show that wind erosion is still a potential hazard and risk in sandy soils, if parallel tillage is performed.
Keywords: Wind erosion, Dust, Simulation, Modeling, Hazard, Risk, Tillage, Nutrient, Denmark