Methods for Measuring Wind Erosion

Wind erosion silently degrades millions of hectares of agricultural land every year. According to the FAO, soil erosion – of which wind is a major driver – can lead to up to 50% loss in crop yields, and the topsoil lost is not recoverable within a human lifespan. Before scientists and land managers can develop strategies to control wind erosion, they first need to accurately measure it. Two primary approaches have emerged for this purpose: field sand traps, which capture eroded particles under real-world conditions, and laboratory wind tunnels, which replicate and analyze wind erosion in a controlled environment. Each method has its own design, strengths, and applications – and together they form the backbone of wind erosion research.

Why measuring wind erosion matters 🔗

Wind erosion is not just a dust problem. As FAO explains, wind erosion strips the soil of its capacity to store nutrients and water, reduces agricultural productivity, and accelerates desertification – particularly in arid and semi-arid regions where vegetative cover is already sparse. Measurements conducted at Jodhpur, India, showed that on a stormy day, the amount of dust blowing above the surface varied between 50 and 420 kg/ha, with average soil losses in the Jaisalmer region reaching 511 kg/ha. Without reliable measurement tools, it is impossible to validate erosion prediction models, design effective conservation measures, or assess the impact of land management practices.

Research by USDA-ARS scientists Fryrear, Stout, Hagen, and Vories established that field investigations of wind erosion require three groups of instrumentation: those for measuring meteorological variables, those for measuring soil flux, and those for measuring surface soil properties. Soil particles during a wind erosion event move in three modes – creep (large particles rolling along the surface, 1-2 mm diameter), saltation (particles bouncing in short hops below 1 m height, 0.1-1.0 mm diameter), and suspension (fine particles carried long distances, less than 0.1 mm diameter). Measuring each of these transport modes requires specially designed instruments.

Field sand traps: measuring erosion where it happens 🔗

Field sand traps (also called sediment samplers or soil flux samplers) are passive devices placed directly on agricultural land to intercept wind-blown soil particles as they move. They capture sediment in real-time under natural wind conditions, without disturbing the soil surface or altering wind behavior. The data they collect – particularly the horizontal mass flux (HMF), which is the weight of material passing through a given area per unit time – is essential for calculating total soil loss and validating erosion models.

The BSNE sampler 🔗

The most widely used field sand trap is the Big Spring Number Eight (BSNE) sampler, first developed by D.W. Fryrear in 1986. The BSNE is designed to rotate into the wind and features a large inlet opening of 10 cm² (2 cm wide by 5 cm high), which allows it to collect adequate quantities of sediment even during low-intensity erosion events. Its relatively large collection area makes it especially useful when the primary goal is to analyze the chemical composition of eroded material. The BSNE is widely used in field studies due to its simple design, easy operation, relatively high accuracy, and low cost.

In practice, multiple BSNE samplers are stacked vertically at different heights above the ground to capture the vertical distribution of eroded particles. In a field experiment in the Taklimakan Desert, each BSNE station consisted of five samplers, with the lowest at 0.05 m and the highest at 1.0 m above the soil surface, with sediment collected every 24 hours. This vertical arrangement is critical because the average amounts of material collected are highest near the ground (at around 22.5 cm height) and decrease substantially at greater heights, reflecting how wind-blown particles are most concentrated close to the surface.

Other sampler designs: MWAC and beyond 🔗

The Modified Wilson and Cook (MWAC) sampler is another widely used trap, and it holds a key advantage over the BSNE: its collection efficiency remains constant across different wind speeds, whereas BSNE efficiency decreases at higher wind speeds due to increased stagnation pressure at the inlet. This makes MWAC more reliable during high-intensity wind storms. However, the MWAC’s inlet opening is far smaller than the BSNE’s, meaning it collects less total material – a disadvantage when sediment quantities are low or chemical analysis is the goal.

Beyond BSNE and MWAC, researchers have developed a range of specialized traps. Rotary Distance (RD) and Fixed Distance (FD) traps have been shown to have higher suspension particle collection efficiency than BSNE, with efficiency values of 99% and 98% respectively, compared to BSNE’s 65%. Newer designs such as the cyclone-type BEST trap offer higher and more consistent mass flux gauging. More recently, the Mendéz Trap (MT) has been introduced, showing 31.83% higher efficiency than BSNE in field evaluations, particularly valuable at greater heights where only small amounts of material are typically collected.

How trap data is analyzed 🔗

Once sediment is collected from traps at multiple heights, researchers calculate the horizontal mass transport (HMT) – the total amount of soil moved across a unit width of land. This is done by mathematically integrating the mass flux values measured at each height using exponential, power, or logarithmic equations. A power expression is used to describe the variation in suspended material amounts up to a 2-metre height, where the quantity and height of material within the saltation zone follow a defined mathematical relationship based on surface creep and the height below which 50% of total mass flow occurs. These calculations allow scientists to estimate total soil loss from a single wind erosion event and compare the performance of different conservation practices.

Laboratory wind tunnels: controlled simulation of wind erosion 🔗

While field sand traps measure erosion as it actually occurs in nature, they cannot control the variables involved – wind speed, direction, surface condition, or vegetation cover change constantly. This is where laboratory wind tunnels play an indispensable role. A wind tunnel generates a controlled, artificially produced airstream over a soil sample, allowing researchers to isolate specific variables and examine their effects on erosion in a repeatable way.

Wind tunnels are indispensable tools for the controlled analysis of wind-induced soil erosion, sediment transport and deposition, especially given the complexities of direct field observation. Their applications include estimating soil loss under specific wind speeds, determining the threshold wind velocity at which erosion begins, testing the effectiveness of surface treatments and mulches, and evaluating how tillage ridges, crop residues, or roughness elements affect erodibility.

Structure and operation of a wind tunnel 🔗

A standard laboratory wind tunnel consists of a fan unit that generates airflow, a flow-conditioning section (often containing a honeycomb structure to straighten airflow and reduce turbulence), a working section where the soil sample is placed, and a sediment collection chamber at the outlet. Wind is blown at an adjustable speed onto the soil sample, and various methods are used to measure soil erosion – including estimating soil loss, determining the threshold of wind erosion, and examining the effects of different soil covers and vegetation.

Wind speeds in tunnels can be precisely adjusted – field-portable wind tunnels typically operate across a range of 0 to 22 m/s, while fixed laboratory tunnels may extend this range further. In a recently designed research tunnel at Erciyes University, wind speed profiles were carefully measured at 5, 7, and 9 m/s using a pitot-type anemometer, with Reynolds numbers well above 200,000 – confirming a fully turbulent flow regime that closely replicates natural wind conditions.

Portable wind tunnels: bridging the lab and the field 🔗

A significant development in wind erosion research has been the emergence of portable wind tunnels, which bring the controlled conditions of the laboratory directly to the field. Mobile wind tunnels are essential tools for the field examination and quantification of wind erosion processes on natural, non-imitated surfaces under standardized, quasi-natural wind conditions – overcoming one of the biggest limitations of laboratory tunnels, which is that soil samples removed from the field lose their natural structure.

Portable tunnels are typically mounted on a vehicle or carried in sections and placed directly over undisturbed soil. One portable design features a rectangular cross-sectional area of 0.5 × 0.4 m and a working section extending up to 5 m, with a sediment collection trap at the end measuring 3 m wide, 5 m long, and 1 m deep. Another innovative device – the Portable In-Situ Wind Erosion Lab (PI-SWERL), developed at the Desert Research Institute – uses a rotating annular blade inside a chamber to simulate surface wind shear, functioning as a miniature wind tunnel that can be operated by a single person with minimal setup time. This kind of tool dramatically expands the range of environments and surfaces that can be practically assessed for wind erosion potential.

What wind tunnels measure and why it matters 🔗

Wind tunnel experiments generate several critical measurements. The soil loss rate (mass of soil eroded per unit area per unit time) at different wind speeds allows researchers to define the erodibility of a given soil. The threshold friction velocity – the minimum wind speed needed to initiate particle movement – is a key parameter used in erosion prediction models. Tunnel experiments also measure sediment flux profiles, revealing how much material moves at different heights, which directly mirrors what field sand traps measure under natural conditions. The assessment of wind erosion parameters from wind tunnels requires careful attention to both scaling laws and the matching of physical processes to ensure results can be reliably applied to real field conditions.

Wind tunnels have also proven vital in calibrating field samplers. The BSNE and MWAC samplers have been calibrated for catching sand and dust particles by means of wind tunnel simulations, and studies have confirmed that the BSNE achieves an overall efficiency of around 90 ± 5% under tunnel conditions – a figure that researchers use as a correction factor when interpreting field data.

Comparing field traps and wind tunnels 🔗

Both methods are complementary, not competing. Field sand traps provide data on actual erosion under real weather, soil, and crop conditions – essential for long-term monitoring and for validating prediction models such as the Wind Erosion Equation (WEQ) or the Revised Wind Erosion Equation (RWEQ). Wind tunnels, on the other hand, allow precise hypothesis testing: researchers can isolate one variable at a time (soil moisture, surface roughness, vegetation height, mulch type) and quantify its effect on erosion without the noise of constantly changing field conditions.

The combination of both approaches is increasingly the standard in modern wind erosion research. Field traps identify where and when erosion is happening; wind tunnels explain why it is happening and what can be done to reduce it. The findings from such measurement studies provide practical guidance for land managers and policymakers to develop efficient soil conservation measures in areas susceptible to wind erosion, directly supporting global efforts toward sustainable agriculture and food security.

What do you think? Given the advantages of both field sand traps and laboratory wind tunnels, how might combining these two methods improve the accuracy of wind erosion assessments in smallholder farming regions like India’s semi-arid zones? And as portable wind tunnel designs continue to improve, could they eventually replace conventional laboratory tunnels for most practical research purposes?

References

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