Sujung Ahn

Bio: Sujung Ahn is an academic researcher from Swansea University. The author has contributed to research in topics: Soil water & Infiltration (hydrology). The author has an hindex of 8, co-authored 15 publications receiving 225 citations. Previous affiliations of Sujung Ahn include Korea Forest Service & Seoul National University.

Effects of hydrophobicity on splash erosion of model soil particles by a single water drop impact

Abstract: Raindrop impact can be a major contributor to particle mobilization for soils and other granular materials. In previous work, water repellent soils, comprised of hydrophobic particles, have been shown to exhibit greater splash erosion losses under multiple drop impact. However, the underlying principal differences in splash behavior between hydrophobic and hydrophilic granular surfaces have not been studied to date. In this study the effects of particle hydrophobicity on splash behaviour by a single water drop impact were examined using high-speed videography. Water drops (4 mm in diameter) were dropped on beds of hydrophilic and hydrophobic glass beads (sieved range: 350400 m), serving as model soil particles. The drop velocity on impact was 2.67 m s -1 , which corresponds to 30% of the terminal velocity of a raindrop of similar size. The resulting impact behaviour was measured in terms of the trajectories of particles ejected from the beds and their final resting positions. The response to the impacting water drop was significantly different between hydrophilic and hydrophobic particles in terms of the distance distribution, the median distance travelled by the particles and number of ejected particles. The greater ejection distances of hydrophobic particles were mainly the result of the higher initial velocities rather than differences in ejecting angles. The higher and longer

Transitions of water-drop impact behaviour on hydrophobic and hydrophilic particles

Abstract: Extreme soil water repellency can have substantial implications for soil hydrology, plant growth and erosion, including enhanced splash erosion caused by raindrop impact. Previous studies of water droplet impact behaviour on man-made super-hydrophobic surfaces, with which water-repellent soil shares similar characteristics, revealed three distinct modes of splash behaviour (rebound, pinning and fragmentation) distinguished by two transition velocities: rebound-to-pinning (vmin) and pinning-to-fragmentation (v*). By using high-speed videography of single water droplet impacts we show that splash behaviour is influenced by the hydrophobicity of immobile particles, with hydrophobic glass spheres exhibiting all three modes of splash behaviour in the hydrophobic state but hydrophilic spheres exhibiting solely pinning behaviour. We found that increasing the particle size of fixed glass spheres increases vmin. A study of droplet impact on hydrophobic sand shows that the increased roughness of the immobile particles makes impacting droplets more likely to fragment at slower impact velocities. The mobility of the particles influenced droplet impact behaviour, with loose, hydrophobic particles displaying significantly greater vmin values than their fixed analogues. The surface tension of the water droplet also lifted loose, hydrophobic particles from the surface, forming highly mobile ‘liquid marbles'. Water-repellent soil was also shown to form ‘liquid marbles' at both the slow (approximately 0.3–2.1 m/s) and fast (about 7 m/s) droplet impact velocities studied. The observation of very mobile liquid marbles upon water droplet impact on water-repellent soil is significant as this provided a mechanism that may enhance erosion rates of water-repellent soil.

Effect of Particle Size on Droplet Infiltration into Hydrophobic Porous Media As a Model of Water Repellent Soil

Abstract: The wettability of soil is of great importance for plants and soil biota, and in determining the risk for preferential flow, surface runoff, flooding and soil erosion. The molarity of ethanol droplet (MED) test is widely used for quantifying the sever ity of water repellency in soils that show reduced wettability and is assumed to be independen t of soil particle size. The minimum ethanol concentration at which droplet penetration occurs w ithin a short time ( ≤ 10 s) provides an estimate of the initial advancing contact angle at which spo ntaneous wetting is expected. In this study, we test the assumption of particle size independence u sing a simple model of soil, represented by layers of small ( ~0.2 to 2 mm) diameter beads that predict the effect of changing bead radius in the top layer on capillary driven imbibition. Experimental results using a three-layer bead system show broad agreement with the model and demonstrate a dependence of the MED test on particle size. The results show that the critical initial advancin g contact angle for penetration can be considerably l ess than 90° and varies with particle size, demonstrati ng that a key assumption currently used in the MED testing of soil is not necessarily valid.

Comparing groundwater recharge and base flow in the Bukmoongol small-forested watershed, Korea

Abstract: Groundwater recharge and base flow using different investigated methods are simulated in the 15-ha Bukmoongol small-forested watershed located at the southern part of Korea. The WHAT system, PART, RORA, PULSE, BFI, and RAP software are used to estimate groundwater recharge or base flow and base flow index from the measured streamflow. Results show that about 15–31 per cent of annual rainfall might be contributed for base flow. The watershed groundwater recharge proportions are computed to about 10–21 per cent during the wet period and 23–32 per cent for the remainder periods. Mean annual base flow indices vary from 0.25 to 0.76 estimated using different methods. However, the study found out that all methods were significantly correlated with each other. The similarity of various methods is expressed as a weighted relationship provided by the matrix product from the principal component analysis. Overall, the BFI and WHAT software appeared consistent in estimating recharge or base flow, and base flow index under Korea’s conditions. The case study recommends the application of different models to other watersheds as well as in low-lying areas where most observation groundwater wells are located with available streamflow data.

Soil-related sustainable development goals: Four concepts to make land degradation neutrality and restoration work

Abstract: In the effort to achieve the Sustainable Development Goals (SDGs) related to food, health, water, and climate, an increase in pressure on land is highly likely. To avoid further land degradation and promote land restoration, multifunctional use of land is needed within the boundaries of the soil-water system. In addition, awareness-raising, a change in stakeholders’ attitudes, and a change in economics are essential. The attainment of a balance between the economy, society, and the biosphere calls for a holistic approach. In this paper, we introduce four concepts that we consider to be conducive to realizing LDN in a more integrated way: systems thinking, connectivity, nature-based solutions, and regenerative economics. We illustrate the application of these concepts through three examples in agricultural settings. Systems thinking lies at the base of the three others, stressing feedback loops but also delayed responses. Their simultaneous use will result in more robust solutions, which are sustainable from an environmental, societal, and economic point of view. Solutions also need to take into account the level of scale (global, national, regional, local), stakeholders’ interests and culture, and the availability and boundaries of financial and natural capital. Furthermore, sustainable solutions need to embed short-term management in long-term landscape planning. In conclusion, paradigm shifts are needed. First, it is necessary to move from excessive exploitation in combination with environmental protection, to sustainable use and management of the soil-water system. To accomplish this, new business models in robust economic systems are needed based on environmental systems thinking; an approach that integrates environmental, social, and economic interests. Second, it is necessary to shift from a “system follows function” approach towards a “function follows system” one. Only by making the transition towards integrated solutions based on a socio-economical-ecological systems analysis, using concepts such as nature-based solutions, do we stand a chance to achieve Land Degradation Neutrality by 2030. To make these paradigm shifts, awareness-raising in relation to a different type of governance, economy and landscape and land-use planning and management is needed.