Advances,in,soil,erosion,research:,Mechanisms,,modeling,and,applications,-,A,special,issue,in,honor,of,Dr.Mark,Nearing

Long-term sustainability of soils is one of the critical challenges facing humanity.Soil erosion is an intrinsic geomorphological process,which if accelerated can threaten natural ecosystems and agriculture through loss of productivity,sedimentation,and associated pollution.Conservation efforts to mitigate these problems require changes in management practices and large investments.This puts special emphasis on understanding physical processes driving soil erosion, developing techniques for accurate estimation of erosion rates and its on-site and off-site impacts, as well as developing new modeling, prediction, and management tools.

This Special Issue was organized to honor Dr.Mark Nearing and his valuable contributions to the study of soil erosion and soil conservation.Mark Nearing received a Ph.D.degree in Civil Engineering (1986) and an M.S.degree in Soil Science (1984)from Purdue University, and worked as a scientist for the United States Department of Agriculture Agricultural Research Service for 35 years.He studied a broad spectrum of soil erosion and conservation topics, including mechanics of erosion processes, field measurement of erosion rates, development and testing of erosion prediction models, and evaluating the effects of climate change on soil degradation.Dr.Nearing"s most recent work was focused on semi-arid rangeland environments.His vast body of knowledge and research findings is reflected in nearly 300 scientific publications.

This Special Issue is a multidisciplinary publication combining articles focusing on the latest developments in soil erosion research in the following four themes: understanding physical erosion processes; quantifying soil erosion rates in the field; runoff and soil erosion modeling;and climatic trends,ecosystem change,and conservation practices.The issue brought together 14 contributions by a diverse group of researchers from six countries in Asia,the Americas, and Europe.

1) Understanding physical erosion processes.

Soil erosion encompasses detachment,transport and deposition of soil particles by raindrops and surface flow.These are fundamental concepts and the foundation of soil erosion research.The articles in this theme cover new developments in our understanding of the mechanics of soil detachment by raindrops, sediment transport,flow hydraulics of eroding surface,infiltration processes,and landform evolution.

Zhang(2022)(A in Table 1)investigated raindrop impact effects on the sediment entrainment process.The author partitioned interrill erosion into direct raindrop detachment and flow transport caused by shock waves, which propagate in a thin water layer around the impact point.Estimating the relative weight of these sub-processes is important for understanding rainfall energy transfer to the soil.Once the flow becomes concentrated a different set of erosive mechanisms come into play.Carollo et al.(2023)(B in Table 1) explored rill morphology and flow path tortuosity and the relationship of the latter with flow resistance.They established important correlations between the Darcy-Weisbach friction factor,Reynolds number,and rill tortuosity parameter.Looking at erosion processes at the landscape scale Wang et al.(2022a) (C in Table 1)investigated internal gully erosion and development mechanisms in relation to rainfall characteristics and hydrological triggers.Their findings will help pave the way for a better gully stabilization and restoration approaches.

2) Quantifying soil erosion rates in the field.

Accurate spatially-distributed data on erosion rates forms the basis for modeling and implementation of soil conservation programs.Yet,such data remains limited and often laborious to obtain.The articles in this group addressed a range of erosion measurement techniques including surveys, artificial tracers, fallout isotopes, innovative optical and imaging methods, or combinations of the above at a variety of spatial scales.All of them add new options to the research toolbox.

Villela et al.(2023) (D in Table 1) used montmorillonite-bound Rare Earth Element (REE) tracers to estimate erosion and deposition rates on natural rainfall plots, then compared the results with WEPP model estimates.This innovative clay-tracer combination may overcome the preferential transport issues reported for REEs in prior studies.Another soil tracing study (Benaud et al.,2023) (E in Table 1) took a different approach combining REEs with SfM (Structure from Motion) photogrammetry.It provided a novel platform for understanding of spatial patterns of soil loss and deposition at much finer,rill and interrill scales.Fallout isotope137Cs was taken beyond its typical application by Xie et al.(2023)(F in Table 1),who estimated the spatial distribution of a soil productivity index in a grain production region in Northwest China.The authors made future soil productivity predictions comparing different conservation scenarios.Dong et al.(2023) (G in Table 1)compared two surveying approaches, grid and sampling, to measure soil erosion rates at a regional scale.The authors discussed advantages of both methods and provided recommendation for their use, tying their findings with estimates of soil loss tolerances.

Table 1 Articles collected in the special issue.

3) Runoff and soil erosion modeling.

Runoff and erosion rates predicted from models are important quantitative indicators for ecosystem health and a tool for assessing the effectiveness of conservation practices.The need for prediction technology is paralleled by the need for decision tools and information delivery mechanisms.

WEPP is one of the most commonly used physically-based soil erosion models, which is periodically being updated.Wang et al.(2022b) (H in Table 1) evaluated the most recent version of WEPP for runoff and soil loss predictions using natural runoff plot data and provided a comprehensive set of performance metrics.Landemaine et al.(2023) (I in Table 1) in their study addressed a major challenge in soil erosion modeling.Namely, accurate representation and parametrization of infiltration-excess runoff resulting from rainfall induced soil crusting.They showed that soil surface saturation, often overlooked by modelers, is an important process to account for.Marchezepe et al.(2023) (J in Table 1) explored a traditional approach (Gunsky"s method) to establish rainfallrunoff relationships for a large group of ungauged watersheds in Brazil.This simple and easy-to-use method demonstrated consistently good results at inter-annual and monthly scales.It might be an invaluable tool for fast decision-making or in the situation of input data scarcity.A comprehensive assessment of soil degradation risks at the global scale was undertaken by Borrelli et al.(2023)(K in Table 1).They used a state-of-the-art multi-model approach to estimate spatial patterns of water, wind, and tillage erosion.Their results enhance our knowledge on the geography of soil erosion and are a valuable resource for decision and policy makers.4) Climatic trends,ecosystem change, and conservation practices.

Climate change presents a major challenge to sustainable land management.The magnitudes and extent of increased rates of soil erosion and runoff that could occur under future precipitation regimes are large.Analyses have shown that changes in precipitation are already happening with a clear trend toward more extreme events.Articles under this theme focus on addressing these challenges and providing conservation practice lessons and perspective.

Fullhart et al.(2023)(L in Table 1)used an innovative parametrization technique combined with the CLIGEN weather generator to map precipitation extremes for South America and Africa which are sparsely covered by gage networks and/or have insufficient historical records.Detailed data is freely available and will be useful for assessing major erosion events in a variety of modeling applications.Long-term precipitation trends in Northeast China at daily and hourly scales were studied by Wang et al.(2023) (M in Table 1).They discovered and quantified increasing trends over the recent decades for a number of precipitation metrics: amount, intensity, energy, erosivity.The data will provide valuable input for planning and conservation measures in one of the world"s most productive grain growing regions.Nichols et al.(2023) (N in Table 1)studied the effect of artificial berms in an arid environment constructed to increase infiltration and improve a grass ecosystem.The study revealed the ability of conservation structures to increase landscape complexity by creating a patchwork of vegetation, bare soil,flow paths,and diversions.The authors emphasized the importance of proper planning and maintenance of these structures to avoid unintended negative consequences.

Conflict of interest

This manuscript has not been published and is not under consideration for publication elsewhere.We have no conflicts of interest to disclose.All authors have read and approved the final version of the manuscript.

Acknowledgements

The Guest Editors are grateful to all the authors for their contributions.We thank Editors-In-Chief Dennis Flanagan, Baoyuan Liu,and Michael Maerker and the Executive Editor Paige Chyu for their support and encouragement in organizing this special issue.

Viktor Polyakov*

Southwest Watershed Research Center, USDA-ARS, 2000 E Allen Rd,Tucson, AZ, 85718, USA

Claire Baffaut

Cropping Systems and Water Quality Research Unit, USDA-ARS, 241 Ag Engineering Building,University of Missouri,Columbia,MO,65211,USA

Vito Ferro

Department of Agricultural, Food and Forest Sciences, University of Palermo, Viale Delle Scienze, Building 4, 90128, Palermo, Italy

Scott Van Pelt

Cropping Systems Research Laboratory, USDA-ARS, 3810 4th St,Lubbock, TX, 79401, USA

*Corresponding author.E-mail address: viktor.polyakov@ars.usda.gov (V.Polyakov).

17 August 2023

Available online 26 August 2023