Editing Soil erosion (section)

==Monitoring, measuring and modelling soil erosion==
[[File:Landscape Madagascar 06.jpg|thumb|[[Terrace (agriculture)|Terracing]] is an ancient technique that can significantly slow the rate of water erosion on cultivated slopes.]]
{{See also|Erosion prediction}}
{{Expand section|date=April 2012}}
Monitoring and [[Simulation|modeling]] of erosion processes can help people better understand the [[Soil erosion#Factors affecting soil erosion|causes of soil erosion]], make predictions of erosion [[Scenario planning|under a range of possible conditions]], and plan the implementation of [[Soil erosion#Prevention and remediation|preventative and restorative strategies for erosion]]. However, the complexity of erosion processes and the number of scientific disciplines that must be considered to understand and model them (e.g. climatology, hydrology, geology, soil science, agriculture, chemistry, physics, etc.) makes accurate modelling challenging.<ref>{{cite journal|last1=Evans|first1=R|title=Assessment and monitoring of accelerated water erosion of cultivated land – when will reality be acknowledged?|journal=Soil Use and Management|date=2012|volume=29|issue=1|pages=105–118|doi=10.1111/sum.12010|s2cid=98809136}}</ref><ref>{{cite book|author=Blanco, Humberto|author2=Lal, Rattan|name-list-style=amp|chapter=Modeling water and wind erosion|title=Principles of Soil Conservation and Management|publisher=Springer|year=2010|isbn=978-90-481-8529-0|chapter-url=https://books.google.com/books?id=Wj3690PbDY0C&pg=PA81}}</ref><ref>See also: {{cite book|author=Shai, Yaping|title=Physics and Modelling of Wind Erosion|publisher=Springer|year=2008|isbn=978-1-4020-8894-0|url=https://books.google.com/books?id=XSwwVeraxjcC}} and {{cite book|author1=Harmon, Russell S.  |author2=Doe, William W. |name-list-style=amp |title=Landscape Erosion and Evolution Modeling|publisher=Springer|year=2001|isbn=978-0-306-46718-9|url=https://books.google.com/books?id=RltaPlIHlrAC}}</ref> Erosion models are also non-linear, which makes them difficult to work with numerically, and makes it difficult or impossible to scale up to making predictions about large areas from data collected by sampling smaller plots.<ref>{{cite book|author=Brazier, R.E.|chapter=Scaling soil erosion models in space and time|editor=Morgan, Royston P.C.|editor2=Nearing, Mark|title=Handbook of Erosion Modelling|publisher=John Wiley & Sons|year=2011|isbn=978-1-4051-9010-7|page=100|chapter-url=https://books.google.com/books?id=pSO4X3XbhJIC&pg=PA100|display-authors=etal}}</ref>

The most commonly used model for predicting soil loss from water erosion is the [[Universal Soil Loss Equation]] (USLE). This was developed in the 1960s and 1970s. It estimates the average annual soil loss ''A'' on a plot-sized area as:<ref>{{cite book|author=Ward, Andrew D.|author2=Trimble, Stanley W.|name-list-style=amp|chapter=Soil conservation and sediment budgets|title=Environmental Hydrology|publisher=CRC Press|year=2004|isbn=978-1-56670-616-2|page=259|chapter-url=https://books.google.com/books?id=yANwmTjf588C&pg=PA259}}</ref>
: ''A = RKLSCP''
where ''R'' is the ''rainfall erosivity factor'',<ref>[http://esdac.jrc.ec.europa.eu/themes/rainfall-erosivity-europe rainfall erosivity factor]</ref><ref>{{cite journal|author=Panagos, P. |display-authors=etal |title=Rainfall Erosivity in Europe|journal=Sci Total Environ |year=2015|doi=10.1016/j.scitotenv.2015.01.008|pmid=25622150|volume=511|pages=801–814|bibcode=2015ScTEn.511..801P|doi-access=free|hdl=10261/110151|hdl-access=free}}</ref> ''K'' is the ''soil erodibility factor'',<ref>{{Cite journal|last1=Panagos|first1=Panos|last2=Meusburger|first2=Katrin|last3=Ballabio|first3=Cristiano|last4=Borrelli|first4=Pasqualle|last5=Alewell|first5=Christine|title=Soil erodibility in Europe: A high-resolution dataset based on LUCAS|journal=Science of the Total Environment|volume=479–480|pages=189–200|doi=10.1016/j.scitotenv.2014.02.010|pmid=24561925|bibcode=2014ScTEn.479..189P|year=2014|doi-access=free}}</ref> ''L'' and ''S'' are topographic factors<ref>[http://eusoils.jrc.ec.europa.eu/library/themes/erosion/Topography/ topographic factors]</ref> representing length and slope,<ref>{{cite journal|author=Panagos, P.|author2=Borrelli, P.|author3=Meusburger|title=A New European Slope Length and Steepness Factor (LS-Factor) for Modeling Soil Erosion by Water|journal=Geosciences |publisher=Geosciences, MDPI|year=2015|volume=5 |issue=2 |pages= 117–126|doi= 10.3390/geosciences5020117|bibcode=2015Geosc...5..117P|doi-access=free}}</ref> ''C'' is the cover and management factor<ref>{{Cite journal|last1=Panagos|first1=Panos|last2=Borrelli|first2=Pasquale|last3=Meusburger|first3=Katrin|last4=Alewell|first4=Christine|last5=Lugato|first5=Emanuele|last6=Montanarella|first6=Luca|title=Estimating the soil erosion cover-management factor at the European scale|journal=Land Use Policy|volume=48|pages=38–50|doi=10.1016/j.landusepol.2015.05.021|year=2015|doi-access=free}}</ref> and ''P'' is the support practices factor.<ref>{{Cite journal|last1=Panagos|first1=Panos|last2=Borrelli|first2=Pasquale|last3=Meusburger|first3=Katrin|last4=Zanden|first4=Emma H. van der|last5=Poesen|first5=Jean|last6=Alewell|first6=Christine|title=Modelling the effect of support practices (P-factor) on the reduction of soil erosion by water at European scale|journal=Environmental Science & Policy|volume=51|pages=23–34|doi=10.1016/j.envsci.2015.03.012|year=2015|doi-access=free}}</ref>

Despite the USLE's [[Universal Soil Loss Equation#Description of USLE|plot-scale spatial]] basis, the model has often been used to estimate soil erosion on much larger areas, such as [[Drainage basin|watersheds]], [[continent]]s, and globally. One major problem is that the USLE cannot simulate gully erosion, and so erosion from gullies is ignored in any USLE-based assessment of erosion. Yet erosion from gullies can be a substantial proportion (10–80%) of total erosion on cultivated and grazed land.<ref>{{cite book|author=Boardman, J.|author2=Poesen, J.|chapter=Soil erosion in Europe: major processes, causes and consequences|title=Soil Erosion in Europe|publisher=Wiley, Chichester|year=2006|pages=479–487|doi=10.1002/0470859202|isbn=9780470859209}}</ref>

During the 50 years since the introduction of the USLE, many other soil erosion models have been developed.<ref>{{cite book|author=Jetten, V.|author2=Favis-Mortlock, D.|chapter=Modelling soil erosion in Europe|title=Soil Erosion in Europe|publisher=Wiley, Chichester|year=2006|pages=695–716|doi=10.1002/0470859202|isbn=9780470859209}}</ref> But because of the complexity of soil erosion and its constituent processes, all erosion models can only roughly approximate actual erosion rates when [[Forecast verification|validated]] i.e. when model predictions are compared with real-world measurements of erosion.<ref>{{cite book|author=Favis-Mortlock, D.|chapter=Validation of field-scale soil erosion models using common datasets|title=Modelling Soil Erosion by Water|publisher=Springer-Verlag NATO-ARS Series 1–55, Berlin|year=1998|pages=89–128|chapter-url=https://www.springer.com/us/book/9783642637872|isbn=9783642637872|series=Nato ASI Subseries I}}</ref><ref>{{cite journal|author=Jetten, V.|author2=De Roo, A.P.J.|author3=Favis-Mortlock, D.T.|title=Evaluation of field-scale and catchment scale soil erosion models|year=1999|journal=Catena|volume=37|issue=3–4|pages=521–541|url=http://www.sciencedirect.com/science/journal/03418162/37/3-4|doi=10.1016/s0341-8162(99)00037-5|bibcode=1999Caten..37..521J }}</ref> Thus new soil erosion models continue to be developed. Some of these remain USLE-based, e.g. the G2 model.<ref>{{Cite journal|last1=Karydas|first1=Christos G.|last2=Panagos|first2=Panos|title=The G2 erosion model: An algorithm for month-time step assessments|journal=Environmental Research|volume=161|pages=256–267|doi=10.1016/j.envres.2017.11.010|pmid=29169100|pmc=5773245|bibcode=2018ER....161..256K|year=2018}}</ref><ref>[https://esdac.jrc.ec.europa.eu/themes/g2-model G2 model]</ref> Other soil erosion models have largely (e.g. the [[WEPP|Water Erosion Prediction Project model]]) or wholly (e.g. RHEM, the Rangeland Hydrology and Erosion Model <ref>[http://apps.tucson.ars.ag.gov/rhem/  Rangeland Hydrology and Erosion Model]</ref>) abandoned usage of USLE elements. Global studies continue to be based on the USLE.<ref name="auto"/>

On a smaller scale (e.g. for individual [[Channel (geography)|channels]], [[Dam|dams]], or [[Spillway|spillways]]), there are erosion rate models available based on the [[Sediment transport#Critical shear stress|critical shear stress of erosion]] as well as the [[Erodability|erodibility]] of the soil. These can be measured using [[geotechnical engineering]] methods such as the [[hole erosion test]] or the [[jet erosion test]].<ref>{{Cite journal |last1=Hanson |first1=G J |last2=Cook |first2=K |date=2004 |title=Apparatus, test procedures, and analytical methods to measure soil erodibility in-situ |journal=Applied Engineering in Agriculture |volume=20 |issue=4 |pages=455–462 |doi=10.13031/2013.16492 |via=Elsevier Science Direct}}</ref>