Editing Erosion (section)

===Rainfall and surface runoff===
[[File:Water and soil splashed by the impact of a single raindrop.jpg|thumb|right|[[Soil]] and water being [[Splash (fluid mechanics)|splashed]] by the impact of a single [[raindrop]]]]
[[Rain]]fall, and the [[surface runoff]] which may result from rainfall, produces four main types of [[soil erosion]]: ''splash erosion'', ''[[sheet erosion]]'', ''[[Rill|rill erosion]]'', and ''gully erosion''. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four).<ref name="toy-2002-p1"/>{{rp|60–61}}<ref>{{cite book|author= Zachar, Dušan|chapter=Classification of soil erosion|title=Soil Erosion|volume=10|publisher=Elsevier|year=1982|isbn=978-0-444-99725-8|page=48|chapter-url=https://books.google.com/books?id=o8ny2dUkpM8C&pg=PA48}}</ref>

In ''splash erosion'', the [[Rain#Raindrop impacts|impact of a falling raindrop]] creates a small crater in the [[soil]],<ref name="Fig. 4">See Figure 1 in {{cite journal |title=Confined Shocks inside Isolated Liquid Volumes – A New Path of Erosion?|journal=Physics of Fluids|volume=23|issue=10|pages=101702|year=2011|arxiv=1109.3175|bibcode=2011PhFl...23j1702O|last1=Obreschkow|first1=D.|last2=Dorsaz|first2=N.|last3=Kobel|first3=P.|last4=De Bosset|first4=A.|last5=Tinguely|first5=M.|last6=Field|first6=J.|last7=Farhat|first7=M.|doi=10.1063/1.3647583|s2cid=59437729}}</ref> ejecting soil particles.<ref name="Hysteretic sediment fluxes in ra"/> The distance these soil particles travel can be as much as {{Convert|0.6|m|ft|abbr=on}} vertically and {{Convert|1.5|m|ft|abbr=on}} horizontally on level ground.

If [[Surface runoff#Saturation excess overland flow|the soil is saturated]], or if the rainfall rate is [[Surface runoff#Infiltration excess overland flow|greater than the rate at which water can infiltrate]] into the soil, surface runoff occurs. If the runoff has sufficient [[Fluid dynamics|flow energy]], it will [[Sediment transport|transport]] loosened soil particles ([[sediment]]) down the slope.<ref name="FAO-1965-pp23-25">{{cite book|author=Food and Agriculture Organization|chapter=Types of erosion damage|title=Soil Erosion by Water: Some Measures for Its Control on Cultivated Lands|publisher=United Nations|year=1965|isbn=978-92-5-100474-6|pages=23–25|chapter-url=https://books.google.com/books?id=6KeL3ix6ZqQC&pg=PA23}}</ref> ''[[Sheet erosion]]'' is the transport of loosened soil particles by overland flow.<ref name="FAO-1965-pp23-25" />
[[File:Rummu aherainemägi2.jpg|thumb|A [[spoil tip]] covered in rills and gullies due to erosion processes caused by rainfall: [[Rummu]], [[Estonia]]]]
''[[Rill]] erosion'' refers to the development of small, [[ephemeral]] concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of the order of a few centimetres (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit [[hydraulic]] physics very different from water flowing through the deeper, wider channels of streams and rivers.<ref>{{cite journal | last1 = Nearing | first1 = M.A. | last2 = Norton | first2 = L.D. | last3 = Bulgakov | first3 = D.A. | last4 = Larionov | first4 = G.A. | last5 = West | first5 = L.T. | last6 = Dontsova | first6 = K.M. | year = 1997 | title = Hydraulics and erosion in eroding rills | journal = Water Resources Research | volume = 33 | issue = 4| pages = 865–876 | doi=10.1029/97wr00013|bibcode = 1997WRR....33..865N | doi-access = free }}</ref>

{{Anchor|gully erosion|ephemeral gully erosion}}
''[[Gully erosion]]'' occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth.<ref name=Boardman>{{cite book|editor-last1=Boardman|editor-first1=John|editor-last2=Poesen|editor-first2=Jean|title=Soil Erosion in Europe|date=2007|publisher=John Wiley & Sons|location=Chichester|isbn=978-0-470-85911-7}}</ref><ref>{{cite book |author=J. Poesen |author2=L. Vandekerckhove |author3=J. Nachtergaele |author4=D. Oostwoud Wijdenes |author5=G. Verstraeten |author6=B. Can Wesemael |chapter=Gully erosion in dryland environments|pages=229–262|editor=Bull, Louise J. |editor2=Kirby, M.J.|title=Dryland Rivers: Hydrology and Geomorphology of Semi-Arid Channels|publisher=John Wiley & Sons|year=2002|isbn=978-0-471-49123-1|chapter-url=https://books.google.com/books?id=qjHoYZXQee0C&pg=PA229}}</ref><ref>{{cite book|author=Borah, Deva K.|chapter=Watershed sediment yield|editor=Garcia, Marcelo H.|title=Sedimentation Engineering: Processes, Measurements, Modeling, and Practice|publisher=ASCE Publishing|year=2008|isbn=978-0-7844-0814-8|page=828|chapter-url=https://books.google.com/books?id=1AsypwBUa_wC&pg=PA828|display-authors=etal}}</ref> A gully is distinguished from a rill based on a critical cross-sectional area of at least one square foot, i.e. the size of a channel that can no longer be erased via normal tillage operations.<ref>{{Cite journal|last1=Vanmaercke|first1=Matthias|last2=Panagos|first2=Panos|last3=Vanwalleghem|first3=Tom|last4=Hayas|first4=Antonio|last5=Foerster|first5=Saskia|last6=Borrelli|first6=Pasquale|last7=Rossi|first7=Mauro|last8=Torri|first8=Dino|last9=Casali|first9=Javier|last10=Borselli|first10=Lorenzo|last11=Vigiak|first11=Olga|date=July 2021|title=Measuring, modelling and managing gully erosion at large scales: A state of the art|url=https://linkinghub.elsevier.com/retrieve/pii/S0012825221001379|journal=Earth-Science Reviews|language=en|volume=218|pages=103637|doi=10.1016/j.earscirev.2021.103637|bibcode=2021ESRv..21803637V|hdl=10198/24417|s2cid=234800558|hdl-access=free}}</ref>

Extreme gully erosion can progress to formation of [[badlands]]. These form under conditions of high relief on [[Erodability|easily eroded bedrock]] in climates favorable to erosion. Conditions or disturbances that limit the growth of protective vegetation ([[biorhexistasy|rhexistasy]]) are a key element of badland formation.<ref>{{cite journal |last1=Moreno-de las Heras |first1=Mariano |last2=Gallart |first2=Francesc |title=The Origin of Badlands |journal=Badlands Dynamics in a Context of Global Change |date=2018 |pages=27–59 |doi=10.1016/B978-0-12-813054-4.00002-2|isbn=9780128130544 }}</ref>