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{{Short description|Shallow channel cut by water}}

{{wiktionarypar|rill}}

{{for|grooves on the moon|Rille}}

[[Image:IMG 0670.JPG|thumb|200px|right|Picture of a rill.]]

{{wiktionaryparWiktionary|rill}}

A '''rill''' is a narrow and shallow incision into soil resulting from [[erosion]] by [[overland flow]] that has been focused into a thin thread by soil surface [[roughness]]. '''Rilling''', the process of rill formation, is common on agricultural land and unvegetated ground.

[[File:Rill network from Tyrone, Ireland.jpg|thumb|right|A downslope view of part of the [[erosion|eroding]] rill network from [[County Tyrone]], [[Northern Ireland]]. See below for a close-up view of a single rill]]

Some rills will continue to grow, as they are widened and/or deepened by the runoff which flows through them. Other rills may decline in importance as [[sediment]] is deposited within them. Eventually, a hydrologically efficient rill network will be formed. The development of such a network is an example of [[self-organisation]].

In [[Geomorphology#Hillslope processes|hillslope geomorphology]], a '''rill''' is a shallow [[Channel (geography)|channel]] (no more than a few [[inch]]es/[[centimeter]]s deep) cut into [[soil]] by the [[erosion|erosive]] action of [[overland flow|flow]]ing [[surface water]]. Similar but smaller incised channels are known as microrills; larger incised channels are known as [[gully|gullies]].

'''Artificial rills''' are [[Channel (geography)|channels]] constructed to carry a [[water supply]] from a distant water source. In landscape or [[garden]] design, constructed rills are an [[aesthetic]] [[water feature]].

In a few cases, rills may continue to grow and become [[Gully|gullies]] or [[rivulet]]s then [[stream]] and so on to a [[river]].

==Rills created by erosion==

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[[File:Rill close-up from Tyrone, Ireland.jpg|thumb|right|Water flowing in an actively-[[erosion|eroding]] rill on bare [[farmland]] in [[County Tyrone]], [[Northern Ireland]]]]

Rills are narrow and shallow [[Channel (geography)|channels]] which are [[erosion|eroded]] into unprotected [[soil]] by hillslope [[Surface runoff|runoff]]. Since soil is regularly left bare during [[agriculture|agricultural operations]], rills may form on [[farmland]] during these vulnerable periods. Rills may also form when bare soil is left exposed following [[deforestation]], or during [[construction]] activities.

Rills are fairly easily visible when first incised, so they are often the first indication of an ongoing [[erosion]] problem. Unless [[soil conservation]] measures are put into place, rills on regularly eroding areas may eventually develop into larger [[erosion]]al features such as [[gully|gullies]] or even (in [[semi-arid]] regions) into [[badlands]].

===Rill initiation===

Rills are created when fire erodes the soil [[topsoil]] on hillsides and so are significantly affected by seasonal weather patterns. They tend to appear more often in rainier months.<ref>Fullen, M.A. & A.H. Reed. 1987. Rill Erosion on Arable Loamy Sands in the West Midlands of England. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 85-96.</ref> Rills begin to form when the runoff [[shear stress]], the ability of surface runoff to detach soil particles, overcomes the soil's shear strength, the ability of soil to resist force working parallel to the soil's surface. This begins the erosion process as water breaks soil particles free and carries them down the slope.<ref>Torri, D., M. Sfalanga & G. Chisci. 1987. Threshold Conditions for Incipient Rilling. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 97-105.</ref> These forces explain why sandy, loamy soils are especially susceptible to the formation of rills, whereas dense clays tend to resist rill formation.<ref>Loch, R.J. & E.C. Thomas. 1987. Resistance to Rill Erosion: Observations on the Efficiency of Rill Erosion on a Tilled Clay Soil Under Simulated Rain and Run-On Water. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 71-83.</ref>

[[File:Rill_initiation,_finger_pointing_at_a_newly-developed_headcut.jpeg|thumb|left|Rill initiation: the finger is pointing at a [[headcut]] which has just been incised by [[Surface runoff|runoff]] which is flowing from right to left]]

Rills cannot form on every surface and their formation is intrinsically connected to the steepness of the hillside slope. Gravity determines the force of the water, which provides the power required to start the erosional environment necessary to create rills. Therefore, the formation of rills is primarily controlled by the slope of the hillside. Slope controls the depth of the rills, while the length of the slope and the soil's permeability control the number of incisions in an area. Each type of soil has a threshold value, a slope angle below which water velocity cannot produce sufficient force to dislodge enough soil particles for rills to form.<ref>Planchon, O., E. Fritcsh & C. Valentin. 1987. Rill Development in a Wet Savannah Environment. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 55-70.</ref> For instance, on many non-cohesive slopes, this threshold value hovers around an angle of 2 degrees with a [[shear velocity]] between 3 and 3.5&nbsp;cm/s.<ref>Rauws, G. 1987. The Initiation of Rills on Plane Beds of Non-Cohesive Sediments. Catena Supplement 8. W. Germany:Catena Verlag. 107-118.</ref>

After rills begin forming, they are subjected to variety of other erosional forces which may increase their size and output volume. Up to 37% of erosion in a rill-ridden area may derive from mass movement, or collapse, of rill sidewalls. As water flows through a rill, it will undercut into the walls, triggering collapse. Also, as water seeps into the soil of the walls, they weaken, amplifying the chance of wall collapse. The erosion created by these forces increases the size of the rill while also swelling its output volume.<ref>Govers, G. 1987. Spatial and Temporal Variability in Rill Development Processes at the Huldenberg Experimental Site. Catena Supplement 8. W. Germany:Catena Verlag. 17-33.</ref>

Less commonly, [[Dissolution (chemistry)|dissolution]] of [[limestone]] and other soluble rocks by slightly [[Rainfall#Acidity|acidic rainfall]] and [[Surface runoff|runoff]] also results in the formation of rill-like features on the surface of the rock.<ref>Ford, D.C. & J. Lundberg. 1987. A Review of Dissolutional Rills in Limestone and Other Soluble Rocks. Bryan, R.B. (ed). Rill Erosion: Processes and Significance. Catena Supplement 8. W. Germany:Catena Verlag. 119-139</ref>

===Significance of rill erosion===

[[file:Волжские утесы с Дурман горы.jpg|thumb|Landscape shaped by rill erosion. [[Volgograd Oblast]], Russia.]]

Although rills are small, they transport significant amounts of soil each year. Some estimates claim rill flow has a carrying capacity of nearly ten times that of non-rill, or interrill, areas. In a moderate rainfall, rill flow can carry rock fragments up to 9&nbsp;cm in diameter downslope. In 1987, scientist J. Poesen conducted an experiment on the [[Huldenberg]] field in [[Belgium]] which revealed that during a moderate rainfall, rill erosion removed as much as 200&nbsp;kg (in submerged weight) of rock.<ref name=autogenerated1>Poesen, J. 1987. Transport of Rock Fragments by Rill Flow—A Field Study. Catena Supplement 8. W. Germany:Catena Verlag. 35-54.</ref>

Unfortunately, rills' considerable effect on landscapes often negatively impacts human activity. Rills have been observed washing away archaeological sites.<ref name=autogenerated1 /> They are also very common in agricultural areas because sustained agriculture depletes the soil of much of its organic content, increasing the erodibility of the soil. Agricultural machines, such as tractors, compact the soil to the point where water flows over the surface rather than seeping into the soil. Tractor wheel impressions often channel water, providing a perfect environment for generating rills. These rills may erode considerable amounts of [[Arable land|arable]] soil if left alone.<ref>Fullen, M.A. & A.H. Reed. 1987. Rill Erosion of Arable Loamy Sands in the West Midlands of England. Catena Supplement 8. W. Germany:Catena Verlag. 85-96.</ref>

Under proper field management, rills are small and are easily repaired by [[Contour plowing|contour tilling]] the soil. This will prevent, for a time at least, the rills from growing and eroding the landscape more rapidly with time.<ref>{{cite web |url=http://dhn.iihr.uiowa.edu/runoff/erosion.htm |title=The Erosion Process |access-date=2010-10-07 |url-status=dead |archive-url=https://web.archive.org/web/20100628182919/http://dhn.iihr.uiowa.edu/runoff/erosion.htm |archive-date=2010-06-28 }}</ref>

==See also==

* [[Aqueduct (water supply)|Aqueduct]]

* [[Chute (gravity)]]

* [[Flume]]

* [[Gully]]

* [[Leat]]

* [[Ravine]]

* [[Valley]]

* [[Pedosphere]]

* [[Zanja Madre]]

==References==

{{reflist|2}}

{{Rivers, streams and springs}}

[[Category:Fluvial landforms]]

[[Category:Soil erosion]]

[[Category:Geomorphology]]

[[Category:Soil science]]

[[Category:Irrigation]]

[[Category:Irrigation canals]]

[[Category:Garden features]]

[[Category:Persian gardens]]

[[Category:Islamic architectural elements]]

[[Category:Architectural elements]]