SoilErosion_WEP

SoilErosion_WEP - Hillslmpe Erosion by 1'i’ilate-r In the earl}r 1940’s experimental work by W D Ellison demonstrated the role of raindmps in

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Unformatted text preview: Hillslmpe Erosion by 1'i’ilate-r In the earl}r 1940’s, experimental work by W. D. Ellison demonstrated the role of raindmps in splashing soil particles into motion. Earlier work in Europe and the United Slates had pointed out that the raindrops eomposing a storm of moderate size and inlenaily [emanate tremendous amount:: of kinetic energy by virtue of their mass and their velmity of fall {kinetic energy — 115 X mass X velocitfi}. Ellison showed that a signifieanl part OE'this energj,r mobilizes soil, and is the major proeess initiating soil erosion at both the gonlogiealljr normal and accelerated rates In areas subject to Horton runoff. He took high-speed photographs of the rainsplash erosion. as shown in Figure [5—4. It is possible to see elearly from the photographs that the impael ul‘ the drop generates a small explosion of soil and water. Large soil aggregates are dispersed and the smaller particles are splashed Figure l5--'l Highsspeed photograph ot'a raindrnp impact on a soil surface. Soil particles are splashed in all directions. but those traveling downhill rntwc farther than those splashed uphill. tL'SIlA —5oi| {‘onsenation tiers-ice} over several feet. The soil particles splashed downhill tras'ei farther than those that more uphill. causing a net downslope transport of soil. Repeated billions of times per rainstorm. this downslope transport is what we measure as rrtt'nspt'usl: erosion. Alter centuries ol‘ concern the Fundamental cause ol‘ soil erosion finalis- became apparent from Ellison‘s work. To appreciate this process. you mas- recall occasions wlten you have walked in the desert or on a construction site during a hears- rainstorm. ‘rour trouser legs record a profile of the heights to which soil particles can he splashed by raindrops. Alter the rainstorm each area of soil protected by a pebble stands up on a small earth pillar. indicating the depth of soil mos-ed by rainsplasb. Rainsplash erosion is particularly important on steep slopes ties-old ot‘ vegetation. just the conditions created during parts ol" the year on agricul- tural fields. toad cuts. and mining and construction sites. A tltiek 1vegetation coeer intercepts virtually all the kinetic energy of rainfall tsee Figure Iii-fit and herein lies the critical and dominant role of vegetation in reducing soil erosion. Man;-r cultts-ated crops leave the soil esposed to raindrop impacts [Figure 15—61. In general. the denser the vegetative cot-er. the lower the rate ol'erosion: this factor. which is controlled in tttrn by climate and land use. dominates the ell'eets ol' all other controls such as raint'all energy and hill- slope gradient. In Fact. the area of the United States that experiences the heaviest rainfalls. namely the southeast. also has one of the lowest geologic rates ol'crosion becaUse ot' the thick natural forest cover. When the forests ot‘ the region were cleared for planting corn. tobacco. and cotton in the late if t’ Hrt'ilsi'rtgm- l’rrtt '1 ' site I.' Figure I55 Thick ground cover of guns and ulnwr. A [hick vegetation 01}ch protects 1hr: mil very L-mdunuy Tram th: klnctit. cum-.1123 "I" raindrops. Few if any drops mach 1J1: 50]]. Lin-mil}r hul rathcr drop rm": 3. Small thghL ultcn on“; urgun'u: mm. {USDA Sui] Cnnscnala'tm Service.) Figure 15-6 Unch a corn crop but: gmqu i5 :xpflfled tfl raindrop J'Tl'lpilcl and Shmmnsh. I|1 lh'rs picrurc m-crland flnw 1's rumflng. 0H' 3 hlllilnp: on a Maryland Farm. {USDA— Snll Curmcn'aliun Hewitt} Figure I55 Thick ground cover of guns and ulnwr. A [hick vegetation 01}ch protects 1hr: mil very L-mdunuy Tram th: klnctit. cum-.1123 "I" raindrops. Few if any drops mach 1J1: 50]]. Lin-mil}r hul rathcr drop rm": 3. Small thghL ultcn on“; urgun'u: mm. {USDA Sui] Cnnscnala'tm Service.) Figure 15-6 Unch a corn crop but: gmqu i5 :xpflfled tfl raindrop J'Tl'lpilcl and Shmmnsh. I|1 lh'rs picrurc m-crland flnw 1's rumflng. 0H' 3 hlllilnp: on a Maryland Farm. {USDA— Snll Curmcn'aliun Hewitt} eighteenth and early nineteenth centuries. however. the result was enta- strophic accelerated erosion. Parts of Brazil. the it'lalagasfgr Republic. Malawi. and many other tropical and subtropical regions have sulferetl the same fate. Another important factor controlling the intensity of rainsplash erosion is the resistance of soil to dispersal. High organic content and moderate amounts ol'elag.r and calcium seem to he the factors promoting the develop- ment of stahle soil aggregates. Si]: and sand are more easin splashed. and man}- tillage operations break down the aggregates to the detriment of soil resistance. The factors controlling resistance to splash are mirrored by Bljl'illl {I963}. M detained in Chapter 9, in areas where rainfall intensity,r exceeds the infiltration capacity of the soil. water accumulates on the land surface and runs downslnpe as an irregular sheet. The depth and velocity of this water increase downsiope as more water is generated by precipitation excess. until the force applied to the soil by the water is sufficient to exceed the resis- tance ol'soil to erosion. The intensity of erosion depends on the product of water depth and hillsiope gradient. it mag.r increase or decrease with distance downslopc depending on the combination of water depth and hillsiopc gradient along the hillslnpe profile. it mmmon situation is for erosion to increase with distance as the gradient steepens dowuslopc. and then for erosion to decrease. or even for some deposition to take place on the gentle footslope. These relationships are summarized in Figure 15-1. Our picture F e it Belt of no 1 F J R. Erosion decreasing shutout-h : Erosion or deposition emdrm : ind-easing occurring F I . '- / ’ Zf: II I; i I 2- 'L/ if %7 ' Z agar)???" 19/594,; / / / / my! I _ .1 .r/,- //. s pep? , , y/fie p/fi/// // - I" - _.r emf/essays ///5/C”JrZs/ficé/ e/a Figure 15-? The relative magnitudes ofthe ecu-din! force ofsheetwesh {F} and the resistance {R} of the soil so creation determine the width of a belt of no shmwmhflosinnnearlhempol'altil sheetweshemsiondependsonfltcuuiw Ft’lr'hiclisptoportimalmthe product ei'hiltshpegadient and water depth}. Nine titer rainsplestt erosio- three oerurintbebellofnoshecmlshmion. 5:3 Harrier: Processes Fla-Int“ Ahflhkcouadcrgoapnpidnmmmdshmwnhmmuamoh ofbta-ip gum; mmpmdhummp Ila-r mots-oil sunning undct each that bdtlh‘lhfllbtrhl .‘hxhhnapradwuflonh-nomcnnhnlotIant tapioll dining Ilic Lot 3cm. In the Wound. thc soil our“ in“ opt of Input mfacc wash. of a uniform slice! Hr n'atcr iii an idcait'zntion. In fact. tht: shcct Domains tin}- 5trcatns and thrcada ol' watcr that am slightly dccpcr and l'aattrl‘ than tltc ai-cragc. The}; moi-t.- hack and forth amiss thc hilhlopc during the rain- storm and tngcthct with rain-splash atc rct-ponsihlc tor mott ol' thc erosion and tramptntanon ot' mi! from tho hillflup-c. Wu.- nill rcrcr to this opinion [um-$5. htmcvcr. an aha-town caution. Manse the act cll'ccl. of tho mow:- tncnt ol‘thosc thrcacia. through many rainstonns is to rcntovc a morn or too. uniform dcpth of soil from the hiiislopc. flt‘tcn tho lflm ahcotwaah cnnion cncotnpam rainipiaah ctocion hccanw tho two arc ditticult to Mpamtt.‘ in thc field. but wc hat-c choscn to dixcnw thcrn scparatclp hcrc. Tttc cfi‘ccti'. of oomhtncd 5hcctwash and l'ilil'h'piiflh crotion arc apparcnt in Fignrc tins. Some at tho oontrots ol‘ I'ilil'iSphhh cmaion also att‘cct ahcctwaah crow-ion. Rainfall 'tntcnaitg:r tictcrrninca the amount and ran: ol‘ precipitation cacti-as and thcroftn'c of the amount and ticpth or runoil'. chctation supplies otpa nic matth to thc soil. pnwitlcs fat-oratiic conditions for soil microfauna. which with plant roots prornotc lotwc lop-mil wilh a lush infiltration capacity and lint talc: ol' ahat'ctwaxh. Sui'l tchturc 'ts anotth dctcnninant ol‘infiltra- lion capacity. as or; ammo in Chaptcr ii. The mistam'c or soil to mansion is anothcr important rant-tow. though onc that is not dearly andomood. Largo mil particica are ntotc dillicolt to moi-c than Iincr onus. hot that ccry Iincat trill and clay particlox arc morn ditticult to mow than sand. A 3.9m! soil structurc in which particch arc hold togctlicr Fla-Int“ Ahflhkcouadcrgoapnpidnmmmdshmwnhmmuamoh ofbta-ip gum; mmpmdhummp Ila-r mots-oil sunning undct each that bdtlh‘lhfllbtrhl .‘hxhhnapradwuflonh-nomcnnhnlotIant tapioll dining Ilic Lot 3cm. In the Wound. thc soil our“ in“ opt of Input mfacc wash. of a uniform slice! Hr n'atcr iii an idcait'zntion. In fact. tht: shcct Domains tin}- 5trcatns and thrcada ol' watcr that am slightly dccpcr and l'aattrl‘ than tltc ai-cragc. The}; moi-t.- hack and forth amiss thc hilhlopc during the rain- storm and tngcthct with rain-splash atc rct-ponsihlc tor mott ol' thc erosion and tramptntanon ot' mi! from tho hillflup-c. Wu.- nill rcrcr to this opinion [um-$5. htmcvcr. an aha-town caution. Manse the act cll'ccl. of tho mow:- tncnt ol‘thosc thrcacia. through many rainstonns is to rcntovc a morn or too. uniform dcpth of soil from the hiiislopc. flt‘tcn tho lflm ahcotwaah cnnion cncotnpam rainipiaah ctocion hccanw tho two arc ditticult to Mpamtt.‘ in thc field. but wc hat-c choscn to dixcnw thcrn scparatclp hcrc. Tttc cfi‘ccti'. of oomhtncd 5hcctwash and l'ilil'h'piiflh crotion arc apparcnt in Fignrc tins. Some at tho oontrots ol‘ I'ilil'iSphhh cmaion also att‘cct ahcctwaah crow-ion. Rainfall 'tntcnaitg:r tictcrrninca the amount and ran: ol‘ precipitation cacti-as and thcroftn'c of the amount and ticpth or runoil'. chctation supplies otpa nic matth to thc soil. pnwitlcs fat-oratiic conditions for soil microfauna. which with plant roots prornotc lotwc lop-mil wilh a lush infiltration capacity and lint talc: ol' ahat'ctwaxh. Sui'l tchturc 'ts anotth dctcnninant ol‘infiltra- lion capacity. as or; ammo in Chaptcr ii. The mistam'c or soil to mansion is anothcr important rant-tow. though onc that is not dearly andomood. Largo mil particica are ntotc dillicolt to moi-c than Iincr onus. hot that ccry Iincat trill and clay particlox arc morn ditticult to mow than sand. A 3.9m! soil structurc in which particch arc hold togctlicr by organic material and cations into a porous aggregate is known to resist shcctwash erosion as it resists dispersal by rainsplash. It moderate amount of clay in the soil binds sand particles together, but high clay content lowers the infiltration capacitglr and promotes runoff. tiilislope gradient afiects sheetwash erosion by increasing the erosive force the runofl' applies to the soil surface and hillslope length is important because. as indicated in Figure li—“l', the depth of overland flow increases with the distance downslope and its abilityr to erode is proportional to its depth. At the top of a hillslope, the depth of the sheet is not sufficient for the eroding force to overtitrme the resistance of the soil to erosion. There will he a zone near the top of the slope in which sheetwash erosion cannot Occur. though rainsplash erosion can. The relative importance of rainsplash and the force applied by sheetwasb changes with the distance downslope. Cln teen,r long hillslopcs with lowr infiltration rates. sheetwash usually becomes the dominant process~ although rainsplash is still exceedingly important in mobilising soil which is then transported by the sheetwash. The combination of the two processes can be appreciated by closelyr observing surface runoff during a lull in a storm when abundant water is moving over the surface. but onlyr an occasional large raindrop falls. The drop passes through the sheet of overland flow and splashes soil up into the flow. A small. faint plume of sediment can be seen drifting downslopc with the sheetwash. If the minute streams of water out separate channels, as shown in Figure IS—El'. the process is known as rill erosion. The concentration of runoff in Figure 15-9 Kill Unbliifl mi an agricultural hold in Illlnoix. tL'SDA —Sanil CUTIWB'ELIIHTI ficrs‘ict‘J these small channels causes an increase in the etfieieney and intensity ol‘soil removal. Cln agricultural fields. tillage operations may obliterate the rills each year, causing their significance to he overlooked. If the rills become engraved into the land surface to depths of more than about one foot. and especially if they are not obliterated by cultivation and do not migrate back and forth across the hihslorpe. they are generally classi- fied as gullies [Figure 15-11)}. Gall].- erosion produces incisions ranging in size from a foot in depth and width and a few feet long to several tens of feet deep. hundreds of feet wide and miles long. (ieomorphologists do not yet understand exactly hov.r rilis and gullies form and remain stable, but they are generally associated with very intense water erosion under the most severe oombinations of the oontrolling factors listed above. The amount of soil exported by gully erosion is usually small by omnpar— ison with that removed by rainsplash and sheetwash erosion. but the process can do great damage to an individual piece of land. In a heavily cut and grazed region in northern Kenya, gully erosion measured by Duane has mobilized only I percent as much sediment as sheet erosion. Leopold et a]. [Who] found that gully groth supplied only L4 percent of the sediment yield of a small EIWJBd catchment in a semi-arid area near Santa Fe. Rain- splash and sheetwash erosion supplied 913 percent and soil creep EL? Figure lS—llll {tally erosion after land was cleared and loaded for housing construction. In a single winter the main gully was eat to a depth oft; feet and an average vothh of 501m and was more than SLI'J feet long. Juanita Creek basin: Kirkland. Washington. {USDA—Soil Conservation Service.) Figure [5-11 Coma-notion sites undergo intense shoemash and gully erosion because the vegetative oover is removed and the sortaee is continually stocpencd. churned. and compacted. [Hansen] Conservation ServieeJ percent. Brune {1950} measured gully contributions of 3.4 to 45] percent In an agricultural basin in Illinois; sheet erosion supplied 19—96 peroent. with the remainder coming from erosion in the valley bottom. and probably from soil creep. Glymph [1957} compared gully and sheetwash erosion at many localitiw in the agricultural lands of the east and central United States. Gullies supplied from I} to 8‘} percent of the total sediment yield. but most ol‘ the high values came {rorn one region of Mississippi where the pmcess is particularly severe. Seventy-five percent of Ulymph’s values were less than 343 percent. We do not yet have any quantitative estimates of the contributions of rill and gully erosion on urban mnsttuctlon sites. road cuts. or mined areas and spoil heaps, where conditions favor all types of water erosion. As we will show in the next section. rates of soil loss from such areas are very high bewase the vegetative cover is removed. the permeable topsoil is removed. and the dense subsoil is exposed. compacted. and graded into steep slopes and is frequently disturbed by heavy machinery. Some of the conditions encountered on these sites are shown in Figure 15-11. Mined areas and their spoil heaps sufi‘er rapid water emsion for the same reasons. Even in formed and agricultural areas. roads are important sources of sedintent and need constant care to minimize damage to stream systems {Figure 15-12). More will be said in Chapter 11" about the efiect of urbaniza- Lion and other types of land use on the sediment yields of drainage basins. Figure 15-12 Sediment washing from a mud road in eastern Kenya. Rates of Water Erosion Because of the difl'ieuities of separating rainsplash. sheetwash. and rill erosion. their efl'eets are usually measured together by monitoring the amount ofsedimertt lost from hillside plots like those shown in Figure 1543. or from small wtehrnents. Another method of obtaining the same data is by repeated measurement or the exposure of stakes or nails (Figure 1314]. A detailed discussion of techniques for evaluating various kinds of erosion is provided by Leopold el al. {igfifij and by Dunne HEW}. Thousands of measurements of rates of soil loss have been made from hillslopes with different gradients, length. vegetative savers, and soil eon— servation techniques. We make no attempt to summarize this literature. except to present a few representative measurements under dilTerent types of land use. They simply emphasize the overriding importance of vegetative cover and therefore of land use. Figure li-lfita] is perhaps the most Ilivider quoted summary of soil loss from agricultural plots in the midwestern United States. Figure lfi-lfith} shows a similar compilation from studies in Tanzania. The results speak liar themselves in view of our foregoing dis.- eussion. A sample of data is listed in Table I5—l. figure 15-13 Experimtniul pluh 2hr mausun'ng runflfi' and soil CTflfiil'ITI Lind.“ Furiuui- EmE‘IS and cultivaunn prawns a: MmJb-um Ncu- June}: The 111;: lamks Trap Ihi‘ watcr and HflJil'ntl'll Hugh! in the tfnugh HL lhl: hulluln of filth plfll {USU-"n Sail CUT-IMH- wutjun Sen-mm Figure 35-M- Fmfimn Flflfi. an a hillsidx: near Sana] Fe. New Mexico. Nails and Wafihtrh' an: phL-cd in fines an tin: slopc. and th: hmghl r‘rm-n [he mp of 1m nail to :he washer |5 measumd am‘maH}I with :1 millimclrr .IiL'aJc. k Soil inst by erosion Items per acre] W Water luel by runvncfl' [percent of rainfallj - _ .El} tens ' .122 .04 tons 6.5% 3-1.6 Inns .+:._— .:. 59.2 tans Baretl‘ailew} l3} Figure 5-15 Results from plot studies of runefi' and erosion under various types of land use. {at} Midwestern Llnilfil Sluice. [Sail Ctmxtrvafinn Sen-let} {1}] Mpwttpwn. Tunmnin. {From Rapp er al. 19H.) I'M 15-]. Sum: measurements elf unl lesa. l'rnm hillside pints. 5011 1:055 {TOHSIM'REIvn} [I TGNNEIHEITT tRE ] LAND use chTmN = 0.4414 I'thIM'RE HULIRCF. PURE-ST Primeval Oklahoma [IUD] Smith and Stanley U965} Hunted arm'uall}r Oklahoma 111] Smith and Stamev {1965‘} Primeval North {Interline 0.1112 Smith and Blarney “9195] Primele Kenya 0119 Dunne. unpubliahetl Burned semiannunfl}: North Carolina 3118 Smith and Stumey {[9155] Second growth Ivory {Toast 0.40 Nye anfi Greenland {1950} 'anun-rilanel1 protected Texas 0.135 Smith and Smmey {1965} wall-dlflfld.‘ burned Texas 0.35 Smith and Stanley {1965} annually iii"rtetrlltmnil1 protected Ohio [LDI Smith and Blame}; {1965} Woodland, protected North Carolina [LBS Dils ["5533 rmtr'nued TM '54. t'mmurd LAND lihE AGHK'IEIJHHE. {'UIJ lVATI-JII Gnmtnum Bluegrass Alfalfa Clover and gun Bermuda gnu Flume gram Iiuyiand Hayiand Tmpicnl PCNI'II'Iill-I yams Truphul imam Hayluntl Tmpical pattIurt: 5mm“ Dubgrus Urns:- AGII'I. lJLTUlh ('IO'PLAN I}! Ran: fallurw Ban.- rllluw Ban: I'atlluw Barn I'nllnw Ban.- fitth- Maize (mm: Mail: Mail: Mair: Maia- Milli}! Hill rice Hill rice Ilill nut Hill rice Ari-tat grain Coup-taut Urid 5min Peanuts IJ'K'ATIHH Midtt-fltcm [1.5. Midwcttcrn [3.5. vttginia 5W. Unitctl Suits- {Infirgih Washinglnn Earth Cum-HM Puma Rim Put-rm Rim lndta India lndta India Georgia ‘I'nnntnin Iii-arty Cum Midwestern US. India indin India hitdtn-cntt-rn LLS. Rhodes“ hfitltwnltm L'.5. Tu nianiu lndin India hut Gullah India India India indI-‘t “III. Ill'k‘i: [IlINSIrAleITII I Ittsutii'ln‘t'nttr: '- 0.4474 'tnhsn'tcnl. 9.92 4134 (“33 1.115 CLO! 410? noon [0 0.29 Illfll 13.03 [LEI LII} 0. IE L05 5.30 0.20 {1.41 |.4- LE- [Ell 59.2 H Hail-lint 4.1 "154 I156 2.1-4.5 '33.! 3M: L0 931 | [.2 {14- Hi 5.5 I .9 21.0 (it! Smith and Stamuy {Haiti Smith and Ethnic} [I'Ntl‘i Smith and Sumcy {lilfifii Smith and Slamcy “96$; Bunch 1 I965! Smith and Stanley “9155) Smith and fitantey i I965} Smith and Elamty {I965} Smilh and Emmett 1 I965} h'a-tudc'miah :1 II. [19:55] Vatudtwatnh et al. {I965} Italian-nt- and Rm “969] Linich Natinmilvfin Bnmctt “965i Rnpp ttl til. “912i Dubitt {[959} Bcnnnll [[939] United Matias-Lt H95” Bntlnnar and RH?“ ([969) Vasudcvttiith :1 nl. ilih‘rfi} limit-on :1 al. “9631' Human and Jackson “959:! Ht:an 1 H.391! Rnpp H in. tlifll} Batu-mar and RN “WI Vusutlct-ninh ut ul. {[9155} N}: and Gtuniand mam Nye and Greenland H9611] Hatuwnt and Rm [1909! BalIaWJr anti Ran “969i h'atudt't'fiah at al. ([965! Vmudn'nuh it: nl. {lgnfii .—.,_.__._._._ ruminant! IJZI] 'l'ablo 15-1. ronfl'arrm' L'AHIJ L'SI: HANULLANW Dry woodland and rangelaml Dry woodland and rangeland. after lire Dry woodland and Iangeland Sparse gl'afiland Dry woodland and rangeland. heavily eat and grazed lGrass and scrub URBAN Huznl gills. Building sites EnablingE sites HINING Land deyegetaled hy- smelter Fumes Spoil bank RURJ'LI. Hit-1D.“- Forest roads in a Jammer tm'n Forest roads Rural road Prediction of Soil Erosion “JET thh ['I'HN'W I“ Rf 9TH] i.('.fi'l'lfll~' [ ' Southern California South ern California N ew M as ieo Mb: rla Kenya Indla {jeorgaa Maryland Maryland lOntario Ohio Idaho Idaho N. Kenya l 'IoNNI-atlaa IMH- |J.44'.-'4 l'tm'fi'fht'lth sol. t-U.'L 2.7 Kramntes 11913433 24.? Kramrnes tl'i'aill 11.2 [.eurmlcl :t al. [mm T-‘J' Campbell (PNEU- If'l T15 Dunne. measurement LS I.l‘i ElnllEt‘l NflElUi'lfi [195” T913? Diseker and Riehardson tl'énl} ll’i—ll"? Wnlman and St‘hiek [WM] 139 Guy {1955} Elinl Pearce {[939} El" fie-inform [HT], [113;] 29.? Megahan and Kidd H.972] 19 (‘ope-lnnd 119135} 53.7 M. Nmtnn—Uriflilltfi. In the planning of agricultmal lantl use. rural development schemes: and small water-resource developments. il is frequently necessary to estimate the rate at which soil is being lost From llillslupes. or the run: at which it will he lost il'a new crop is introduced. ifsoll conservation programs are begun, or if there is a IlueLuaaon ot' the weather. saeh as a run of very war years. This is being done oath inereasing Frequency. eare. and Sumeria- as we ae- eumttlate unfortunate eaperienees about the [MlEIlEiEtl impaet of” soil envision iipim agrieulturai Eiriuluetioii and engineering works. and as the teelinitlties for making predictione of erosion iittpre'i'e. The heat method of predlellng mil loss is to have some loenl tield data that are representative ot‘ the range of eotidiiions found in the area of in- terest. hlmt planners are not in a pofittion to monitor soil erosion in the field. 'I'hey shtiuld he “Ware. however. that some agrieulltiral engineers. gewniorpliologists. and hydride-gist; do monitor soil loss. and that data are ai-aiiahle in some areas. This is particularly true in agrieultural regions where the Soil Conservation Hewit‘e. and the Agricultural Heseareh fien'iee iii the L't'iited States. and similar organizations Clficwhii'l'l’.‘ hater.- aeeumiilaied a great deal of infunnation ahotit soil loss from eultii'ated tieltls. Planners are often in :i [firiillllll to encourage further eiilleetitiit ttl'stteh tltiltt tti dt‘ICll- them both the impaet if human aetieities and of“ natural fluettiiitiens ot‘ weather. The necessary fieldwork is not expensive and can yield Villuill'llt.‘ tnliii‘n‘ialitin ‘rflil' planning. A semnd method of predicting erosion is by the use ot'niullieariate equa- tions deeeloped I'nit'n data eolleeted at large numbers of experimental sites. Many examples oi" multiple regression equations are available in the liter-'1- ture. but their are only useful in the area for whieh they were develop-ed. The prediction—equation approaeh has been most highly and usefully developed by the Soil t'iiusereatioti lien-ice and the Agricultural Reseiireh :sen-iee over the past 35 years. and inaiii.‘ predietit'e equations have emit-ed. The mmt mliipreltensii'e and useful of these has become knot-uni as the Unite-mill Suit Lass Equation [Wisehmeier and .‘imith I965} and continua to undergo refinement and testing holh oilhin the United States and in other countries {Hattawar and Rao HEN. Hudson I‘JTI i. The technique eari also be used to ptediet mil loss from eonstruetion sites [Wlhflhlllflltfif and Meyer H73}. The equation prediets their the amount of soil tiioeed from its original position on it field or hillside. it does not estimate the net soil erosion result- ing than the ditt‘erenee heiween erosion and deposition. Nor out its results he eompared with the sediment yield ot‘a river hasiii. The following eompu- tittitms provide an index of net soil ereiaioti that aflows the planner to recog- nize sensitive sites. and to plan strategies to minimize soil him. The Univ-flan} Suit-lam; Equation is at — RKLSFP {lS-It where it Mail has [tons per tieret R - the rainfall enmsity intlea K - the soil em-dihility index I. the hillslope‘length faetet S '= the hillslnpe-gradient i‘aetoi' (‘ the eroppitig-iiianagement I'aettit' P — the erosion-control praetiee I‘aeior. 'llie rainfall erosi'eitjt index is ealetilatod from the lijnelie energy t E l of each rainstorm multiplied h}; the maximum Sill-minute intensity of the storm 523 Hill-slope Prm'e'ri'e't 524 Scamp fan-fog [Jim]. The kinetic energy varies with the rainfall intensity [I] according to the relationship a -- sis - 331mg“, r {iii-2t where the energy is expressed in foot—tons per acre per inch of rainfall and the rainfall intensity in inches per hour. The rainstorm is divided into por— tions of uniform intensinr for the application of this equation. The energy per inch of rain is calculated from Equation l5-2 and is multiplied by the amount of rain falling during the time interval. Incremean of energy are then summed to obtain E for the whole storm. The rainfall erosivin index cart then be calculated from the sum of the E X i3“ product for each snirm during the period of interest. such as a year or a season. according to the equation 2 Edna R r '“lm - {ti-3i The factor can be calculated from the rainfall intensity values obtained with a recording gauge. For the eastern and central United States. annual erosiv- ity values have been mapped (see Figure IS—lti]. and graphs of seasonal dis. tribution and probability ofoccurrence ofvarious amounts ol'rainfall energy are given by Wiseltmcier and Smith. The soil credibility factor. K. is the average soil loss. in tons per acre. per lftfl foot‘tons per acre of rainfall erosivin tie. per unit value of the erosivin index given in Fuuation l5-3} when the soil is esptised as cultivated bare fallow under specified conditions of hilisiope length and gradient. 1tl'alues of K 1were obtained by direct field measurement on a fev.‘ agricultural soils in the United States (see Figure l5- H]. and K values lint other soils can he estiv mated from these by comparing their physical properties. For most of the major soil series in the United States such erodibility factors can be obtained from the Agricultural Research Service or from the Soil Conservation Ser— vice. Wischmeier et al. {19H} have extended the method by the develop- ment of Figure IS—IE, which can be used to estimate K values for both agricultural soils and for the dense suhwils exposed on construction sites. The soil properties needed for the newer technique are the percentage ot'soil particles between 0.002 and lill mm. the percentage greater than {Ll turn. the organic matter mntenL the soil structure. and the permeability. The use of the nomograph is illustrated by the path of the dashed line. The authors of the work claim that about 95 percent man K values estimated in this way should lie within 10.04 of the true value. The length and slope factors are usually evaluated together front Figure I549. [1‘ convert or concave hillslopcs are being studied. the length and slope values used should be modified. If the lower end of the hillslopc is much steeper than the upper 'Ptfl'lltfll, the gradient of the sleeper reaction should be used with the overall length. (in a concave hillslope the gradient and length of the upper. steeper segment only should be used. Emmi 3.55m uauufingu 1.5m .mfi EEE gm} anugflfluflfi Ha Nun H.352. EEEEw 5 EEE mi? in; us :5 fining an 3E :233. 13.33%... 2E :3: :zEm Ea Eufiauma an 55% uflufi HE.“ his". has: an: an. "3:? E: E5 2n: .qu—m Egan 3350 van 2330 3.: 3.. aha hum gag.“ hE €225; E £333.? 232 Eng $5.59.}. 3%— E 3:15.,“ _ 3.1.5. “€33 EES 5:5 . “and: _Il_|_|_ :cm :2“ :=_ _u_ 53f} {immrphnfngt LIA fl? L_I._._. . __t.__t.__t_.__.| {I ‘9 5 e e E E‘s—h: E e 2 i 5 3 .3 g 92 E e 3 g g e E 3- E e 3 E a s Seillentnre flu.” Ifi-l‘l' Summary {If mar-med K value: [er I rang: aim“; in the emern end eerrtral ['nttnl Starter. Each lament-l he indicates an: pl“! mmurement. The mailed area reprereutt. the range while data available tie far. uptut I’mm n 2-ier measuremth m1 II- fiilt hum. [flute I‘runt Wischme'ue: and litmlh meal The creppingmanagemcnt factor. 6‘. includes the etl‘ect nl' vegetative cure-r. the requenee ercmp in a retainer, the stage of the crap, tillage prac- Iiecs. and residue management. Values of F are given in the Wischrneicr and Smith report liar teen-t}r ditl'erent ere-p ruwlions and lb: seven Iield cen- tlitinns meeting lbur crop stages and the falluw period. The C' 1irnhte I'er eech erupt etuge in Table Ill-1 can be weighted by the pmpurtiun ef the annual tulal eresieity for the Irritation {also git-1n fer the United States by 1'.‘|.r'i:..chn-Imeicr and Smith} to calculate a weighted average value of the emp- management Teeter. C... i r.- - m + - ".L___ Thus. (u. -r J" mu I [15-41 where tilt, is the percentage of rainfall cmsiritjr uni'hi occurring in the ith twp stage. The US. Soil Cmsewatiun Service ( I915c} :lw releases: C values for ether agricultural regions. and fer woodland and pnsrure, as illnulrntcd in Tahlee 15-3 and 15-4. 1It‘i’ittehmeier end Meyer [19?3} pit-(wide some guid- ance en C values lin- censtructien sites. :5. .3 a tfitfififi Ears: .5”. n 2 “E333 a. EEEEE .n 333...": said ED arm “Emu 6% Eng unu bu... new. 5.3 mflig =3 a .8... £333..— MHEHEE up: flusofi an... .flisu #251 30.5.3 3:33.35 .3553... :5: E .hdmLMunn—uufi “En .uhauafi AEDH Eu.an umcuma Ln. umfizuflua _AEE QWEE new“ .3 umflnufluh 3:9. 2.: mEEumEaE HEEL Em . a .3 3% EH :2 E «EH H35 flan. flatmfinmn Era. ..E=.uEEt £5.53 a 1. m _ anti E5. :3: HE; 5.5355 new .34 E. n." Bum“: E E: q. E .2 902.”. finazntumufi. uEEm 3min: E $.33— .hufism .mnm high. 5 Ea.»me . £53902; _ L 2 Ed EEw¢Eo= 2% E 3,... E... Fun—“Gun EEEEL =3. at. £333 §-__cm uaE aHnmfifi ______.\_____‘_ $5533. or: H: EumnE bEnEEu mnumfifiu .E fiEmoEoZ 5.2 2%.“. . I X _ 33m hunk-m1 V \ N am . x . .___. x M W. . 5w. um. an. H puns any in}. + m5 1u:r:u'.1d ‘3‘- Ju mnewqxufide mug _ . Rana .5 55.“— 5.33... as... :Ew Rug”. .5 .EEA 5.... .53 3E." E: 5cm use in? _ ._ 4-” x ; ..--' 2", ~-"" if -" "" "" fl...- 3" 1.:- .-"" r" I! ififjof I I'll-'- f f If m --" _..-"" r" i," 2""! xx“! ff in? ,..---""J ..-" xi" I’Kf/ ..-' '1'”! fl! 3",” fjffffz f f fz’ffx’f-bu “#x’ffx" h 1’ fr“ ~31 «Li: .r' {I I. I: «a! 1* g a!" 1% 1 "I. ‘53 """ .2"! if .E w .H" ..-"' p" 3. 1, a!" K; a fix f .z". 1 Eu If E a r - u H .1 n. *‘" ____, .u-fi ['15 '5 I'd-“Hf 1"" If. 1 ,_..--' fix .u-F" ,u-F ' w“!!! air-yr“...- 02 - r“ ...-"" 1.5-” ____,.~---"'" if” fr Eff-I! Hf! ff! 0.] *"fi _ “#- 3*”! H} 2"] '10 60 “It 2CD HI] 6C0 [DU] 131]} Hillslope lcngth [fuel] Flgnre 15-19 Chart for m1uath'lg th: 1mgm—slnpe l'aclnr. LS. in U11: Univmal Soil-[ms Equatiun. '11“: solid ljncs rcptumt conditian within [111: range of dam from which lb: was were dtrin'ui. Th: dashed lines Lune based on extrapolaljmus and: shnuhi he used with cart. [Fa-mu U5. Sail Quasarvaunn 3m 191511.} Table 15—1 Exfl'l'lplfi of cmpplng—managcmmt meters. If in th: Univemal Soil-an Equation. f'nr agricultural land. Many nlhfl crop umbinnliuns an: Lisltd in the original Mama: as we“ as in mhnifiul releases for particulatr Icgiflns. The crop 51354;; are also dcfinnd in the original source. {From Wisduncicr and Smith 1%5.) __—___—___—————- CRO? STIMIE ______—___—————- HM UR (:RDP FILL WW SEEDLING EST hH-LISIIM I'NT GRUW I'H RESIDL‘ Cum with mnvmlinnal 0.36 0.63 0.50 i116 0.31:} tillug: in rotation with small grains and girls; Almif‘a Table 15-3 Cmpping-managcmnt factors. C in I111: Universal Soil-Loss Equalion. for woodland. .529 {From [1.5. 5011 Cummalian Sen-rial: 192511.] —_—_—_——— '3: m- ARI-EA mutual) a? TREE CANE-w :- 2 niches or (‘3. m ARI-£11] musr LITTER UNDERGRUW'IH C 100-25 100—90 Grazing and burning controlled 0.001 Heavily grand and burned 0.003 0.011 20-40 M25 Grazing and burning anrc-lled 0032-0004 l'lmwily grained and burned 0.01—0.04 35-20 20—40 Grazing and burning Donn-011:0 01113—0009 Heavin granted and burned 0.02—0.0“.':'|I {20 Truatud as grassland nr cropland Talia [5-4 Cmpping—nmnagemflnt Mars. C in the Universal Sailings Equation. far pasium rangthnd. and 101': land. {Frmn 1.1.5. Soil (h‘nwntim Smioe 1925b.) TYPIE I'JJ‘ (TANG? 'I!' AND AVERAGE FALL CANOPY FERCENT GRDUNI} cull-ER HEIGII‘I' OF WATER COVER [illflLrND ——— DROPS [9'41 count“ 0 20 40 an :10 95— 100 No appreciable camp}- 0 .45 .21] .II] .042 .013 .1113 w .45 .24 .15 .090 .043 .01 l Cannpy of tall weeds 25 G 36 .1'1' .09 .033 .012 .003 or 51mm brush W .30 .20 .13 .0112 .041 .01 I {0.5 In [all 111} 50 G .26 .13 .0? .035 .012 .003 W .20 .10 .I I .075 .0351 .01 I 25 Ci .1? .10 .05 .03! .011 .1113 Appreciabh: bruah 25 G .40 .13 .09 .040 .013 .003 or hnflhea W .40 .22 .14 .035 .042 .011 [2 m 010 Ill} 30 G .34 .10 .035 .0311 .012 .003 W .34 .19 .I3 .011] .04. .011 25 G .20 .14 .00 .036 .0 I2 .003 W .20 .12 .12 .07? .040 .01 I Trma' but no appreciable 25 C- .42 .10 .10 .041 .013 .003 luw brush W .42 .23 .14 .032 .042 .011 [4 m 0111 In} 50 G .39 .113 .09 .040 .013 .003 W .39 .21 .14 .035 .042 .01 I 1'5 (3 .314 . I 1" .00 .039 .012 .003 W .34: .20 .13 .0113 .041 .01 I ‘G : [Comer at surface Ls grass, grasslikc Flam; dccaying mmpacicd duff. m‘ litter al least 2 imhcs dccp. W — Come: a: surface is .Efl-Dsll)‘ trundle-Jr hflhamus Flam: {as wards WIIJI liltlc lateral-root nctworb: ncfil th: sufl‘auc. undccaycd rcudut. or both}. .5 3 ti Germ orpholngv Tillie 15-5 Erosiian-conIJ-ol practice fas‘ttn IP} for the Universal Soil-Loss Equation. (From L'.S. Soil Conservation Service H.75er P VALUES EillN'l'flUfl ('(‘IN 1'0le LAND SLOPE 511th inmosrm i‘i’cl t'.‘DNTflURINr5 caor-vmo' riiintnws Tcnawmtd 2.0—? {150 13.25 1125 U. til Hfl- 12 can [L30 1131] Fl. I: [3.13- 13 Ml} [L4G H.443: El. 1 ti 193—24 0.90 1145 0.45 El. 1 it ‘Using a dnyear mention of maize. small grain. meadow. and meadow. For preilii-iiun of enatnhution tn ofl'—iiclcl sediment load. The erosion—control practice factor. F. varies with such techniques as contour cultivation, strip cropping, and terraeing. Recommended values are given in Table I5-5. The Universal Soil-Loss Equation can be extended from individual fields and hillsiopcs to small drainage basins by dividing the basin map into areas of uniform soil type. topography. and agronomic conditions, and computing the soil loss for each combination. In the United States. a concept has been developed of a “tolerable soil— low rate." which is the erosion rate below which land productivity can be maintained. The values of tolerable erosion estimated for different soils in the United States vary from about one to six tons per acre per year. In plan- ning agricultural development. therefore, this rate can be substituted For A in Equation 15—}. and for the fixed values of R. K. L. and 3 for a field. the values of C and P can be manipulated to balance the equation. this exercise indicates the management options available lcr keeping the erosion rate below the tolerable level. and policies can then be followed to encourage the adoption ol' these practices among farmers. it should be realized. however. that the values decided upon for the agricultural areas of the United States refer mostly to good conditions of soil and climate. a high level of manage- ment skills on the farm. and a situation in which fertilizers and energy inputs have been very cheap. These last two conditions are not likely to last much longer. even in North America. In many other countries with a soil—erosion problem, conditions are even less encouraging For all the factors referred to. It is likely, therefore. that tolerable ratm of soil erosion in some areas are lower than one ton per acre per year. [in urban construction sites. road cuts. and rural roads+ the Univetsat Soil-loss Equation can be used to predict rates ot‘sediment production and their impact upon turbidity levels of receiving streams. as well as to design strategies for sediment control within the disturbed area {Wischmeier and Meyer 19?} }. The approach incorporated in the equation has many possibilities for application and refinement. tts application to other countries is limited by lack of field data. but this might be remedied by a survey of the few such data that exist in a country [see Fournier 1961'. Temple IWZ, and Hudson lil'l'l for valuable summaries of past work on soil erosion in tropical coun- tries}. A relatively few field measurements and some educated guesses by experienced soil scientists. agricultural engineers. geomorphologists. and hydrologists could also be used to extend the approach. Valuable informa— tion of this ltind is now beginning to accumulate in several countries and is published in such outluLs as the Journal of Soil and 1Water Conservation in India. the lnterafrican Soils Conference. and the Soils Bulletins of FAU. Battawar and Rao {well}. for example. have evaluated the trend of C values through the cropping stages of several important crops in lndlt’t. Even where it is not [mss'tble to make such quantitative estimates of soil loss, it is still possible to map the distribution of erosion intensity in a region. to gain insights into the major controls, and to make predictions of the rela— tive soilsloss rates to be expected from some change of land use. This can be done by means of an erosion surrey. in which the degree of erosion is esti- tttated visually on an ordinal scale, as indicated in Ta hle lfi-ti. The scale could. of (nurse. be altered to suit any regional conditions. Such a survey can be carried out rapidly over large areas Itvith individual fields or bill; slopes. or over areas of up to “It acres classified together. At each site. other factors cart be noted. such as average land gradient. drainage density. roth type. soil type. vegetatitm. land use. and clintate. By means of a graphical portrayal or statistical analysis. the intensity of erosion in a region can be related to it»; controls and the results can be used to make semi—quantitative predictions of what to expect under various management alternatives. An early example of this methodology was given by Renner [1936}. and in the rush to practice multiple regression analysis on two or three years of plot measurements of soil loss, the potentialities of the erosion survey for use in planning ever larger areas has been neglected. Significance of Soil Erosion The total cost of accelerated soil erosion. either in monetary terms or in human suffering. has never been calculated and probably never could be. in the United States during the t93fl‘s. soil erosion tvas recognized as a major threat to the continued productivity of the land. and a great deal of research and investment went into soil conservation. Important advances were made and the productivity of most of the nation’s agricultural and grazing lands was stabilized and often irtereased. This was accomplished. however. with the aid of vast amounts of cheap energy and fertiliser. The elTects of the rising wsts of these commodities on the soil conservation pro- gram arc uncertain. More recently. concern about soil erosion has shifted to .531 H illsl'ope Prat“ eats er ...
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This note was uploaded on 08/06/2008 for the course ESM 203 taught by Professor Dozier,dunne during the Fall '07 term at UCSB.

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SoilErosion_WEP - Hillslmpe Erosion by 1'i’ilate-r In the earl}r 1940’s experimental work by W D Ellison demonstrated the role of raindmps in

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