Published April 1985. Facts and recommendations in this publication may no longer be valid. Please look for up-to-date information in the OSU Extension Catalog: http://extension.oregonstate.edu/catalo

Istok, Jonathan D.

Oregon State University. Extension Service

Solutions to Environmental & Economic Problems (Organization)

Vomocil, James A.

Undetermined

ScholarsArchive@OSU

it--COPIIPPRePAIIIIreu3 "P IMOO	 ii POssSP-ate VA , IAPvi —VI — 00 st K INFORMATION FOR LEADERS IN LAND M GOEleAResearch and Extension in land manag	 cfdit,32tetC>„,conservation of soil and water.SR 736 / April 1985gy for farm profits andSOME WILLAMETTE HILLSLOPE EROSION RATESThe ProblemErosion destroys land, increasesfertilizer costs, decreases yields, andcauses water pollution . when sediment-laden runoff flows into streams andrivers. Rates of erosion are oftenestimated indirectly. Estimates depend onthe method used and may vary by 200tons/acre or more. Which numbers arecorrect for a given soil and managementscheme? To answer this question, theDepartment of Soil Science at OregonState University has been measuringerosion rates by various methods onselected soils in western Oregon forseveral years.The ResearchErosion was measured by four differ-ent methods over a four-year period fromOctober 1977 to June 1981. One methodused standard'erosion plots. These plotsare 6 feet wide, 73 feet long, with atotal area of 1/100 of an acre. All ofthe water and soil leaving the downslopeend of the plot was collected in a largepan and measured. A second method usedsmaller erosion plots (called "mini"plots) with an area of 1/500 acre. Athird method used II-flumes to measure thetotal soil loss from watersheds as largeas one square mile. Runoff was collectedat an outlet and measured; samples weretaken to determine the content ofsuspended solids.A fourth eNperimental method thathas recently been used in Oregon involvesestimating erosion rates by measuring thedepletion. of radioactive cesium in soils.Testing of atomic weapons from 1954 to1965 deposited a measurable amount ofradioactive cesium. As the cesium fell,it became tightly bound to the soil nearthe surface. When soil is lost from alandscape by soil erosion, the amount ofcesium decreases. Therefore, by measur-ing cesium levels in soils, we car. tellhow much of the surface soil has erodedsince the cesium. fell, over 20 years ago.This technique is experimental and re-quires certain assumptions, but it isimportant because it estimates long-termrates of erosion as opposed to short-termestimates obtained from most erosion plotstudies.Results reported here are all fromgrass or winter wheat fields and all hadappreciable cover by December 1. Theaverage total September, October, andNovember rainfall is about 11 inches andpan evaporation is about 7 inches at thelocations studied. Therefore, thesesoils are not likely to be extremely wetbefore December 1, which limits earlyrunoff and erosion.Erosion Plot Results A summary of the available erosionplot data is shown in Table 1.AOREGON STATE UNIVERSITY EXTENSION SERVICETable 1.	 Erosion rates for five soilseries in western Oregon.	 Measure-ments were made with erosion plots.nial grass isson why grassin controllinThe small mewheat cover onprising becauhas	 longer,well known and is one rea-waterways are so effectiveg the formation of gulliessured losses under winterthe Jory soils were sur-e the Silverton hills areateeper	 slopes	 and	 higherSoilRange intotalannual soilSeries' Slope Crop	 eresion3 rainfall	 tha	 the	 other	 sites.	 Fromtons/acre/yr observations	 lade during the winter, thelow erosion r tes were attributed to theWillakenzie 6-7 winter	 0.7-1.5 strong surface soil structure and to thewheat2 large	 water	 holding	 capacity	 of	 theHelmick 7-9 winter	 0.6-5.0deep, well-drained profile.wheat Sediment Losses for	 10	 Watersheds:	 Asummary	 of	 25 watershed-years	 of	 dataDupee 7 winter	 3.8 for	 ten	 locations	 in	 Polk	 and	 Bentonwheat counties is shown in Table 2.	 Measuredsediment losses ranged between 0 and 20Jory 7-12 winter	 0.1 tons/acre/year,	 and in all but two in-wheat stances were less than 8 tons/acre/year.12 perennial The largest annual loss was measured ongrass Watershed 1,	 a 9-acre watershed on theHazelair	 soil.	 Seventy	 percent	 of	 theNekia 7 perennial	 0.1 20	 tons/acre	 annual	 total	 was	 lostgrass during a single storm in November 1977.The	 fall-planted	 ryegrass	 had	 estab-1The plots on Jory and Nekia soils (2 years ofdata) were located in Marion County. The others(4 years of data) were located in southern PolkCounty.2All plots under winter wheat cover were installedwith the drill rows parallel to the long sides ofthe plot ("downhill"). Conventional tillage wasused.3T values, that is, the tolerable soil loss rates,calculated to equal the estimated soil formationrates for these soils are as follows: Willakenzie- 2 tons/acre/year, Helmick - 5, Dupee - 4, Jory -3, and Nekia - 3.Measured erosion rates ranged between 0and 5 tons/acre/year. The highest losses(3-5 tons/acre) occurred on Helmick andDupee soils in fields fall-planted towinter wheat. The lowest erosion rates(0-0.1 tons/acre/year) occurred underperennial grass cover on Jory and Nekiasoils. The reduced erosion with peren-lished a low amount of cover and thesoils were frozen with a thin cover ofsnow. The warming rains which followedremoved large amounts of sediment. Thefollowing year, the established ryegrassoffered greater protection with sedimentlosses reduced to 0.2 tons/acre.The lowest sediment losses weremeasured from Watershed 4, which hasdeep, well-drained soils. Adjacentareas of Willakenzie soils (Watersheds5, 6, and 8) had higher losses becauseof poorer natural drainage resultingfrom a clay layer about 3 feet from thesurface.Comparing Tables 1 and 2, we seethat erosion plots give somewhat lowererosion rates than watersheds but bothtypes of data are generally in the range0 to 10 tons/acre/year.Table 2. Sediment losses fr om 10 watersheds in Polk and Benton Counties, Oregon.WatershedYears	 ofSize	 Data(acres)Principal	 soil	 series,	 slopes	 and	 Range	 in totalcropping	 annual sedimentyield'tons/aere1 9 2 Hazelair,	 12-207, fall planted ryegrass 0.2-20.02 234 1 Willamette, Dupee, Hazelair, 	 7-20%,annual ryegrass, woodland0.4-0.63 40 1 Dupee, Willamette, Hazelair, 3-15%,annual orchard grass0.564 1 Willakenzie, 3-7%, winter wheat 0-0.35 4 4	 Willakenzie, Helmick, 0-7%, winter wheat 1.1-7.56 15 4	 Helmick, Willakenzie, 3-12%, winter wheat 2.3-9.67 704 4	 Willakenzie, Helmick, Waldo, 0-20%, winterwheat pasture, orchard, woodland0.2-1.38 6 Willakenzie, 3-7%, winter wheat 1.89 222 Steiwer, Helmick, Hazelair, 3-20%, winterwheat, woodland0.4-1.010 148 2	 Willakenzie, Bellpine, Hazelair, 3-20%,winter wheat, pasture, woodland0.2-0.61T values, that is, the tolerable soil loss rates, calculated to equal the estimated soil formation rate forthese soils are as follows: Hazelair - 2 tons per acre per year, Willamette - 5, Dupee - 4, Willakenzie - 2,Helmick - 5, Waldo - 5, Steiwer - 3, and Bellpine - 3.• The Cesium Tracer Study Cesium distribution was measured onWatershed 6, to calculate long-termerosion rates. The results ranged from 1to 12 tons/acre/year, which is consistentwith short-term measurements made witherosion plots and flumes. The cesiumdata also show that toeslopes are activeportions of the landscape, serving asboth temporary storage sites of erodedsediment and source areas for activeerosion. Sideslopes and stableridgetops, however, show no detectabledifference in erosion rates.Significance of the Results The four largest watersheds (2, 7,9, end 10) had somewhat smaller sedimentlosses than the smaller watersheds.There are two possible reasons: (1)some redeposition of sediment occurredin the swales, and (2) the larger water-sheds containea a greater proportion ofwoodland, which is resistant to erosion.Redistribution of sediment withinthe watershed will not affect waterquality in major streams, but can havea tremendous impact on land productivity.A slight decrease in depth of a shallowsoil will decrease yields far more thana slight increase in the depth of a deepsoil will increase yields. Under mostcircumstances, the consequences of anappreciable redistibution within a water-shed will he a productivity decrease.The average annual rate of soilloss that can occur without greatly de-creasing productivity is called the soilloss tolerance, or T value. T valuesfor each of the soils studied are shownin Table 2. In most cases erosion rateswere below T. Watersheds 1, 5, and 6exceeded T at least once; the 20 tons/acre loss for the Hazelair soil (water-shed 1) was 10 times the T value forthat soil.Was the four-year study long enoughto indicate what happens over the longrun, say 20 or 30 years? We know thaterosion is not a uniform process and thatlarge erosion losses occur from a fewstorms. However, the climatic conditions(rainfall amount and intensity, tempera-ture, etc.) during the study were fairlyclose to the long-term averages for thisarea. Also, the cesium tracer methodverified the erosion plot and watershedresults. We, therefore, have confidencein the measured rates of erosion.What About Other Soils and Crops? These measurements show the range ofexpected erosion losses on certain soilsof the hilly uplands of the WillametteValley. Obviously, ten watersheds andsix sets of erosion plots do not repre-sent the wide range of soils and croppingpractices found in western Oregon. Therates of erosion measured using erosionplots, H-flumes, and a cesium tracer werefairly consistent. Because of many fac-tors, including the natural variabilityof the soil and local climatic differ-ences, we must be careful not to extrapo-late the results of this study to othersoil situations. The information pre-sented here is applicable to the condi-e measurements and shouldonservation decision-makingThe Oregon State University Extension Service pro-vides education and information based on timelyresearch to help Oregonians solve problems anddevelop skills related to youth, family, commun-ity, farm, forest, energy, and marine resources.Extension's agr culture program provides educa-tion, training, and technical assistance to peoplewith agriculturally related needs and interests.Major program emphasis is on food and fiber produc-tion, farm business management, marketing andprocessing of agricultural products, and resourceuse and conservation.This publication: was prepared by Jack Istok,biotechnician, and James A. Vomocil, extensionspecialist in soil science, Department of SoilScience, Oregon State University, Corvallis.Publication was made possible by a grant fromExtension Service, USDA, and contract funds fromthe Soil Conservation Service, USDA, for a specialprogram to extend research results developed by aproject entitled "Solutions to Environmental andEconomic Problems (STEEP)." STEEP is a specialresearch program conducted by USDA-ARS and theAgricultural Experiment Stations of Idaho, Oregon,and Washington.Extension Service, Oregon State University,Corvallis, O. E. Smith, Director. This publica-tion was produced and distributed in furtheranceof the Acts of Congress of May 8 and June 30,1914. Extension work is a cooperative program ofOregon State University, the U.S. Department ofAgriculture, and Oregon counties.Oregon State University Extension Service offerseducational programs, activities, and materialswithout regard to race, color, national origin, orsex as required by Title VI of the Civil RightsAct of 1964 and Title TX of the Education Amend-ments of 1972. Oregon State University ExtensionService is an Equal Opportunity Employer.tions of thhelp guide cin this area.

Some Willamette hillslope erosion rates

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Some Willamette hillslope erosion rates

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