Published February 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

;IMPERVIOUS CLAY LAYER .SR 733 / March 1985"7411-',„PINFORMATION FOR LEADERS IN LAND MANAGResearch and Extension in land management titi'oconservation of soil and water.olo fOr farm profits andDrainage f or Erosion Control in Western OregonTHE PROBLEMLand of the margins of the valley isused for small grains, pasture, hay, andgrass grown for seed, and increasingly,used for fruit and horticultural crops.Many of the soils contain a silty-clay orclay layer at a depth of 2 to 3 feet. Ifthe underlying layers have a high claycontent they can restrict the percolationof rainwater, resulting in "perched" wa-ter tables. We now know that watertables at the soil surface lead to in-creased rates of runoff and erosion.Tile increases the ability of the soil to absorb rainwater: For the last fiveyears, the Department of Soil. Science atOregon State University has been mea-suring erosion rates in western Oregonwith the help of the federally fundedSTEEP program. Since 1976, STEEP(Solutions to Environmental and EconomicProblems) funds have been available toOregon, Washington, and Idaho through theefforts of wheat growers organizationsand the Associations of ConservationDistricts.The major emphasis of STEEP researchat Oregon State University has been themeasurement of erosion rates on the hillywestern margins of the Willamette Valleyin cooperation with the USDA/SCA and thePolk Soil and Water Conservation Dis-trict. One outcome of this work has beenan understanding of the way high watertables increase runoff and erosion onthese lands. When water tables are high,rain falling on hillslopes moves quicklydownslope as runoff and can cause seriouserosion (Figure 1A). When water tablesare absent and the soil is drier, forexample during the early fall or afterdry spells, rainfall does not become run-off unless the rainfall intensity isgreat; it is stored on the hillside inpuddles and furrows until it infiltrates(Figure 1B).Fig. la. When infiltrating water reaches arelatively impervious layer, the temporary watertable may rise to the soil surface. Then runoffoccurs even when there is no concurrent rainfall.OREGON STATE UNIVERSITY : EXTENSION SERVICE':'IMPERVIOUS CLAY LAYER70-RUNOFF 6.7 INCHES	 RUNOFF 3.9 INCHESEROSION 3.6 TONS/ACRE EROSION 1.2 TONS/ACRESCHLSUFIF21Ea_ WATERTABLE  Fig. lb. A more rapid lowering of the water tablecan prevent saturation induced runoff.Erosion plot measurements show that areasof high water tables (near the soilsurface) can produce significantlygreater amounts of runoff than areas oflow water tables (Figure 2).Fig. 2. Measurements in Polk county havedemonstrated enhanced runoff and erosion associatedwith high (near the soil surface) perched watertables.These observations and measurementssuggest that tile drainage, by control-ling soil water tables, can result indecreased rates of erosion. To testthis idea, data were collected over atwo-year period on paired watersheds.Then one watershed was drained, and themeasurements were repeated for two years.THE RESEARCHElkins Road drainage experiment: Thisexperiment was conducted on the ElkinsRoad Experimental Watershed. The 704acre Elkins Road Watershed contains threesmaller watersheds which will be referredto as Watersheds 1, 2, 3 (1.1, 3.5, and14.8 acres, respectively). The southernPolk County site is on rolling terraintypical of the western margin of the Wil-lamette Valley. Total relief in the wa-tershed (which has an average elevationof 262 feet) is about 33 feet, withslopes ranging from near zero to 15 per-cent. The annual precipitation is 40inches, with approximately 70 percent ofthis total occurring from Novemberthrough March.The principal soil series on thestudy area is Willakenzie silt loam; amoderately deep, well drained deposit ofsilty material overlying either a clayeyburied soil or weathered sandstone.Rainfall, watershed runoff, and sus-pended sediment concentrations were mea-sured continuously during the winter rain-fall seasons from October 1977 to June1981. In August 1979, after two wintersof data collection, tile drains were in-stalled on Watershed 2. Flexible plasticdrain tubing (4 inches in diameter) wasinstalled at a depth of 3 to 4 feet and aspacing of 40 feet (Figure 3).Fig. 3. Perforated plastic drain tube wasinstalled at forty foot spacing covering a 2.3acre sub-watershed.A250.2315-BBEFOREDRAINAGEAFTERDRAWEE10	 20The tubing was installed with a drain	 2 and 3 are shown in Figure 5. Afterplow; grade control was maintained by the 	 two rainfall seasons, Watershed 2 hadlaser-plane method (Figure 4). 12.5 inches of runoff, or an average of6.3 inches/year; Watershed 3 had a totalof 20 inches of runoff, or an averageloss of 10 inches/year (Figure 5A). Forthe two years after drainage, Watershed2 had an additional 2 inches of runoff,or 1 inch/year, and Watershed 3 had 9inches of runoff, or 4.5 inches/year.From this we can conclude that runofffrom Watershed 2 was decreased by about65% relative to Watershed 3 after drain-age. For the two years before drainage,the average sediment yields from Water-sheds 2 and 3 were 4 and 6 tons/acre/year, respectively (Figure 5b).Fig. 4. Appreciable disturbance of the soil abovethe tube occurs with this method. Under the rightsoil conditions, topsoil mixing into the "backfill"to maintain sustained high infiltration potentialis appreciable.A hydrologic technique of comparisonwas performed on the flow records forWatersheds 1, 2, and 3 to estimate theamount of reduction of runoff and sedi-ment yield. This method plots the cumu-lative amounts of runoff for one water-shed against another so the relationshipbetween the two may be seen and plottedas a straight line. Any treatment thatchanges the way one watershed responds isclearly seen in the slope of the line.RESULTSTile drainage reduced erosion: Perchedwater tables were lowered and seepagewas reduced on Watershed 2 after tiledrainage. Plots of cumulative watershedrunoff and sediment yield for WatershedsFig. 5a. Relative water runoff was reduced 65%by drain installation.CUMULATIVE SEDIMENT LOSS WATERSHED 3 (TONS/ACRE)Fig. 5b. Relative erosion was reduced 50% bydrain installation.After drainage, the average sedimentyields were 1.5 tons/acre/year forWatershed 2 and 4.5 tons/acre/year forWatershed 3. This means that sedimentyield from Watershed 2 was decreased byabout 55% relative to Watershed 3 by in-stalling subsurface drains. Similar com-parisons with Watershed 1 gave reductionsof about 50% and 40% for watershed runoffand sediment yield, respectively. Thereductions in runoff and sediment yieldoccurred because the tile drains, by low-ering water tables, gave the soil a lar-ger rainfall absorption and storage capa-city.CONCLUSIONSTiling for erosion control: We mustremember that the results of this exper-iment were measured at only one loca-tion. Additional erosion research iscontinuing to determine if the resultsapply to a wider area than the one stu-died. Other tile spacings and layoutsare being examined at a Columbia Countysite to find the best combination foreconomical erosion control. But based onresults from Polk County, it is apparentthat tile drains should be consideredwhen planning conservation practices forwestern Oregon lands. More complete in-formation is contained in a research pa-per. If you are interested in furtherinformation, write to the author for acopy of the reference listed below (1).LITERATURE(1) Istok, J. D. and G. F. Kling.1983. Effect of subsurface drain-age on runoff and sediment yieldfrom an agricultural watershed inwestern Oregon. J. Hydrol. 65:279-291.The 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 agriculture 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, ExtensionSoils Specialist, Department of Soil Science,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 IX of the Education Amend-ments of 1972. Oregon State University ExtensionService is an Equal Opportunity Employer.

Drainage for erosion control in western Oregon

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Drainage for erosion control in western Oregon

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