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o. Reprinted from the _ Proceedings of the Sympl)sium on WATERSHED MANAGEMENT '80 ASCEIBoise./D/Ju/y 2/-23. /980 ESTIMATING EROSION 165 impact and shear of surface runoff this affords, grazing and trampling by cattle, and the importance of channel erosion processes. 1n this paper, some'sediment yield formulae are tested against sediment accumu- lation data ,from nine small watersheds in the Walnut Gulch Experimental Watershed. ESTiMATiNG EROSION AND SEDiMENT YiELD FROM RAN6ELAND cOMMON TECHNiqUES FOR ESTiMATING SEDIMENT YIELD By Kenneth G. Renardi, M. ASCE i NTRODUCTl ON Sediment yield estimating techniques vary appreciably in their complexity depending in a large part upon the objective of the investi- gation and the availabi1ity of data. The methods commonly used in esti- mating sediment yield and some comments regarding problems of using each are: 1. The sediment rating-curve/flow duration method. This method is highly;dependent upon the accuracy of sediment concentration meas'urements at field locations. Meaningfu'l dat.a are difficult to obtain for this method on small watersheds because discharge~ of water and. sediment vary rapidly and there is not enough time to sample accurately without sophisticated permanent sampling equipment. 2. The sediment delivery ratio method. Sediment delivery ratio is a percentage relationship between sediment yield and gross ero- sion in a watershed. Sediment del ivery ratios have been deter- mi ned for ,many areas of the country and have been found 'to be related .to the size of the drai nage area. The method requires estimation of the erosion from all sources in the watershed~ a value difficult to obtain. 3. Reservoir sediment deposition surveysw Rate of accumulation of sediment in a reservoir can be detennined when the time period between consecutive surveys is knownw Sediment yield is esti- mated by ,adding the estimated amount of sediment that' passed through the reservoir, based on the reservoir's trap efficiency. These estimates can be used to relate sediment yield to drainage area. The approach provides good information on magnitude and variation-of average annual sediment yield ~ithin a phYSiograph- ic region, but has little value for forecasting sediment yield over a short time. 4. Field measurements of erosion and deposition. The difference between erosion and deposition estimates or measurements can be used to estimate sediment yield. The uncertainty of both the erosion and deposition measurements can lead to large error. 5. Bedload relationships. The ~oarse fraction of sediment yield can be estimated using bedload relaHonships. Most of these relationships ."ere developed primarily from laboratory flume studies, and they often give widely differing estimates for the same set .of field conditions. Furthenoore,. separate estimates must be made to include the fine sediments in transportw 6. Mathematical silllJlation node1s. Such models us'e relationships for the processes of soil detachment, transport and deposition. These relationships are incorporated into a hydrologic rodel to estimate sediment yieldw This method, widely used in research, will undoubtedly be an illlJortant method in the future. However, there are presently many limitations concerning parameter defi- nition, and the method is expensive because of data requirements and computer costs. Sediment yields in many rangeland areas of the. western- United States are larger tha'n might be expected with the low rainfall' generally characteristic of the area. These high yields result from: (1) the general low veget~l density inadequately pre. .:!cts the soil against the erosion forces of raindrops and runoff.; (2) 'land slopes are often steep and infiltration is generally low, which results in -high shear from the water moVing over the land surface; (3) high intensity thunderstorms and their associated high kinetic energy are relatively C01llllOn, which leads to excessive splash erosion and overlarKf runoff; and {4} channel slopes are generally steep and contain 1ar!je amounts of alluvium for transport in the runoff. . The problems ass'ociate~ with ,erosion/sedlment, yield for downstream areas are well documented. The prOblems are important rot only because of deposition in reservoirs <.J.oss of storage) and channels (decreased conveyance capacity)' bU,t .also because the sedf.ments often contain appre- ciable quantities of adsorbed chemicals, which can severely degrade water quality. Recent water quality legis,lation and the mandates to correct pollution problems in the .waters of the United States have created a new emphasis ~n this problem with water quality. Another prOblem ;s that erosion in upland areas often reduces soil productiVity. In western rangeland areas, the problem is often acute because the soil 'profile in such areas ;s already insufficient for ade- quat~ forage production. Soil erosion reduc.es 'a soil's potential for production of forage by: (i) loss of soil water storage capacity; {21 decreased infiltration rate (surface sealing) and increased opportunity for runoff and evaporation; (3) loss of soil nutrients; and to a lesser extent (4) increased weed production; (5) reduced seed germination; and (6) decreased root development. Methods for estimating erosion and sediment yield from rangelands are based pri,.arily upon the principles developed in parts of the U.S., where cultivated agricultural activities are prevalentw Techniques in corporating disturbance of the soil by tillage are not generally appli- cable to rangelands, so the erosion estimating techniques RlIst be appro- priately adjusted to reflect such differe,nces. Typical problems unique to rangelands are those associated with the different soils of ran<jl!, the existence of erosion pavements and the protection from raindrop IThe author is a hydrauliC engineer, U.S. Department of Agriculture, Science and Education Administration, Agricultural Research (USOA-SEA- AR), SQlLthwest Rangeland Watershed Research Center, Tucson, Arizona B5705. 2908 164 , SrI.