RANGELANDS 11(2), April 1989 51 Gullies and Sediment Yield Herbert B. Osborn and J. Roger Simanton Gullyerosion can be a major contributor to rangeland majorchannels as headcuts have moved"upstream" from watershed sedimentyield and stock pond or small reser- the San Pedro River. Second, there is evidence of local- voir sedimentation.There is considerableliterature on ized (small watershed) gullying independentof the gen- qualitativegully development and growth,but little quan- eral downcutting on the watershed. The downcutting of titative information on the extent of gully contribution to the San Pedro River has been blamed primarily on over- sedimentyield and subsequent downstream sedimenta- grazing, climatic change, or a combination of the two tion. The principlesof gully erosion, as defined asearly as factors. Localized gullying is probablydue to a combina- 1939for the Piedmontregion of South Carolina,are also tion of overgrazing, concentrationof flow on trails and pertinentto southwestern rangelands. stock paths, failure of man-made structures,and climatic In southwesternrangelands, acceleratedgully erosion change. This discussion is limited to localized gully appears to be the result of road and trail development, downcutting on four small subwatersheds on Walnut cattle paths, overgrazing andclimatic change (Cookeand Gulch. The gully contribution to watershed total sedi- Reeves 1976). Gullying has occurred in two ways on the mentyield is estimated by proportioningmeasured sedi- Walnut Gulch Experimental Watershed in southeastern mentyield into upland, tributary, andgully sediment sources. Arizona (Fig. 1). First, there has been downcutting in the Study Area Description Authorsare researchhydraulic engineer and hydrologist,respectively, Arid- land WatershedManagement Research Unit, 2000 East Allen Road, Tucson, The 58-mi2 Walnut Gulch Experimental Watershed, in Arizona 85719. southeastern Arizona, is millions of Editor's Note: Readers maywish to see thefollowing articles: representative of "Arroyo Formation,Juab County, Utah,1983" byJames L. Baer,Range/ends acresof brushand grass rangeland found throughout the 7(6):245-248, 1988 between the "Gully Erosion" by ArthurBettis Ill and Dean M. Thompson Range/ends semiarid Southwest, and is a transitionzone 7(2):70-72,1985 Chihuahuanand Sonoran Deserts. annual "Managing RangelandSoil Resources:The Universal Soil Loss Equation"by Average pre- KG. Renardand G.R. Foster, Range/ends 7(3):118-122, 1985. cipitation on the watershed is about 12 inches, with 70% STUDY AREA — - 4 5 SCALE IN MILES WATERSHED BOUNDARY -'-'--—— MAIN CHANNELS LOCATION OF 4z Fig. 1. Location of Walnut Gulch studyarea. U) 52 RANGELANDS11(2), April 1989 occurring during the summer thunderstorm season of Julyto mid-September. This is also theseason when99% of annual runoff occurs. Upland soilson the four small subwatersheds are generally well drained, calcareous, gravelly barns (Rillito Series) with large percentages (>50%) of rock and gravel on the soil surface. Soil in which gullies have formed is a sandy loam (Laveen Ser- ies) with upto 15%fine gravel on the surface.Watershed sideslopes rangefrom 3 to 15%, with abouta 9% average. FL Cattle grazingwas discontinuedon the subwatersheds in 1962,and no revegetation or rangeland renovationtreat- ments were made until 1980. Vegetationcanopy cover is about 30%, and major species include: creosote bush, white-thorn, tarbush, snakeweed,burroweed, blackgrama, blue grama, sideoatsgrama, and bush muhly. Instrumentation In 1963, four small rangeland watersheds, within a 20- acrearea on WalnutGulch, were instrumentedfor hydro- logic studies (Fig. 2). Three weighing-type recording raingages were established, and broadcasted V-notch weirs wereconstructed to measure runoff (Fig.5). Water- shed 101 isa tributary of 103, and 102 is atributary of 104 (Fig. 2). Watershed 103 isdrained by a relatively straight, Incised gully, while watershed 104, adjacent to 103, is FIg. 3. Gully between WalnutGulch 101 and 103 runoff-measuringstations. RANGELANDS11(2), AprIl 1989 53 drainedby a meandering channelwith more gently sloping banks. The V- notch weirfor 101 was installedabout 1.5 ft above the existing channel bot- tom. This silting basinabove the weir allowed "clear water" discharge over the weir, which may have inducedthe active downcutting in the already in- cised gully between 101 and 103 (Fig. 3). A similar, but smaller,silting basin above 102 did not appreciablyaccel- erate downcutting between 102 and 104 (Fig. 4). Rainfall, runoff, and total sediment yield were available for 103 and 104 from 1973 through 1984. In the first few years, suspended sediment was sampled with pump samplers and bed q, Channel betweenWalnut Gulch 102 and 104 stations. load was measured as trapped sedi- FIg. runoff-measuring ment in the silting basins above the V-notchweirs (Fig. 5). Beforethe 1976 runoff season, the V-noteweirs at 103 and 104were replaced with moreaccu- ratesupercritical flumes equipped with total load samplers (Fig. 6), and 13 permanent crosssections were estab- lished in the gully between 101 and 103 (Fig.7). Precise gully cross-section measurements were made at each of the 13 permanentcross sections be- fore the beginning (late spring) and afterthe end (early fall) of each runoff season. Analyses Analyses were based on compari- son of sedimentyield estimates from the Universal Soil Loss FIg. 5. Sediment trap above Walnut Gulch runoff-measuring StdtiOtl Ii)4 (lOuNifig Equation downstream). (USLE),the measured total sediment yield, and gully contributions based on measuredchanges atchannel cross- sections(extrapolated to estimate vol- umechanges between crosssections). USLE The USLE was developed to esti- mate the long-term average soil loss from agricultural fields (Wischmeier and Smith 1978). Although the USLE parameters have been developed pri- marilyfor cultivatedagricultural areas, the physical conditions associated with the parameters, along with limit- ed testing, have permitted extrapola- tion for use in the sparse vegetation conditionsof western rangelands. The FIg. 6. Flumeand totalload sampler atrunoff measuringstation 103, Walnut Gulch. 54 RANGELANDS11(2), April 1989 Weused the USLE to estimatethe sed- 0 iment contributed from the uplands. We assumed measurements of transported the — 1964 sedimentat flumesequaled total sed- 1980 imentyield from thewatersheds. We also 1.5 — — 1976 assumed that P = 1 for ungrazed, un- treated rangeland, that C included rock I- -J fragments as part of ground cover, and La U. that thesoil erodibility nomographdeve- I-I loped for the USLE gave representative La values for K. Valuesfor each of the fac- a 4.5 tors were estimated using field data and Agricultural Handbook No. 537 proce- A was 9. 6.0 duresand tables. The factor calcu- SECTION 3 lated using the nearest recording rain- 8 3.0 60 9.0 20 gage record. Althoughdata were available from 1973 0 a 7. through 1984, portion ofwatershed 104 — 1994 was renovated (converted from brush to 980 — — 976 grass) in a separate study, before the '.5 1981 runoff season. There was good Ui the La correlation between paired water- sheds of annual measured sediment 3.0 and Q. yields between 103 104 from 1973 aUJ through 1980 (Table1 and Fig. 8). Com- parisons of sediment yield before and 4.5 SECTION 6 after renovationsuggested an increase 0 3.0 6.0 0 20 in sediment after treatment on 104 (Fig. 8). 0 — 1984 Erosion 980 Gully — 976 I- Analyses of gullycross-section data of Ui ILL 103 indicatedonly minor changes in the I gully bottom gradient between 1976 and I- 1979. However, increases in gully cross- 3.0 FLUME 103 SECTION I sectional areasindicated that gully banks were lostand towater- 0 3.0 6.0 9.0 '2.0 being contributing WIDTH (FEET) shed sedimentyield. From 1980 through 1984,the moresteeply banked crosssec- to FIg. 7. Location ofcross sections and selected cross sectionsabove flume103. tions on 103 continued to contribute sediment yield by banking sloughing, whilethe shallowersloped bankson the lowercross sections were becoming more stable, and gullies were aggrading (Fig. is: equation 7). A = RKLSCP The greatest gully bank soil losses where A 1 estimated soil loss (tons/acre/year), between 101 and 103, from 1976through A = rainfall erosivity factor (El units/year), 1984,were from the uppermost reaches K = soil erodibilityfactor (tons/acre/El unit), of the gully (Table2). From 1976through LS = slope-length gradient factor, 1979, channel cross-section surveys indi- C = cover and managementfactor, and cated the lower reach of the gully con- P = erosion controlpractice factor. tributed very little to watershed sediment From 1980 through 1984, channel These factors reflect the variableswhich influenceerosion by yield. major aggradationin the lower reach actually rainfall and resultant overlandflow. The equation is based on plot data the factor exceeded channelbank contribution. collected mainly in the eastern United States. Because equation On the bank contribu- in different climatic considerations are upper reach, gully relationshipsvary areas, special tions unit runoff were extend USLE western United States et al. per approximately requiredto the to the (Simanton constant over the 9 of record.The 1980). years RANGELANDS11(2), April 1989 55 Table 1. Annualsediment yield (tons/acre)for twosmall watersheds, 103 and 104, on the Walnut Gulch ExperimentalWatershed. '750 103 104 (9.2 acres) (11 .1 acres) Year Est. Meas. Est.* Meas. U) 1973 0.29 1.24 0.25 0.35 U, 1974 .36 2.17 .30 .75 0-J 1975 .85 3.83 .72 1.42 -J 1976 .14 .12 .31 0 Y0.35+2.28X(WITHOUT 1981) 1.08 U, 1977 .37 3.04 .32 1.33 1978 .21 .17 44Ui .89 .08 1979 .11 z-.. .21 .10 0 4W 1980 .12 .25 .10 .10 I)Qcr 24 Average 0.31 1.59 0.26 0.54 0(I) *From USLE Ui Table2. Cross sectionalestimates of gully contribution tosedIment I— 4 yield betweenwelr 101 andflume 103, Walnut Gulch. 01'S' 1976 Dist. 1976- 1980- 1976 Cross X sectional between 1979 1984 1984 Section area sections vol vol vol (ft2) (fi) (ft3) (ft3) (ft3) Flume I 2 103 WATERSHED 04 ANNUAL SOIL LOSS 1 24.79 37 + 50.4 +106.9 +157.3 (TONS/ACRE/YEAR) 2 29.07 46 - 3.1 + 86.7 + 83.6 3 21.41 37 - 17.2 + 56.2 + 39.0 FIg.