PROCESSES OF EPHEMERAL GULLY EROSION Ja.:er Casali 1 , Sean J. Bennett, and Kerrv M. Robinson ABSTRACT The formation of ephemeral gullies can si gnificantly increase soil loss from agricultural lands and severely impac: farm procuctivity. Erosion prediction technology and conservation management techniques would be izreatl. Improved if the contribution from e phemeral gullies could be more accurately quantified. Field research n Nlississippi. U.S.A. and Spain has revealed three categories of ephemeral gullies. Classic ephemeral zullies tornied by concentrated t10 erosion from runoff occurring within the same field. Drainage ephemeral gullies formed by concentrated flow erosion from runoff originating from areas upstream of where :re gully occurs. Discontinuity ephemeral gullies formed in areas where management prac:ices ha e created a sudden chan ge in slope, such as field boundaries adjacent to roads. Despite the larac differences ri climate, watershed size, hvdroloay, and geography, the ephemeral gullies observed in Spain were mor—phologically similar to those in Mississippi. Using an experimental flume, ephemeral gulls erosion proceeded primarily through bed incision, gully widening, and bank steepening, and total sediment load depended upon whether the flow was detachment- or transport-limited. Key Words: Ephemeral gully: Soil loss; Erosion prediction technology; Conservation management; Bed incisions; Total sediment load I INTRODUCTION Ephemeral gullies are small erosional channels on agricultural landscapes caused by the concentration of overland flow typically between two opposing slopes (a hollow), often formed during a single rainfall event. Since the scoured soil volume is not very large within these gullies, farmers can easily refill them. In general, ephemeral gullies can reappear at or near the same location on a yearly basis because the surface topography of the field does not change appreciably. Most ephemeral gullies occur on cultivated fields with highly erodible soils, with little or no crop residue cover, and where crop harvest disturbs the soil (USDA-NRCS, 1997). Ephemeral gully erosion is not accounted for in current soil-loss assessment programs, but its contribution and importance to total soil losses has long been recognized. The USDA-NRCS estimated the ratio of ephemeral gully erosion to nh and sheet erosion, and values range from 21% for New York and 274% for Washington, v.ith an average of 78% for the 19 states surveyed (Table I). In actively eroding areas, ephemeral gullies typically contribute about 30% to the total soil loss, but can reach as high as 100% (Laflen et al., 1985 b; Spomer and Hjelmfelt, 1986; Thomas et at. 1986; Thorne et al., 1986; Grissinger and Murphey, 1989: Lentz et al. 1993; Table 2). Ephemeral gully erosion also impacts the Loess Belt and Mediterranean region of Western Europe (Poesen and Govers, 1990; Table 2). In the U.S.A. and Europe, ephemeral gully erosion is responsible for at least 10% of total soil losses (Poesen et al., 1996). Ephemeral and permanent gully erosion is also a severe problem in Spain (Sala et al., 1991; Sala and Rubio, 1994; Casali e al., 1998), and parts of Spain are at risk of desertification (Poesen, 1995; Rubio, 1995). Accurate assessment of ephemeral gully erosion is limited by a number of factors. At present, few field studies have provided viable data on gully erosion rates, and these studies tend to be restricted in both time and space. Accurate rainfall data are also needed to constrain erosion rates and projected soil losses. Ephemeral gullies can form by a variety of causes. Smith (1993) identified the following critical Parameters for gully development: (1) a critical slope length and slope gradient that is dependent upon Slope characteristics and crop row direction, (2) occurrence and depth of a fragipan, (3) agricultural Department of Projects and Rural Engineering, Public Universit y of Navarra, 31006 Pamplona, Navarra, Spain National Sedimentation Laboratcrv. USDA-ARS, P.O. Box 1157, Oxford, MS 38655, USA Plant Science and Water Conservation Research Laboratory, USDA-ARS, 1301 N. Western St., Stillwater, OK 74075, USA Note: The manucript of this paper was received in march 1999, Discussion open until March 2001. Inter national Journal of Sediment Research, Vol. 15, No. 1, 2000 9 pp. 31-41 -31 - practices, principall y row direction and timin g of cultivation, and (4) timing and total amount of precipitation. Linear features of the landscape like plot borders, lanes, tractor rows, or furrows can also promote ephemeral gully development (Laflen. 1985; Poesen and Govers. 1990; Casali, 1997; Casali et al.. 1998). Moreover, headcut development appears to be an important triggering mechanism for gully erosion, particularly in classic and discontinuity gullies (see below: Smith, 1993: Casali et al.. 1998). Table I Assessment ofephemeral g ull erosion rates in selected areas of the U.S.A. (from USD.A-NRCS. 997) Estimated Annual Measured Ephemeral Ephemeral Gully Erosion Location Sheet and Rill Erosion Gulk Erosion as a Percentage (%) of (k ,-,!m -V) (ko!ni-v) Sheet and Rill Erosion Alabama 0.573 0.342 60 Delaware 0.038 0.093 245 Illinois 0.261 0. (91 73 Iowa 0.353 0.110 3 Kansas 0.807 0.294 36 Louisiana 0.65. 0.222 34 Maine 0.41 0.189 46 Maryland 0.195 0.147 75 Michigan 0.172 0.045 26 Mississippi 0.646 0.275 43 New Jersey 0.246 0.191 78 New York 0.873 0.185 21 North Dakota 0.277 0.130 47 Pennsylvania 0.093 0.065 70 Rhode Island 0.331 0.136 41 Vermont 0.165 0.224 136 Virginia 0.477 0.470 98 Washington 0.025 0.069 274 Wisconsin 0.289 0.154 53 There is no unique solution to prevent or mitigate ephemeral gully formation and erosion. Reduced tillage is an excellent soil conservation practice that can reduce gully erosion (Laflen et al., 1985a; Spomer and Hjelmfelt, 1986; Dc Ploe y, 1988), but more specific measures such as retention structures, diversions, waterways, terraces, and underground outlets are often required (Foster, 1986; USDA-NRCS, 1997). Vegetation management programs appear to be well suited for controlling ephemeral gully erosion. Smith (1993) noted that residue often helped reduce erosion by clogging rows and gullies and actually inducing deposition. The combination of reduced tillage and vegetation barriers such as stiff grass hedges constructed within gullies have decreased the headward advance of gullies in cultivated fields of northern Mississippi (Dabney et al., 1997). Contoured grass buffer strips and strip-cropping offer additional alternatives to mitigate ephemeral gully erosion (USDA-NRCS, 1997). Table 2. Summary of relevant data concerning ephemeral gully erosion. Methods used to measure gullies include: simple volumetric measurements with profilers and tapes, conventional photography, aerial photogrametrv, and digital terrain models. All studies in U.S.A. except (2) and (6) use Universal Soil Loss Equation to estimate rill and interrill erosion. Data ranges are given in parentheses. Sources of data, soil types, and land management: (1) Miller (1982), soil hydrologic Group A and B; (2) Spomer and Hjelmfelt (1986), loess, conventional till: (3) Laflen (1985), loess and glacial till; (4) Thomas et al. (1986), Thomas and Welch (1988), sandy loam, soybeans, conventional till; (5) Gnissin ger and Murphey (1989). loess, soybean, conventional till; (6) Lentz et al. (1993). loess or g lacial-till- loess, corn and soybeans, conservation till; (7) Smith (1993), boessial silt barns with fragipan. soybeans or corn, conventional till; (8) Moore et al. (1988), bare, salodic loam; (9) Auzet et al. (1993) variable crops and managements; (10) Vandacle (1993), loess, silty loam, variable crops; (11) Vandaele and Poesen (1995) loess. silty loam, variable crops; (12) Poesen et al. (1996), sandy loam, 20-50% rock fragments, inactive; (13) Vandacle et al. (1996) a - loess, silty loam, variable crops; b - lithosol. >30% rocks, winter wheat and barley; (14) Casali et al. (1998) loam or silt-loam, winter grains, conventional till; (15) Hidal go et al (1998), clayey soil. -32- International Journal of Sediment Research, Vol. 15, No. 1. 2000, pp. 31-41 Table 2 Summary of relavant data concerning ephemeral gully erosion Slope (%) Drainatze Soil loss. Total Soil Loss. % of total Watershed Ephemeral rill/sheet soil loss Source Location (W) or gully Area (ha) Gullies Soil loss erosion due to ephemeral (G) (W: G) (ka/m1-v) (k,-,/m -Y) (k/m gullies United States (I) Alabama na. na. 0.80, 1.45 1.34, 2.90 0.54, 1.45 60, 50 (2) Iowa 4.0-14.0 (W) 24.3 W) 1.70, 068 8.9, 0.62 7.20, 0 19, 100 (3) Iowa 2.0-11.0 (W) I (W) 0.19-0.73 0.97-3.75 078-3.02 9-20 (4) Georaia 4.5 (G) 5.3 (W) 4.00. 5.06 10.73 6.20 42 2.0 (G) (5) rvltssss;ppi na. l.9(\) 1.47 2.45 098 60 (6) Minnesota 3.4-6.1 (W) 7.4 (W) 0.30 na. na. na. (0.15-0.54) (7) Mississippi 0.8-2.0 (G) 2.2 (G) 1.68 6.04 4.36 36 (1.2 1-2.02) (4.51-10.3) (0.90- (16-67) 8.74) .-1 ustrulia (8) Australia 12.5 (W) 7.5 (W) 1.30 na. n.a. n.a. Europe (9) France 1.9-7.9 (W) Ca. 650 (W) 0.20 0.29 0.09 72 cu. 410 (G) (0.09-0.70) (0.05-0.93) (0-0.25) (36-100) (10) Belgium gentle 170.0 (W) 0.21-0.35 0.56-0.82 0.35-0.50 37-39 (II) Belgium gentle 25.0 (W) Ca. 0.40 0.85 0.41 52 (12) Spain 3.0-25.0(W) 10.0(W) 1.26 1.52 0.26 80 (I 3a) Belgium gentle 4,000.0(W) 0.15-1.32 0.52-1.90 0.36-0.58 30-69 (I 3b) Portugal gentle 550.0 (W) 0.10-0.68 0.12-0.80 0.02-0.13 83-84 (14) Spain 0.5-9.5 (G) 88.0 (W) 0.87 n.a.