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PROCESSES OF EPHEMERAL

Ja.:er Casali 1 , Sean J. Bennett, and Kerrv M. Robinson

ABSTRACT The formation of ephemeral 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 steepening, and total 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 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 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 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 . 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. n.a. n.a. Ca. 5.0 (G) (0.16-2.66) (IS) Spain n.a. 4.9 (W) 6.49 Ca. 8.83t n.a. ca. 74 4.9 (G) na.: not available t Minimum value, rill and sheet erosion not considered

Despite the significance of ephemeral gully erosion, little data exist on rates of soil losses, physical characteristics of gully systems, and criteria for gully formation. Below we present the main morphologic characteristics of ephemeral gullies from Southern Navarra, Spain (Casali et al., 1998) and from central Mississippi, U.S.A. (Smith. 1993), and describe recent experimental results detailing erosion processes within pre-formed swales. 2 EPHEMERAL GULLIES IN SOUTHERN NAVARRA, SPAIN Field measurements were conducted in Southern Navarra from October 1995 to September 1996 to understand further the characteristics and erosion processes of ephemeral gullies (Casali, 1997). The study site was located 50 km south of Pamplona, and comprised 88 ha of agricultural fields within a 210 ha watershed. The slope of the cultivated fields ranged from I to 14%. The climate is continental Mediterranean, with a mean temperature of 13°C and an annual rainfall about 500 mm (Elias and Ruiz. 1986). The main crop was winter cereal, wheat or barley, sown at the beginning of October. The parent material of the soil was a mixture of clay and barns from Miocene-age sandstones. Soil texture was generally loamy, although silt-loam soils were found in the low-land areas. Soil structure was platy, which reduced rate, and crust (seal) formation was common. These soils can be considered highly erodible (Donézar et al., 1990). Ephemeral gullies were examined during one growing season. During this time, detailed topographic surveys of all gullies, and rates of erosion, growth, and development were documented. Gully cross-sections were measured with a mechanical profiler Consisting of a 1-rn wide frame that held 50 pins. Each field measurement was photographed and later digitized. A more detailed description and discussion of these results can be found in Casali et al. (1998).

International Journal of Sediment Research, Vol. 15, No. 1, 2000, pp. 31-41 -33 - Three distinct types of ephemeral gullies were observed within this region in Spain and these are referred to as classic ephemeral gullies, draina ge ephemeral gullies, and discontinuity ephemeral gullies. Classic ephemeral gullies formed by concentrated (low erosion from runoff occurring within the same tield. These trapezoidal gullies formed on 6% slopes, and had top widths oO.5 m, bottom idths of 0.1 m, depths ofO.2 m, len g ths of43 rn. and width-to-depth ratios (W/D) o4 (Table 3)

rahk 3 Main characteristics olephemeral gullies from studies in Southern Navarra, Spain based on one year oF data Casali et al.. 1998) and Mississipni. U.S.A. based on three years of data (Smith, 1993). Mean values are shown, and minimum and maximum values are ziven in parentheses

Ephemeral UI L111V characteristic Southern Navarra. Spain Mississippi, U S A. Ephemeral gulk t.pe Classic Drainaee Discontinuity Classic Numbcr of sites 4 Number of gullies examined 4 5 Near-surface soil texture loam- loam- loam- Silt loam silt loam silt loam silt loam Mean annual rainfall mm) 50)) 500 50)) 1.400 Gulls watershed area (10 m-) 7.9 na. na. 1.9 (2.6-17.4) Total gully watershed area 10 m) 39.4 na. na. 21.5 Gully slope (%) 6.3 3.9 3.7 1.3 (3.0-9.5) (2.9-5.0) (0.5-9.5 Top width (m) 0.53 0.73 0,76 0.7! (0.34-0.77) (0.63-0.90) (0.67-0.90) (0.5 1-0.81) Bottom width (m) 0.14 0.19 0.28 0.12 (0.08-0.20) (0.08-0.35) (0.18-0.36) (0.07-0.17) Depth (m) 0.16 0.19 0.25 0.14 (0.11-0.22) (0.14-0.29) (0-14-0.37) (0.06-0.25) Cross-sectional area (m) 0.08 0.08 0.11 0.06 (0.05-0.11) (0.05-0.10) (0.04-0.18) (0.02-0.11) Width-to-depth ratio 4.0 5.1 3.6 7.7 (2.0-5.9) (4.1-6.2) (2.2-6.1) (2.5-14.5) Bank angle (degrees) 47 53 42 61 (25-68) (40-69) (34-6!) (44-74) Gully length (m) 43 107 20 30 (18-80) (52-240) (5-43) (10-57) Eroded volume per gully (m3) 3.18 7.56 2.46 1.73 (0.83-5.50) (2.43-14.73) (0.66-7.89) (0.19-3.73) Total eroded volume (mi) 0.4 30.2 12.3 6.2 (4.1-9.1) Soil toss per unit gully length (kglm) 117 117 171 77 (63-174) (72-154) (101-283) (48-102) Soil loss rate (kg/m-y) 0.87 Ca. 2.60 n.a. 1.68 (0.16-1.74) (1.21-2.02) Total soil loss due to ephemeral na. na. na. izullies (°/) using USLE 36 (16-67) na: not available

Typically a headeut or formed, and this area of local scour probabl y enlarged and deepened tormin g a gully. Five small watersheds covering 3 . 94 ha were affected by classic ephemeral gully ero s ion during the study period. Total volume of soil eroded by classic gullies was 10.4 m 3 . Although cross-sections were trapezoidal, maximum cross-sectional area tended to occur in the central part of the gull. This may be due to the persistence of detachment-limited flows in the upstream reaches compared to transport-limited flows in the downstream reaches. Drainage ephemeral gullies formed by concentrated flow erosion from runoff ori g inating from areas upstream of where the gully occurs. Typically, gully incision and development began at the upper boundary of cultivated plot downstream of the runoff source region. These trapezoidal gullies generally -34- International Journal of Sediment Research, Vol. 15, No. 1, 2000, pp. 31-41 formed on 4% slopes, and had top widths of 0.7 m. bottom widths of 0.2 m, depths of 0.2 m, lengths of 107 m. and W/D values of 5 (Table 3). Four izullies ol this class were monitored during the study period. Drainage gullies were the most active gully type in the field area, eroding 30.2 m ofsoil material. These gullies were trapezoidal in shape, but typically had greater cross-sectional areas at the most upstream section. and gully area decreased downslope. Discontinuity ephemeral gullies were found in areas where management practices created a sudden chan ge in slope, such as field boundaries adjacent to roads. Ihese trapezoidal gullies generally formed on 4% slopes, and had top widths of 0.8 m. bottom widths of 0.3 m. depths of0.3 m, lengths of 20 m, and W/D values o14 (Table 3) These gullies were generall y triggered by headcuts related to small piping lubes or seepae zones across the discontinuity (Gutirrez et al.. 19% Del Valle de [.ersundi and Del Val, 1990). Discontinuity gullies accounted for 12.3 in nt soil erosion, were trapezoidal. and showed little ari:ition in cross-sectional area along their length. Oascd on field obscratiuns in this re g ion of Southern Navarra. most gullies were formed or were i g nitieantiv reactivated during one intense winter rainfall of short duration. Althou gh near normal rainfall was ohsers ed durin g the studs period (450 mm/v). only one rainfall event \•vas associated with g ull erosion: .lanuary 22. 906 where peak rainfall rate was 54 mm h. At that time, elevated soil moisture contents facilitated runoU, and hare soil surfaces eroded most dramaticall y . Such observations support the use of a rainstorm intensity and duration index for gully erosion modeling. This also hi ghli ghts the importance of recording hi gh-intensit y rainfall data in areas prone to gully formation. All gullies examined had W/D values between 3 and 6 (Table 3). In central Bel g ium. Poesen and Govers (1990) observed that intense rains of short duration favor the formation of gullies with W/D > 1. while frequent long rains of small intensity produce narrower gullies with W/D < I . In general, broader gullies cause greater environmental degradation because losses in productive topsoil are the greatest and these soils tend to be richer in fertilizers and pesticides. Results presented here are consistent with the proposal of Poesen and Govers (1990). Classic ephemeral gullies had smaller W/D values in intermediate positions, whereas drainage ephemeral gullies had minimum W/D in upstream sections. Maximum soil erosion occurred when W/D was a minimum, illustratin g that bed incision and flow concentration were the critical parameters for high erosion rates. In order to characterize the susceptibility of a watershed to classical ephemeral gullies, a simple topographic index based on the area-weighted mean slope of the region, AS, has been defined by Casali et al. (1998):

A,•S1

AS= A" (I) where the slope of every tract with uniform characteristics S, is weighted by its respective area A, and multiplied by the total area A. Thorne etal. (1986) established a similar topographic index for identifying landscapes susceptible to ephemeral gully development. Figure 1 demonstrates the application of this index in Southern Navarra where greater soil losses are associated with higher values of AS.

3 EPHEMERAL GULLIES IN MISSISSIPPI, U.S.A. Smith (1993) conducted a detailed investigation of ephemeral gullies at four sites in central Mississippi (U.S.A.) near Jackson during October 1983 to April 1985. Climate is subtropical with a mean annual rainfall of 1400 mm. During the study period, annual rainfall rate ranged between 1590 and 1640 mm. Soils were loessial silt-barns (Memphis, Loring. and Providence series), were cultivated with soybeans or corn, and winter cover was grass and wild peas. A fragipan layer was present at most gully locations at an average depth of about 0.35 m. At each site location, Smith (1993) obtained gully cross-sections. and

International Journal of Sediment Research, Vol. 15, No. 1, -000,1 pp. 3 1-41 -35.

final yearly measurements were made in May just before plowing. These final yearly data are the focus of the discussion herein.

6L I A? - L

T - L • LaAbejera 1, 1995-96 -2 3 L v Cobaza 1. 1995-96 = L U La Matea 1, 1995-96 2 • La Matea II, 1995-96 A La Abejera 0, 1995-96 ...- •

01 I 1 200 400 600 800 1000 1200 1400 AS Fig. 1 Relationship between total soil losses and index AS for the period October 1995-September 1996 for five different gullied watersheds in Southern Navarra (Spain). Soil losses at La Abejera 0 watershed were estimated based on measurements, daily rainfall data, and management practices (from Casali et al., 1998).

Flow Control Valve

Section A-A Compacted Cohesive Soil

Fig. 2 Large-scale outdoor flume at the USDA-ARS location in Stillwater, OK showing flow system, inlet region, and pre-formed swales.

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fl All gullies described by Smith (1993) are considered classic ephemeral gullies. These trapezoidal gullies generall y formed on 1% slopes, and had top widths of 0.7 m, bottom widths of 0.1 m. depths of 0.4 m, lengths of 30 m, and W/D values of 8 (Table3). Many eullies initiated one-third to one-half theay d own the slope, and then extended and grew in both directions. In general, maximum ephemeral gully erosion correlated with maximum precipitation events. Gully erosion was divided into two phases during the year (Smith, 1993). Following May plowing, rainfall in June promoted gully erosion prior to the development of crop canopy. From July to October, canopy cover and crop growth allowed the gullies to refill and anneal. After the November harvest, gullies were most vulnerable and could reactivate. From November to April, maximum gully erosion and extension occurred, coinciding with the hi ghest and most frequent precipitation of the year. Fifteen relatively small watersheds covering 2.2 ha were affected by ephemeral gully erosion and total volume of soil eroded avera ged 6.2

0.24

0.24

0.00Section 2 10.7 in

0.24

0.08 0.00 Th-7SectionT 3; 13.7mI C 0.24

0.00 Section 4; I6I rn 0.24

Original : After i.i8hj 0.00: - -After 2.07 h Section 5; 18.3I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Distance from a datum (m)

Fig. 3 Time variation in ephemeral gully morphology for the 6/1/98 experiment. The position of each cross-section is given as distance downstream.

4EROSION PROCESSES WITHIN EPHEMERAL GULLIES To examine the erosion processes within ephemeral gullies, the authors recently conducted a series of Oxpe riments using a 29 m long, 1.8 m wide, and 2.4 m deep horizontal test flume located at the USDA- ARS location in Stillwater, Oklahoma, U.S.A. (Fig. 2). A red clay soil (CL) with a moisture content of Inter national Journal of Sediment Research, Vol. 15, No. 1, 2000, pp. 31-41 -37- 9.1% was incrementally packed into the flume to a dry density of 1.7 Mg/M3 and a bed slope of 1%. Using a purpose-built tiller, two channels were cut into the packed soil bed. Each trapezoidal was approximately 0.6 m wide at the top, 0.1 m wide at the bottom, and 0.125 m deep with W1 4.8 and 2:1 sideslopes (27). These dimensions were chosen to replicate closely the physical characteristics of ephemeral gullies described in Table 3. Clear-water flows were measured with an orifice meter and an air-water differential manometer. Test flow durations were typically 2 h. The time variation of channel erosion was closely monitored; morphologic and sediment load data were collected at five cross sections, and bed profiles were measured along the channel centerline at the start and end of each experiment.

6/2/98 0.00 Section 1; 9.1 m

0.24 0.16 0.08 0.00 Section 2; 10.7m E 0.24

cs 0.16 0.08 0.00 ^ 31 3.7 rn .2 0.160.24

0.00 Section 4;16.8m

0.24

0.00 - -After 2.07 It Section 5; 18.3 rn

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Distance from a datum (m)

Fig. 4 Time variation in ephemeral gully morphology for the 6/2/98 experiment. The position of each cross-section is given as distance downstream.

Results from two experiments are shown in Figures 3 through 6. Using a flow rate of 70 gpm (0.00441 m3!s; 6/1/98), channel erosion occurred along the entire channel (Figs. 3 and 5a). Sediment load increased with distance along the channel, and total load decreased with time (Fig. 6). For this experiment, the following morphologic adjustments were observed: (1) the channel bed deepened by 16%, (2) the bottom width enlarged by 350%, (3) the 530 channel sidewalls steepened to 70° in the upstream reaches and to in the downstream reaches, and (4) W/D ratio decreased by 14% to 4.1 (Figs. 3 and 5a). No change was observed in channel top width. Using a higher flow rate (150 gpm or 0.00943 m 3/s), similar but not identical results were observed for the 6/2/98 experiment. Sediment load increased with distance along the channel and total load decreased with time, but total load was greater for the higher flow rate (Fig. 6). Moreover after 2 h, sediment load did not change appreciably with distance. In the

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upstream reaches, channel depth increased by 26%, bottom width increased by S00%, top width increased by 12%, W/D decreased by 13% to 4.2, and the sidewalls steepened to 67 (Figs. 4 and Sb). In the downstream reaches, depth decreased by 17%, bottom width increased b y 420% , and the sideatIs steepened to 49. Clear-water discharge into these pre-formed swales caused systematic gully erosion: bottom width increased by 350 to 500%, gully depth increased by 16 to 26%, and the gully sidewalls steepened to a maximum o170.

°

(a) 6/1/98 Q=O.00441 in

0.6

05

0-4

> 6/2/98 Oip; Q=0.00943 4113/S

0.6

0.5 Original bed profile -- Final bed profile

0.4 10 12 14 16 Distance dow nstream (m

Fig. 5 Longitudinal profiles of the ephemeral gully bed before and after the (a) 6/1/98 and (b) 6/2/98 experiments. Also given is the flow rate (Q) used.

The transport capacity of the flow played an important role in gully erosion and morphologic adjustment. During experiment 6/1/98, the same ma gnitude of bed adjustment occurred along the entire channel length (Figs. 3 and 5a), and sediment load increased with distance downstream (Fig. 6). Although sediment load decreased with time, one can conclude that the flow was detachment limited, i.e. the flows transport capacity was never attained during the experiment. In experiment 6/2/98, the higher flow rate caused greater rates of erosion, increasing both the channel width and depth (Figs. 4 and 5b). Excessive rates, however, caused the formation of a migrating aggradational (depositional) wedge starting at a distance of 6 m downstream (Fig. 5b), and after 2 h, sediment load reached a constant or asymptotic value at a distance of 7 m from upstream (Fig. 6). Clearly, the flow reached transport capacity sometime between 1 and 2 h, exacerbated by the upstream erosion due to flow entrance effects. Thus the higher sediment yields modulated further channel adjustment in the downstream reaches. Such Interactions between soil erosion and transport capacity would be greatly complicated in gullies on agricultural plots where source areas, flow discharges, and sediment yields are temporally and spatially Varied (see also Thorne et al., 1986).

CONCLUDING REMARKS Ephemeral gully erosion is a significant problem in many geographic regions. The contribution of ephemeral gully erosion to total soil loss typically ranges from 10 to 30% in agricultural regions. Yet erosion prediction technology cannot determine where ephemeral gullies will form and how much soil Will be eroded.

International Journal of Sediment Research, Vol. 15, No. 1, 2000, pp. 3141 -39-

ER 0.04

CM Ch (Y03

— 0,02

s-I

5) Cr 0.0!

0.0) 0 2 4 6 8 I!) 32 14 16 Distance downstream (m)

Fig. 6 At-a-point measurements of sediment load for the ephemeral gully erosion experiments 6/1/98 and 6/2/98, and their temporal variation. Measurements were obtained at each gully cross-section, and are plotted relative to distance downstream.

Despite the large differences in climate, watershed size, hydrolo gy, and geography, the ephemeral

C observed in Spain were morphologically similar to those in Mississippi. On average, Navarrese gullies were sli ghtly longer, and their cross-sections slightly larger, deeper, and wider at the bottom. Gullies from Mississippi were wider at the top. These differences may be related to smaller curvature (concavity) of the hollow, smaller watershed slopes, presence of a fragipan, and higher peak flows after higher intensity rainfall events observed in Mississippi. Higher W/D values in Mississippian gullies (7.7 versus 4.0) appear to be related to intense rainfalls of short duration (Poesen and Govers, 1990). The average volume eroded per gully in Spain was greater (3.18 m) than gullies in Mississippi (1.73 m3 ), and this is presumably due to larger watershed areas with steeper slopes. However, Mississippi experienced much higher rates of erosion (1.68 kg/m 2-y) as compared to Southern Navarra, Spain (0.87 kg/m2-y). Although soils at both localities are considered highly erodible, this difference in erosion rate must be related to the precipitation regime. Each ephemeral gully examined in Mississippi exceeded the tolerable soil erosion limit of 1.2 kg/m-y (USDA-SCS, 1973), and most but not all gullies in Spain also exceeded this limit. The striking similarity of ephemeral gullies in two different geographic locations suggest that gully erosion processes are common, and that technology developed to mitigate such erosion processes may be widely applied. Laboratory experiments revealed that ephemeral gully erosion occurs by bed incision, channel widening, and bank steepening. The flows transport capacity controlled the magnitude of soil erosion or deposition as well as the time-variation in gully morphology.

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