Antecedent water content effects on runoff and sediment yields from two Coastal Plain Ultisols

Agricultural Water Management 98 (2011) 1189–1196

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Antecedent water content effects on runoff and sediment yields from two Coastal Plain Ultisols夽 C.C. Truman a,∗ , T.L. Potter a , R.C. Nuti b , D.H. Franklin c , D.D. Bosch a a b c

USDA-ARS, Southeast Watershed Research Laboratory, P.O. Box 748, 2375 Rainwater Rd., Tifton, GA 31793, USA USDA-ARS, National Peanut Laboratory, Dawson, GA 39842, USA USDA-ARS, J. Phil Campbell, Sr., Natural Resource Conservation Center, Watkinsville, GA 39842, USA

a r t i c l e

i n f o

Article history: Received 30 September 2010 Accepted 8 March 2011 Available online 9 April 2011 Keywords: Erosion Infiltration Strip-tillage No-tillage Rainfall simulation Soil water

a b s t r a c t The highly weathered, low-carbon, intensively cropped, drought-prone Coastal Plain soils of Georgia are susceptible to runoff and soil loss, especially at certain times of the year when soil water contents are elevated. We quantified the effects of antecedent water content (AWC) on runoff (R) and sediment (E) losses from two loamy sands managed under conventional- (CT), strip- (ST), and/or no-till (NT) systems. Two AWC treatments were evaluated: field moist (FM) and pre-wet (PW), created with and without post pesticide application irrigations (∼12 mm of water added with the rainfall simulated over 30 min) for incorporation. Treatments (5) evaluated were: CT + FM, CT + PW, ST + FM, ST + PW, and NT + PW. Field plots, each 2-m × -3 m, were established on each treatment. Each 6-m2 field plot received simulated rainfall at a variable rainfall intensity (Iv ) pattern for 70 min (site 1) or a constant rainfall intensity (Ic ) pattern for 60 min (site 2; Ic = 50.8 mm h−1 ). Adding ∼12 mm of water as herbicide incorporation increased AWCs of the 0–2 (3–9-fold) and 2–15 (23–117%) cm soil depths of PW plots compared to existing field moist soil conditions. Increase in AWC increased R (as much as 60%) and maximum R rates (as much as 62%), and decreased E (at least 59%) and maximum E rates (as much as 2.1-fold) for corresponding tillage treatments. Compared to CT plots, ST and NT plots decreased R (at least 2.6-fold) and maximum R rates (as much as 3-fold), and decreased E (at least 2.7-fold) and maximum E rates (at least 3.2-fold). Runoff curves for pre-wetted CT and ST plots were always higher than corresponding FM curves, whereas E curves for field moist CT and ST plots were always higher than corresponding PW curves. Changes in AWC and tillage affected detachment and transport processes controlling runoff and sediment yields. A more accurate measure of rainfall partitioning and detachment and transport processes affecting R and E losses was obtained when commonly occurring field conditions (increased AWC with irrigation; Iv pattern derived from natural rainfall; commonly used tillage systems) were created and evaluated. Published by Elsevier B.V.

1. Introduction Coastal Plain soils in Georgia have traditionally been intensively cropped under conventional tillage practices, have relatively sandy surfaces, tend to be drought-prone, and are susceptible to compaction and runoff, sediment, and chemical losses. Rainfall in the Coastal Plain region (∼1250 mm yr−1 ) tends to have short durationhigh intensity, runoff producing storms followed by extended drought periods during the crop growing season. Thus, crop pro-

夽 Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by U.S. Department of Agriculture. ∗ Corresponding author. Tel.: +1 229 386 7174; fax: +1 229 386 7294. E-mail address: [email protected] (C.C. Truman). 0378-3774/$ – see front matter Published by Elsevier B.V. doi:10.1016/j.agwat.2011.03.001

duction at times requires supplemental irrigation to prevent yield limiting water stress. Conservation tillage (strip tillage, ST) adoption in Georgia has increased because ST systems enhance infiltration, and reduce runoff, sediment, and supplemental irrigation amounts/costs (Potter et al., 2008; Truman and Nuti, 2010). Conservation tillage also accumulates residue and increases organic carbon at the soil surface, which helps dissipate raindrop impact energy, reduces evaporation, and retains more water near the soil surface. Two factors that influence runoff, sediment, and chemical losses from Coastal Plain soils are rainfall intensity characteristics of runoff producing storms, and antecedent water content of the near surface soil at the time these intense storms begin. Rainfall intensity affects processes controlling infiltration, runoff, soil detachment, and sediment and chemical transport (Leonard, 1990; Truman and

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Bradford, 1993; Truman et al., 1998). Natural rainfall is variable, spatially and temporally (Flanagan et al., 1988; Bosch et al., 1999; Frauenfeld and Truman, 2004). The frequency of severe rainfall events has increased throughout the U.S., including the Southeast, mainly in the form of increased intensity of extreme rainfall events (Karl and Knight, 1998; Groisman et al., 2001; Nearing et al., 2005; Todd et al., 2006). Changes in rainfall intensity within a storm affect how rainfall is partitioned into infiltration and runoff and subsequent sediment and chemical transport (Flanagan et al., 1988; Romkens et al., 2001; Frauenfeld and Truman, 2004; Nearing et al., 2005; Truman et al., 2007; Franklin et al., 2007; Potter et al., 2008). Antecedent water content (AWC) of the near surface soil is a dynamic variable that influences detachment and transport processes controlling rainfall partitioning and sediment delivery, yet opposing AWC effects have been reported (Farres, 1987; Kemper et al., 1987; Bradford et al., 1990; Govers, 1991; Truman and Bradford, 1993; Reichert and Norton, 1994; Meyles et al., 2003; Castillo et al., 2003; Seeger et al., 2004; Lado et al., 2004; Hawke et al., 2006; Wei et al., 2007). For example, studies have shown that increased AWC significantly decreased aggregate stability and infiltration; and increased runoff, soil detachment, and sediment delivery (Al-Durrah and Bradford, 1981; Francis and Cruse, 1983; Luk, 1985; Froese et al., 1999; Wangemann et al., 2000). Le Bissonnais et al. (1995) found that seal formation and subsequent runoff was delayed, resulting in less total runoff and erosion from air-dried plots compared to field moist plots. Conversely, studies have shown that increased AWC significantly decreased soil detachment, runoff, and sediment delivery (Truman and Bradford, 1990; Le Bissonnais and Singer, 1992; Rejman et al., 2001; Lado et al., 2004). Vermang et al. (2009) reported no runoff from the 0.19 m3 m−3 AWC treatment; the most runoff from the 0.12 m3 m−3 AWC treatment; and intermediate runoff amounts from the 0.04 m3 m−3 AWC (air-dry) treatment. Soil loss behaved differently as the most soil loss occurred from the 0.04 m3 m−3 AWC treatment; intermediate soil loss amounts occurred from the 0.12 m3 m−3 AWC treatment; and no soil loss occurred from the 0.19 m3 m−3 AWC treatment. Mixed pesticide loss results have also been reported due to AWC changes resulting from post-application irrigation incorporation (Evans et al., 1998; Liu and O’Connell, 2002; Smith et al., 2002; Potter et al., 2008; Lewan et al., 2009). Irrigation (4–18 mm) for incorporation applied <24 h after simazine, atrazine, and metolachlor application reduced their runoff loss from simulated rainfall (Liu and O’Connell, 2002; Smith et al., 2002; Potter et al., 2008). Conversely, Smith et al. (2002) showed that watering in atrazine increased its runoff from simulated rainfall 8 and 15 days after treatment. Evans et al. (1998) attributed increased diazinon runoff after irrigation for incorporation to increased AWC due to the irrigation. Changes in AWC thus can increase or decrease rainfall partitioning into infiltration and runoff (timing, amount), sediment, and chemical losses, depending on soil characteristics, wetting method and rate, and chemical/contaminant properties. Producers in Georgia incorporate applied herbicides with irrigation, especially in conservation tillage, to limit runoff losses and increase weed control. This management practice increases AWC and could increase runoff, sediment, and chemical losses if an intense, runoff producing rainfall event occurred close to the time of incorporation. Our objective was to quantify AWC effects on runoff and sediment losses from two loamy sands managed under conventional- (CT), strip- (ST), and/or no-till (NT) systems. Differences in AWC were created with and without post pesticide application irrigations for incorporation.

Table 1 Summary of treatment combinations for both sites studied. Year

Tifton loamy sand Site 1

Faceville loamy sand Site 2

2006

CT + FMa (n = 2) CT + PW (n = 4) ST + FM (n = 2) ST + PW (n = 4) CT + FM (n = 2) CT + PW (n = 2) ST + FM (n = 2) ST + PW (n = 2) NT + PW (n = 2) CT + FM (n = 3) CT + PW (n = 3) ST + PW (n = 3) CT + PW (n = 3) NT + PW (n = 3)

CT + FM (n = 3) CT + PW (n = 3)

2007

2009

2010 a

CT = conventional till; ST = strip till; NT = no till; FM = field moist; PW = pre-wet.

2. Materials and methods 2.1. Experimental sites and treatments Field site 1 was located near Tifton, GA (N 31◦ 26 , W 83◦ 35 ) on a Tifton loamy sand (Typic Kandiudult; 82% sand, 7% clay; pHH2 O = 5.9; 2% slope), which represents over 762,000 farmable ha in the Coastal Plain region of Georgia. Site 1 has been managed under CT and ST systems in a cotton (Gossypium hirsutum)–peanut (Arachis hypogea) rotation since 1998 (Potter et al., 2008; Truman and Nuti, 2010). CT consisted of fall disking, winter rye (Secale cerale) cover, followed by spring disking and cultivator levelling. Rye cover was incorporated 10–15 cm. ST consisted of planting a winter rye cover after crop harvest in the fall and killing the rye chemically 30 days before planting the next year’s row crop. With ST, a 10-cm wide tilled strip was used to plant the crop into. Site 1 was cropped to peanuts (planted 15 May, 2009; 0.9 m row spacing). In 2010, site 1 was cropped to pearl millet (Pennisetum glaucum L.R., Br.) (planted 18 May; 0.15 m row spacing), and was managed under no-till (NT) conditions (same as ST without tilled strips; pearl millet planted directly into chemically burned surface residue). Surface residue cover for CT, ST, and NT treatments were <1, 52, and 83%, respectively. Field site 2 was located near Dawson, GA (N 31◦ 46 , W 84◦ 31 ) on a Faceville loamy sand (Typic Kandiudult; 71% sand, 16% clay; pHH2 O = 6.5; 1% slope), which represents over 87,000 farmable ha in the Coastal Plain region of Georgia. Site 2 was managed to CT in a cotton-corn (Zea mays)–peanut rotation (2002–2008) (Truman and Nuti, 2009). After crop harvest each fall, stubble was disked twice and a rye or wheat cover crop planted. In the following spring, CT consisted of disking the cover crop twice, field cultivating, and bedding. Treatments consisted of field moist (FM) and pre-wet (PW) CT, ST, and/or NT tillage. A summary of treatment combinations evaluated is given in Table 1.

2.2. Rainfall simulations Rainfall simulation plots (2-m wide, 3-m long) were established on each tillage–AWC treatment combination at planting in May (2006) for sites 1 and 2, and May (2007, 2009, 2010) for site 1. An area surrounding each 6-m2 plot was treated like the test area to allow soil material to be splashed in all directions. Antecedent water content was determined gravimetrically (Gardner, 1986) from samples taken five locations around each 6-m2 plot just prior to simulating rainfall (0–2 and 2–15 cm depths).

Rainfall Intensity (mm h-1)

C.C. Truman et al. / Agricultural Water Management 98 (2011) 1189–1196

180

intervals from 0 to 20, 20 to 40, and 40 to 70 min. Sediments for each time interval were sieved through a nest of nine sieves with openings of 19, 9.5, 4.76, 2, 1, 0.5, 0.25, 0.1, and 0.05 mm. Shake time for the nest of sieves was 5 min. Material remaining on each sieve and material <0.05 mm was collected and weighed. Surface soil (0–2 cm) samples of the CT and NT plots were also collected, air-dried for 72 h, and sieved with the same procedure as the sediments.

Site 1

160 140 120 100 80

Site 2

60

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40 20

2.4. Data analysis

0 0

10

20

30

40

50

60

70

80

Time (min)

Fig. 1. Simulated rainfall intensity patterns evaluated (site 1: Iv , variable rainfall intensity; site 2: Ic , constant rainfall intensity).

At both sites, simulated rainfall was applied with an oscillating nozzle rainfall simulator (Truman et al., 2007; Potter et al., 2008; Truman and Nuti, 2010) that used 80150 Veejet nozzles (median drop size = 2.3-mm). The simulator was placed 3 m above each 6-m2 plot. Simulated rainfall was applied at a variable rainfall intensity pattern (Iv ) for site 1 (Tifton loamy sand) and at a constant rainfall intensity (Ic ) at site 2 (Faceville loamy sand) (Fig. 1). The Iv pattern was developed from measured 5- and 1-min natural rainfall data (30 yr) collected at Tifton, GA. Natural rainfall during March, April, and May were analyzed to determine the pattern that occurred most frequently during the row-crop planting season. Parameters (maximum intensity, time to maximum intensity, maximum precipitation, duration) were then averaged for the group of natural storms occurring most often during this 3-month period (91 storms). The individual storm with the most parameters similar to the average of the entire group was then selected and its pattern programmed into the simulator on a 1-min basis as the Iv pattern. The pattern selected does not represent the highest intensity observed (183 mm h−1 ), but does represent 27% of the spring time storms sampled from the 30-yr data record. Statistical average of the Iv pattern was 57 mm h−1 . Rainfall duration for each Iv simulation was 70 min. At site 1, the total rainfall volume applied to each 6-m2 plot was repeatable (average Iv = 1227 mL, CV = 4%). At site 2, simulated rainfall was applied to each 6-m2 plot at a target I of 50 mm h−1 for 60 min (average Ic = 50.8 mm h−1 ; CV = 1%). Well water was used in all simulations, and had an average pH of 7.6 (CV = 0.8%), electrical conductivity of 0.002 S cm−1 (CV = 3%), and temperature of 22.4 ◦ C (CV = 3%). For the pre-wet treatment, 12.7 mm (25 mm h−1 for 30 min) of water was added to each PW plot with the rainfall simulator immediately after plot preparation and herbicide application and 24 h before simulating rainfall (site 1: average water added 11.9 mm, CV = 6%; site 2: average water added = 12.0 mm, CV = 6%). The 12.7 mm of water was the recommended amount of irrigation water needed for herbicide “activation”. The field moist treatment was the soil water content at the time of rainfall simulation. Runoff (R) and sediment (E) were measured from the down slope end of each 6-m2 plot at 5-min intervals throughout each simulation, and determined gravimetrically. Based on time to runoff, sediment during the first 10 min of each run was assumed representative of splash sediment amounts among treatments. Infiltration (INF) was calculated by difference (rainfall − runoff), and the parameter d(INF) was calculated by difference (maximum INF − minimum INF). 2.3. Sediment size analysis For site 1 (Tifton loamy sand) in 2010, oven-dried sediments from CT + PW and NT + PW plots were combined from rainfall time

Means, coefficient of variations (CV, %), and standard error bars are given for measured data. We performed unpaired t-tests (twotailed distribution) to determine significance among treatment means using SigmaStat 3.1 (Systat, 2004). All test statistics were evaluated at P = 0.05, unless otherwise noted. All data analysis was conducted with Microsoft Office Excel 2007. 3. Results and discussion 3.1. Runoff Increased antecedent water content (AWC) decreased infiltration and increased runoff (R) among tillage treatments (Table 2). Adding ∼12 mm of water (CT + PW, ST + PW plots) for herbicide incorporation increased AWCs of the 0–2 and 2–15 cm soil depths by 3–9-fold and 23–117% compared to existing soil conditions when rainfall was simulated (CT + FM, ST + FM plots). For site 1 (Iv pattern), CT + PW and ST + PW plots had 60 and 42% more R than CT + FM and ST + FM plots. As for tillage, CT + FM plots had 2.6-fold more R than ST + FM plots; CT + PW plots had 2.8-fold more R than ST + PW plots and 3.4-fold more R than NT + PW plots; and ST + PW plots had numerically (16%) more R than NT + PW plots. For site 2 (Ic pattern), CT + PW plots had 26% more R than CT + FM plots. Tillage (ST, NT) and AWC (+FM) treatments significantly decreased runoff. For site 1, R curves had similar shapes as the Iv pattern (Ivmax = 160 mm h−1 at 20 min) (Fig. 2). The first 15 min of rainfall produced <15 mm h−1 of R for all plots. After 15 min, R rates for all plots increased (at different rates) until each reached its peak. Peak R for all tillage–AWC plots occurred 5 (CT + FM, CT + PW, ST + FM, ST + PW) or 10 min (NT + PW) after the 20 min Iv peak. Maximum 5-min runoff rate (Rmax ) for CT + PW plots was 62% higher than Rmax values for CT + FM plots (Table 2). The Rmax value for ST + PW plots was numerically (18%) higher than that for ST + FM plots. Rmax values for CT + FM and CT + PW plots were 1.9- and 3-fold higher than Rmax values for ST + FM and ST + PW plots. After reaching their respective Rmax values, R curves declined almost as sharply as they increased to their peak. At 40 min, R curves gradually declined for the remaining 30 min duration. Even though R curves had similar shape, R curves for CT + PW and ST + PW plots were always steeper than corresponding FM curves. For site 2, R rates steadily increased to or near the end of the 60 min simulation. Runoff rates for CT + FM plots reached a quasisteady-state rate (32 mm h−1 ) during the last 15 min; R rates for CT + PW plots never reached steady-state. Rmax for CT + PW plots was numerically (9%) higher than that for CT + FM plots (Table 2). 3.2. Sediment yield Increased AWC decreased sediment (E) yields among tillage treatments (Table 2). For site 1 (Iv pattern), CT + FM and ST + FM plots had 72% and 1.9 times more E than CT + PW and ST + PW plots. As for tillage, CT + FM plots had 2.7-fold more E than ST + FM plots; CT + PW plots had 3.1-fold more E than ST + PW plots; 3.3-fold more E than NT + PW plots. The ST + PW plots had only numerically (5%) more E than NT + PW plots. For site 2 (Ic pattern), CT + FM plots

a AWC = antecedent water content (%); Int = rainfall volume (mL); INF = infiltration (mm h−1‘ ); R = runoff (mm h−1 , % value is % of simulated rainfall); Rmax = maximum 5 min runoff rate (mm h−1 ); E = soil loss (g); Emax = maximum 5 min soil loss rate (kg m−2 h−1 ); [E] = sediment concentration (g L−1 ); Ss 10 = splash sediment during the first 10 min of rainfall (g). b CT = conventional till; ST = strip till; NT = no till; FM = field moist; PW = pre-wet. c Mean (coefficient of variation). d *, ** and *** indicate statistical significance at the 0.05, 0.01, and 0.001 levels, respectively. NS = not significant. e Statistical significance between NT + PW and ST + PW treatments.

29.8 (28) 46.8 (07) * 15.8 (03) 8.2 (02) *** 0.59 (17) 0.28 (02) * 33 (04) 40 (03) * 925 (01) 927 (01) NS 5.2 (10) 11.3 (02) ***

33 (05) 30 (02) *

16 (01) 20 (02) **

33 (04) 35 (03) NS

1534 (08) 966 (10) **

7.0 (16) 4.9 (09) *** 7.8 (62) 4.1 (25) ** 4.2 (11) NS 0.69 (19) 0.67 (17) NS 0.21 (54) 0.18 (55) NS 0.25 (12) NS 38 (18) 61 (04) *** 14 (14) 20 (17) * 18 (14) NS 1201 (04) 1232 (04) NS 1305 (01) 1207 (02) *** 1122 (06) ** 8.8 (09) 10.9 (04) *** 7.0 (10) 9.9 (14) ** 17.3 (60) NS

35 (11) 23 (10) *** 49 (02) 45 (04) ** 47 (03) NS

21 (18) 34 (07) *** 8 (14) 12 (17) * 10 (14) NS

68 (02) 110 (08) *** 31 (11) 36 (12) NS 35 (19) NS

1767 (10) 1028 (14) *** 641 (57) 326 (51) NS 310 (12) NS

[E] (g L−1 ) Emax (kg m−2 h−1 ) E (g) R (%) Rmax (mm h−1 ) R (mm h−1 ) INF (mm h−1 ) Int (mL) AWC 15 cm (%) AWCa 2 cm (%)

Tifton loamy sand (site 1) 1.1 (05)c CT + FMb CT + PW 10.1 (10) ***d ST + FM 3.3 (16) ST + PW 12.6 (10) *** NT + PW 21.7 (03) ***e Faceville loamy sand (site 2) CT + FM 2.2 (06) CT + PW 19.3 (02) ***

Treatment

Table 2 Hydrology and erosion parameters for each treatment studied.

8.0 (10) 12.9 (11) *** 4.4 (02) 7.9 (07) ** 7.6 (11) NS

C.C. Truman et al. / Agricultural Water Management 98 (2011) 1189–1196

Ss 10 (g)

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Fig. 2. Runoff rates from each tillage (CT = conventional till; ST = strip-till; NT = notill) and antecedent water (FM = field moist; PW = pre-wet) treatment during the 70 min of simulated rainfall (bars = standard error, S.E.).

had 59% more E than CT + PW plots. Tillage (ST) and AWC (+PW) treatments significantly decreased sediment losses. Sediment rates for each tillage–AWC treatment are given in Fig. 3. For site 1, E curves had similar shapes as the rainfall and runoff pattern (Figs. 1 and 2). For all plots, the first 10 min of rainfall produced essentially no runoff and relatively little sediment. Sediment measured during the first 10 min of each run was assumed splash sediment (Table 2). After 10 min, E rates for all plots increased (at different rates) until each reached its peak. Sediment curves for CT + FM and CT + PW plots had similar shapes, whereas E curves for ST and NT plots had similar shapes. Peak E rate for all plots occurred 5 (CT + PW, ST + PW, NT + PW) or 10 min (CT + FM) after the 20 min Iv peak, except for the ST + FM treatment (peak = 20 min). Maximum 5-min sediment rate (Emax ) for CT + FM and ST + FM plots were numerically (3% and 17%) higher than those for corresponding PW plots (Table 2). Emax values for CT + FM and CT + PW plots were 3.2and 3.7-fold higher than Emax values for ST + FM and ST + PW plots. After reaching their respective Emax values, E curves declined less sharply as they increased to their peak. At 40–45 min, E curves gradually declined for the remaining 30 min duration. Also, E curves for CT + FM and ST + FM plots were always higher than corresponding PW curves. For site 2, E rates for CT + FM plots steadily increased during the 60 min duration, whereas E rates for CT + PW plots steadily increased during the first 20 min of rainfall, then gradually increased to its peak at 40 min. Sediment rates for CT + FM plots reached a quasi-steady-state sediment rate (∼0.5 kg m−2 h−1 ) during the last 5 min; E rates for CT + PW plots stayed at a quasisteady-state rate (∼0.23 kg m−2 h−1 ) after reaching its peak. Emax for CT + FM plots was 2.1 times higher than that for CT + PW plots (Table 2).

C.C. Truman et al. / Agricultural Water Management 98 (2011) 1189–1196

Fig. 3. Sediment yield rates from each tillage (CT = conventional till; ST = strip-till; NT = no-till) and antecedent water (FM = field moist; PW = pre-wet) treatment during the 70 min of simulated rainfall (bars = standard error, S.E.).

3.3. AWC–tillage interactions Adding ∼12 mm of water to PW plots as a herbicide incorporation measure 24 h before simulating rainfall increased AWCs in the 0–2 and 2–15 cm soil layers. As a result, PW plots had decreased INF, increased R, and decreased E values compared to corresponding FM plots. Differences in R and E occurred because AWC and tillage affected soil detachment and sediment transport processes throughout the rainfall duration. For the Tifton loamy sand (site 1, Iv pattern), relatively little R and E were lost during the 0–10 min time period (Figs. 2 and 3). As the soil surface began to wet up beyond PW or FM initial conditions, rate of wetting was different for FM and PW treatments which influenced splash processes between FM and PW soil surfaces. Splash detachment by raindrop impact was estimated as sediment loss in the first 10 min of rainfall. Splash detachment (Ss 10) for CT + PW and ST + PW plots was 61% and 79% more than that for CT + FM and ST + FM plots (Table 2). Also, CT plots had at least 63% higher Ss 10 values than ST and NT plots. Surface residue cover values for CT, ST, and NT (site 1) were <1, 52, and 83%, respectively. Thus, PW and CT treatments increased detached soil amounts potentially available for transport. From 10–30 min, sediment increased with runoff, with Rmax and Emax occurring at 5–10 min after peak Iv , as detachment and transport processes were active. Then, R and E rates steadily decreased through 40–45 min as detachment and/or transport capacity decreased. From 40 to 45 min until the end of the 70 min

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duration, R and E rates continued their gradual declines indicating a further decrease in detachment and/or transport capacity. Thus, detachment and transport processes were relatively active in the first half (10–35 min) of Iv events; while from 35 to 70 min, the capacity to detach soil and transport sediment became less, especially approaching the end of each simulation. These trends vary in magnitude and rate with AWC and tillage as R rates for PW plots were higher than those for FM plots; E rates for FM plots were higher than those for PW plots; and R and E rates for CT plots were higher than those for ST or NT plots. Also, E amounts from CT + FM, CT + PW, ST + FM, ST + PW, and NT + PW plots during the first half (0–35 min) of their respective Iv events were 68, 83, 69, 84, and 88%, respectively; compared to 32, 17, 31, 16, and 12%, respectively during the second half (35–70 min) of those same events. Compared to FM plots, PW plots had on average ∼21% more sediment lost during the first half of the event and ∼89% less sediment lost during the second half of the event. Frauenfeld and Truman (2004) studied the same soil and Iv pattern under laboratory conditions (CT only), and showed that splash detachment rates also increased to a peak at ∼25 min, then decreased sharply to the 40 min rainfall duration. They also showed that Iv events had more splash detachment during the first half of the 70 min simulation than the second half. Truman et al. (2007) found similar results for Tifton loamy sand managed to CT and ST under the same Iv pattern; that is, R and E values from the 0 to 35 min time period was significantly greater than corresponding values from the 35 to 70 min time period. Differences in detachment and transport processes and subsequently differences in INF, R, and E from both soils can be partially explained by differences in how the interactions between the two AWCs and three tillage systems partition rainfall as a result of soil surface alteration (surface sealing). Values for d(INF) (INFmax − INFmin ) have been related to the degree of surface sealing (Truman and Bradford, 1993; Frauenfeld and Truman, 2004; Truman and Nuti, 2009, 2010) with larger d(INF) values proportional to greater surface sealing. For the Tifton loamy sand (site 1), CT + FM plots had d(INF) values that were 1.8-fold greater than those for ST + FM plots; CT + PW plots had d(INF) values that were 1.6-fold greater than those for ST + PW plots. Thus, CT + FM and CT + PW plots experienced more surface sealing, R, and E than corresponding ST (and NT) plots (Table 2). However, opposite trends in d(INF) values were obtained for FM vs. PW plots. For example, CT + FM plots had d(INF) values that were 1.7-fold greater than those for CT + PW plots, yet CT + PW plots had 62% more R, 61% more detachment (Ss 10), and 1.7-fold less E than CT + FM plots (Table 2, Figs. 2, 3). Also, there were no differences in d(INF) values for CT plots on the Faceville loamy sand (site 2). Thus, changes occurred in the soil surface due to surface sealing, but surface sealing only partially contributed to differences in INF, splash, R, and E among treatments. The sand fraction of the Ap horizon of both soils influence detachment and transport processes and subsequent R and E losses. The Ap horizon of the Tifton loamy sand (∼30 cm; 85% sand) and Faceville loamy sand (∼20 cm; 71% sand) has 45% and 31% of their respective sand fractions as either medium, coarse, or very coarse sand (Perkins et al., 1986). These materials are easily detached; require much energy to be transported, and can create transportlimiting conditions. Small changes in micro-topography and/or transport capacity (runoff) make these sediments susceptible to deposition, which impacts measured E values (Tables 2, 3). Sandy Ap horizons also allow water to infiltrate quickly to subsurface horizons. Bulk density and porosity values for the Ap horizon of the Tifton loamy sand were 8% less and 10% greater (P = 0.005) than corresponding values for the argillic (Bt) horizon (30–51 cm) (unpublished data). Also, hydraulic conductivities for the Ap and Bt horizons ranged from 5 to 15 cm h−1 and 2.5 to 5 cm h−1 , respectively (unpublished data). As a result, infiltrating water is “backed

NT + PW

a

11 0.48 0.31 0.43 0.42 0.35 0.31 0.35 15.1 0.9 (04) 4.1 (03) 9.2 (21) 1.0 (22) 2.9 (02) 2.8 1.9 (11) 0.1 14.5 2.1 (07) 7.3 (04) 9.4 (10) 5.5 (09) 7.2 (04) 7.5 8.1 (05) 1.5 34.3 7.5 (09) 20.1 (04) 24.2 (06) 23.7 (10) 23.1 (04) 26 28.9 (05) 28.3 27.5 9.0 (10) 20.6 (03) 23.9 (06) 26.0 (05) 22.6 (05) 27.6 32.1 (02) 42.9 8.1 7.5 (13) 13.6 (06) 16.7 (06) 18.3 (08) 9.1 (6.5) 12.7 14.3 (08) 16.8 0.5 3.2 (09) 12.0 (16) 8.2 (12) 13.7 (11) 3.1 (05) 6.4 5.7 (02) 4 3 3 3 3 3 1 3 1

24 69.8 (14)d 22.5 (28) 8.6 (58) 11.9 (62) 32.1 (13) 17.1 9.1 (42) 6.5

V. fine (0.1–0.05 mm) (%) Fine (0.25–0.10 mm) (%) Medium (0.5–0.25 mm) (%) Coarse (1–0.5 mm) (%) V. coarse (2–1 mm) (%)

PSDc SOIL 20 40 70 SOIL 20 40 70 CT + PWb

b

68 8.3 7.5 4.9 7.0 4.6 3.9 2.9

Cc Cu D50 a (mm) Silt + clay (<0.05 mm) (%) Sand (2–0.05 mm) Fragments (>2 mm) (%) n Time (min) Treatment

Table 3 Size distributions for the Tifton loamy sand (site 1) and sediments from each treatment combination studied in 2010.

D50 = aggregate diameter (mm) corresponding to 50% passing; Cu = coefficient of uniformity = D60 /D10 ; Cc = coefficient of curvature = (D30 )2 /(D10 /D60 ). CT = conventional till; NT = no till; PW = pre-wet. c PSD = particle size distribution (Perkins et al., 1986) of the Tifton loamy sand; SOIL = dry sieved surface soil (0–2 cm) from CT and NT treatments. The 20, 40, and 70 min times refer to rainfall time intervals 0–20, 20–40, and 40–70 min, respectively. d Mean (coefficient of variation).

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0.94 0.78 1.14 0.81 0.76 0.87 0.93 1.09

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up” during a rainfall event as it reaches the Bt horizon, effectively decreasing infiltration and generating runoff. Amount of water in the Ap horizon when rainfall starts affects how much additional water it can hold before water is “backed up” and runoff starts. More water in the Ap horizon, due to increased AWC from incorporating herbicides with ∼12 mm of water, results in less INF, more R, and higher R rates and Rmax values. We have partially explained how increased AWC affects detachment and transport processes and in turn deceased INF, increased R and Ss 10 values, and decreased E values compared to corresponding FM conditions. Initially, one might expect an increase in AWC to increase R and as a result increase E, especially given the high overall correlations between R and E for both soils (r2 = 0.82 for the Tifton loamy sand; r2 = 0.85 for the Faceville loamy sand). Others have shown that increased AWC decreased aggregate stability and infiltration; and increased runoff, soil detachment, and sediment delivery (Al-Durrah and Bradford, 1981; Francis and Cruse, 1983; Luk, 1985; Froese et al., 1999; Wangemann et al., 2000). However, our results show that increased AWC increased R (transport capacity), increased Ss 10 values (soil detachment available for transport), but E was decreased (transport was limited). Vermang et al. (2009) reported runoff decreased in the following AWC order: 0.12 m3 m−3 > 0.04 m3 m−3 (air-dry) > 0.19 m3 m−3 . They also showed that soil loss decreased as AWC increased (0.04 m3 m−3 (air-dry) > 0.12 m3 m−3 > 0.19 m3 m−3 . Sediment delivery decreased with increased AWC, despite different trends in AWC–runoff results. Others have shown that increased AWC decreased soil detachment, runoff, and sediment delivery (Truman and Bradford, 1990; Le Bissonnais and Singer, 1992; Rejman et al., 2001; Lado et al., 2004). Sediments consisted of non-cohesive and aggregated materials. An explanation for our findings is that increased AWC increased the size of aggregated sediment which decreased E, despite increases in Ss 10 and R. Pre-wetting created larger, more stable, aggregated (non-dispersed) sediments because of reduced slaking forces (Truman et al., 1990; Le Bissonnais and Singer, 1992; Vermang et al., 2009) and increased binding forces holding these aggregates together through surface tension of water (Kemper and Rosenau, 1984; Bullock et al., 1988; Truman and Bradford, 1990). Larger sized sediments required more energy (or power) to be transported, and the increased energy associated with increased R was not sufficient enough to transport all of these larger sediments. As a result, less sediment was delivered for the PW treatment. Similarly, Le Bissonnais (1990) reported that pre-wetting produced larger aggregated sediments which was partially responsible for reduced sediment losses. Truman and Bradford (1990) found that pre-wetting increased aggregate size or stability to raindrop impact for a Cecil sandy loam (Georgia Ultisol). They also showed that pre-wetting increased splash (∼10%) and wash (20–42%) aggregated sediment diameter (mm) for the 5 and 20 min sampling times of a 60 min simulated rainfall duration. Partial evidence for this sediment size explanation can be seen in Table 3 and Fig. 4. More non-cohesive and aggregated soil materials were found in the air-dried, sieved CT and NT soil (0–2 cm) than in the particle size distribution (texture) of the Tifton loamy sand for size classes >1 mm. Also, more sediments were found in the 20 min time interval than in the original, air-dried, sieved soil for all size classes except the fragments >2 mm class for CT + PW plots and for all size classes ≤ 2 mm and >0.1 mm for NT + PW plots. Although the FM treatment was not evaluated, an equal or higher percentage of sediments 1 mm or larger was found for the 20 min time interval of both CT + PW and NT + PW plots compared to the 40 and 70 min time intervals. Tillage impacted rainfall partitioning and detachment and transport conditions, thus R and E amounts and rates. For the Tifton loamy sand, CT plots had more surface sealing (greater d(INF)), R, Rmax , and E than corresponding ST (and NT) plots. Also, ST + PW

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ods that often occur in the Coastal Plain region of Georgia during the crop growing season. 4. Summary and conclusions When evaluating the effects of two antecedent water contents (AWC), field moist (FM) and pre-wet (PW), on runoff (R) and sediment (E) losses from two Coastal Plain loamy sands, we found that adding ∼12 mm of water as herbicide incorporation increased AWCs of the 0–2 (3–9-fold) and 2–15 (23–117%) cm soil depths of the PW plots compared to existing field moist soil conditions. Increase in AWC increased R (as much as 60%) and maximum R rates (as much as 62%), and decreased E (at least 59%) and maximum E rates (as much as 2.1-fold) for corresponding tillage treatments. Compared to CT plots, ST and NT plots decreased R (at least 2.6fold) and maximum R rates (as much as 3-fold), and decreased E (at least 2.7-fold) and maximum E rates (at least 3.2-fold). Thus, tillage (ST) and AWC (+FM) treatments significantly decreased R; tillage (ST) and AWC (+PW) treatments significantly decreased E. Runoff curves for pre-wetted CT and ST plots were always higher than corresponding FM curves, whereas E curves for field moist CT and ST plots were always higher than corresponding PW curves. Changes in AWC occur when producers “water in” pesticides to reduce pesticide runoff losses and increase weed control in CT, ST, and/or NT. Our results show that the increased soil water content of the surface 15 cm, resulting from watering-in pesticides, increased runoff but decreased soil erosion under simulated rainfall conditions. A more accurate measure of rainfall partitioning and detachment and transport processes affecting R and E losses was obtained when commonly occurring field conditions (increased AWC with irrigation; Iv pattern derived from natural rainfall; commonly used tillage systems) were created and evaluated. Acknowledgements

Fig. 4. Soil and sediment size distributions for pre-wetted (PW) conventional- (CT) and no-till (NT) treatments from the Tifton loamy sand (2010) (bars = standard error, S.E.). The 20, 40, and 70 values (legend) refer to rainfall time intervals 0–20, 20–40, and 40–70 min, respectively.

The USDA-Agricultural Research Service and the University of Georgia Coastal Plain Experiment Station supported this work. USDA employees Ricky Fletcher, Coby Smith, Margie Whittle, Lorine Lewis, Jess Bolton, Bobby Hagler, Corey Collins, Bryant Luke, and Clay Lott provided expert assistance. References

plots had only slightly more R and E than NT + PW plots, while NT + PW plots had increased AWCs in the top 15 cm soil depth. Surface residue amounts for CT, ST, and NT treatments were <1, 52, and 83%, respectively, and effectively reduced the detrimental impacts of raindrop impact on a soil surface. Based on relatively small hydrology and sediment differences found between ST (52% residue cover) and NT (83% residue cover), one could possibly remove up to ∼30% of the residue cover from NT plots for alternative uses and not create any significant negative hydrology or erosion impacts. We have shown that AWC affects rainfall partitioning, but AWC could also influence plant available water estimates. If one assumes that all INF (Table 2) is available for crop use and an evapotranspiration (ET) value of 7 mm day−1 , plant available water estimates (PAWe), expressed as days of water for crop use, can be calculated for each treatment combination (INF in mm/ET). For the Tifton loamy sand, PAWe values for CT + FM, CT + PW, ST + FM, ST + PW, and NT + PW were 5.9, 3.9, 8.1, 7.6, and 7.9 days of water for crop use, respectively. Pre-wetting reduced PAWe by 2 days for CT and by 0.5 days for ST. Pre-wetting the CT treatment on the Faceville loamy sand reduced PAWe by 9% or 0.4 days. These differences may seem small, however, 0.4–2 more days of water for crop use could make a difference in yield-limiting water stress levels, especially given the low water holding capacity of both soils and extended drought peri-

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