DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION Runoff Features for Interrill Erosion at Different Rainfall Intensities, Slope Lengths, and Gradients in an Agricultural Loessial Hillslope

In agricultural landscapes, factors affecting V under steady-state conditions of infiltration are well docuVarious interactions, particularly those existing between the rainmented (Kinnell, 2000). The effect of slope angle on fall intensity, the slope gradient, the slope length, and the tillage runoff for interrill erosion has also been fully investisupposedly can affect the runoff features for interrill erosion. Despite gated. Runoff may increase at steeper slopes because numerous studies, their effect on runoff production and pathways and the resulting soil losses have seldom been analyzed. This is especially of a decrease of the ponds’ ability to retain water (Fox true under field and tillage conditions. This study investigated the et al., 1997). Several authors have confirmed the influeffect of rainfall intensity, slope length, and gradient on runoff amount ence of slope gradient on soil losses by interrill erosion and pathways for interrill erosion in tilled fields. Runoff features and (Huang, 1995; Fox and Bryan, 1999). The increase of soil losses were evaluated on bounded plots of 1and 5-m length detachment and transport of soil particles with higher located on 4 to 8% slope gradients, and under natural and simulated flow velocity has already been demonstrated by laborarainfalls with intensities ranging from 1.5 to 30 mm h 1. Runoff coeffitory experiments (Torri and Poesen, 1992; Fox and cients (R) ranged from 34 to 98% whereas sediment concentrations Bryan, 1999) and field investigations (e.g., Chaplot and (SC) varied from 2.9 to 49 g l 1. The runoff coefficient was affected Le Bissonnnais, 2000). Fox and Bryan (1999) argued: by all three factors: rainfall intensity (r 0.48; P 0.0001), slope gradient (r 0.51; P 0.0001) and slope length (r 0.29; P 0.02); “For a constant runoff rate rain impacted flow erosion whereas SC was correlated with only rainfall intensity (r 0.48; P increased roughly with the square root of the slope 0.0001) and slope length (r 0.44; P 0.0004). The runoff coefficient gradient (2 to 40%). Soil losses were correlated (r2 and SC ratios between 1and 5-m long plots were systematically 0.81) with runoff velocity.” However, Govers (1990) greater for the intermediate rainfall intensity. Runoff features mainly showed that slope gradient might have a significant negaffected by tillage implements may explain higher interrill erosion at ative effect on runoff and erosion because of differential longer and steeper slopes. Lower differences between 1and 5-m plots soil cracking. Moss and Green (1983) attributed a deat high rainfall intensity may reflect greater ponded runoff absorbing crease of interrill erosion at the steepest slopes to an raindrop kinetic energy and lowering detachment and transport proincrease in the water depth in ponds. When this depth cesses. Finally, the effects of rainfall intensity, slope length, and gradiexceeds the diameter of two drops it protects the soil ent and tillage are discussed in respect of possible erosion processes operating in the experiments. surface from drop impact (Kinnell and Cummings, 1993). It is also well known that R and SC increase with the increase of rainfall intensity (e.g., Wischmeier and W movement within landscapes is fundamental Smith, 1978; Fraser et al., 1999) because of: (i) the augfor the prediction of soil erosion and the conservamentation of runoff fraction of rainfall (Williams et al., tion of water (Mermut et al., 1997). Rainwater not only 2000); (ii) the augmentation of soil detachment with moves up and down through the soil profiles and saproincreasing drop detachment forces (Kinnell, 1990; Torri lites by percolation and evaporation, but also moves and Poesen, 1992; Mermut et al., 1997) and (iii) a better laterally on the surface and through the subsurface. Retransport and remobilization of particles by raindistribution of soil particles by surface flow remains the impacted flow (Hairsine and Rose, 1992; Chaplot and most important factor in tropical and intertropical areas Le Bissonnais, 2000). However, in some conditions, esbecause of extreme rain events (e.g., Puigdefabregas et pecially when a crust surface was formed quickly, no al., 1999). In temperate climates, overland flow predomsignificant relation has been found between soil loss and inates when the natural infiltration capacity of the soil rainfall parameters from 10 to 103 mm h 1 (Uson and surface is altered (Valentin and Bresson, 1992). Ramos, 2001). Soil loss rate (SL, mass per surface and time unit) Limited studies have been conducted to investigate may be defined as: the effect of slope length on runoff for interrill erosion, especially under field conditions. Since overland flow SL V SC [1] velocity is important for soil detachment and transport With V, the volume of surface water per time unit and capacity, the interrill erosion rate should also be strongly SC, the mass of sediment per unit volume of water. influenced because longer slope lengths allow higher runoff velocities. Using field surveys Horton (1945) was V.A.M. Chaplot, IRDAmbassade de France, BP 06. VIENTIANE, one of the first to quantify the effects of the slope steepLAOS PDR. Y. Le Bissonnais, INRA, Science du Sol, Avenue De ness and length. He demonstrated that erosion increases lab Pomme de Pin, B.P. 20619, Ardon, 45166 Olivet cedex, France. Received 28 June 2001. *Corresponding author (chaplotird@laopdr. com). Abbreviations: CEC, cation-exchange capacity; DEM, digital elevation model; SC, sediment concentration; SL, soil loss rate. Published in Soil Sci. Soc. Am. J. 67:844–851 (2003).