Reducing ¯ood risk from sediment-laden agricultural runoff using intercrop management techniques in northern France Philippe Martin * Institut National Agronomique Paris-Grignon, De Âpartement AGER, 75231 Paris, Cedex 05, France Received 18 June 1998; received in revised form 1 April 1999; accepted 14 September 1999 Abstract To reduce muddy ¯ooding in northern France, where soils are typically silty and highly sensitive to crusting, it is necessary to decrease the volumes of sediment-laden runoff coming from upslope agricultural source areas. To investigate the consequences of upslope agricultural practices on downslope muddy ¯ooding, we compared the effects of no-tillage treatment (NT), light-duty mouldboard ploughing (PLOUG), mustard (Sinapis alba L.) intercrop (MUSTA) and super®cial tillage with a cultivator applied either under dry (ECULT) or humid conditions (LCULT). The in¯uence of these different treatments were measured on soil surface modi®cations and on both runoff and sheet erosion during the entire intercrop period. Field trials were conducted in the Pays de Caux (Normandie), over two years (1993±1994 and 1994±1995) under natural rainfall conditions on 20 m 2 experimental plots on Orthic Luvisols. In 1993±1994, the post-harvest ®eld conditions were wheat (Triticum aestivum L.) with pulverized straw, wheat without straw and pea (Pisum sativum L.) without straw. Only the pea post-harvest conditions were used in 1994±1995. The tortuosity index and percentage of surface area covered by vegetation, which were used for soil surface description, indicate the role played by surface cover and soil water during tillage in modifying surface roughness. In 1994±1995, due to speci®c agronomic and climatic conditions, the ®ve intercrop management techniques did not differ signi®cantly at P < 0.10 for runoff and erosion. In 1993±1994, considering average values for both the three post-harvest conditions and 10 recorded rainfall events, NT resulted in low erosion (40 kg ha 1 ) but high runoff (6.1 mm). Compared to NT, MUSTA signi®cantly (P < 0.10) reduced runoff (1.5 mm) without signi®cantly increasing erosion (82 kg ha 1 ); LCULT led to an increase in both runoff (12.4 mm) and erosion (301 kg ha 1 ); PLOUG increased erosion (182 kg ha 1 ) but reduced runoff (3.2 mm) and ECULT increased erosion (247 kg ha 1 ) without reducing runoff (7.1 mm). These results specify the dynamics of runoff and sheet erosion, thus providing an aid for locating intercrop management techniques within a catchment basin according to the local erosive system and agronomic constraints. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Runoff; Sheet erosion; Intercrop management techniques; Soil surface modi®cation 1. Introduction Over the last ten years, ¯ood frequency has increased in northwestern Europe (Boardman et al., 1994). These ¯oods are often called ``muddy ¯oods'' Soil & Tillage Research 52 (1999) 233±245 * Tel.: 33-1-44-08-16-97; fax: 33-1-44-08-16-57 E-mail address: pmartin@inapg.inra.fr (P. Martin) 0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII:S0167-1987(99)00084-7