At European level, the effect of support practices (compulsory for farmers to receive incentives under the CAP-GAEC) on PSI-7977 loss were assessed by P-factor estimation taking into account: (a) contour farming, (b) maintenance of stone walls, and (c) grass margins. P-factor was proposed as a product of those 3 sub-factors by Blanco and Lal (2008); applied by Lopez-Vicente and Navas (2009):equation(1)P=Pc×Psw×PgmP=Pc×Psw×Pgmwhere Pc is the contouring sub-factor for a given slope of a field, and Psw is the stone walls sedimentation sub-factor (known as terrace sub-factor) and Pgm is grass margins sub-factor (known as strip cropping sub-factor and buffer strips). In the same context, Angima et al. (2003) computed the P-factor as a product of individual support practices (contour farming, terracing and strips) that are used in combination to reduce soil erosion in Kenya.
3.1. Contour farming sub-factor
Contouring is a specific support practice applied only in croplands (CORINE land cover classes 21×) which account for around 25.2% of the total European Union land area. Contour farming means that farmers apply certain field practices (ploughing, planting) along contours, perpendicular to the normal flow direction of runoff. Contour cultivation reduces runoff velocity by increasing up- and downslope surface roughness. The increased surface roughness reduces water velocity providing more time for infiltration (Stevens et al., 2009). The effectiveness of contour farming in reducing soil erosion depends on the slope gradient (Table 3).