PATTERNS OF ROCK FRAGMENT COVER GENERATED BY TILLAGE EROSION

Intensively cultivated areas in the upper part of the Guadalentin catc hment (southeast Spain) show a systematic spatial pattern of surface r ock fragment cover (R(c)). The objective of this paper is to quantify and to explain this spatial rock fragment cover pattern. Therefore, a map of an intensively cultivated area of 5 km(2) was digitised, and fo r each pixel total topographic curvature was calculated. Next, rock fr agment cover was determined photographically at 35 sites with a range of total slope curvatures. A linear relation between total curvature a nd rock fragment cover was found, except for narrow concavities. It wa s hypothesised that this pattern can be explained by a significant net downslope movement of rock fragments and fine earth by tillage. The d isplacement distances of rock fragments by tillage with a duckfoot chi sel were measured by monitoring the displacement of tracers (painted r ock fragments and aluminium cubes) on 5 sites having different slopes. The rate of tillage erosion for one tillage pass with a duckfoot chis el, expressed by the diffusion constant (k), equals 282 kg/m for up an d downslope tillage and only 139 kg/m for contour tillage. Nomograms i ndicate that mean denudation rates in almond groves due to tillage ero sion (3 to 5 tillage passes per year) can easily amount to 1.5-2.6 mm/ year for contour tillage and up to 3.6-5.9 mm/year for up- and downslo pe tillage for a field, 50 m long and having a slope of 20%. These fig ures are at least one order of magnitude larger than reported denudati on rates caused by water erosion in similar environments. Hence tillag e erosion contributes significantly to land degradation. The downslope soil flux induced by tillage not only causes considerable denudation on topographic convexities (hill tops and spurs) and upper field bound aries but also an important sediment accumulation in topographic conca vities (hollows and valley bottoms) and at lower field boundaries. Kin etic sieving (i.e. the upward migration of rock fragments) by the tine s of the duckfoot chisel also concentrates the largest rock fragments in the topsoil in such a way that a rock fragment mulch develops in na rrow valleys and at the foot of the slopes. These results clearly indi cate that tillage erosion is the main process responsible for the obse rved rock fragment cover pattern in the study area, Since the study ar ea is representative for many parts of southern Spain where almond gro ves have expanded since 1970, the results have a wider application. Th ey show to what extent intensive tillage of steep slopes has contribut ed to the increase in soil degradation, to changes in hillslope morpho logy (i.e. strong denudation of convexities, development of lynchets a nd rapid infilling of narrow valley bottoms) and to the development of rock fragment cover patterns which control the spatial variability of the hydrological and water erosion response within such landscapes.