Interaction of land use, slope gradient and rain sequence on runoff and soil loss from weakly aggregated semi-arid soils

The effects of land use, tillage methods and slope gradient on soil erosion have been widely investigated. However, there is limited information about their effects on very weakly aggregated soils that can be found in semi-arid, nutrient limited regions of low productivity. This study was carried out to investigate the interaction of land use, tillage method and slope gradient on runoff and soil loss in a weakly aggregated soil in the northwest of Iran. Plots (3 m x 10 m) with three treatments and two replicates were installed in each of four slopes gradient categories (12.6, 15.3, 17.0, 19.4%). The three treatments were pasture with poor vegetation cover (27%) and rainfed wheat either cultivated up to down slope or along the contour. Wheat was sown in early autumn and runoff and soil loss was measured under natural rainfall during the eight months long growing period. Out of 53 precipitation events, 8 storms were classified erosive. The susceptibility of all plots to produce runoff and soil loss increased pronouncedly and highly significantly with subsequent rainfalls. The potential of a rainfall to produce runoff and soil loss was thus highly dependent on preceding rainfalls. Wheat cultivation increased runoff 13 times and soil loss 60 times compared to pasture. Runoff and soil loss increased about 5.5 and 35 times in plots cultivated up to down slope as compared with contour cultivated plots. The effects of land use increased pronouncedly with increasing slope gradient. These results showed that erosivities of rainfalls separated by several months were not independent from each other and that the effects of land use and slope gradient interacted. Also the effect of slope gradient on soil loss was much larger than estimated by current equations. All effects could be explained by the extremely weak aggregates. Soil erosion was thus not detachment limited but transport limited. Transport limitation decreased in the sequence of rainfalls due to loss of surface roughness and with increasing slope gradient due to increasing failure of roughness elements. Under conditions leading to extremely weak soil aggregates, land use and slope gradient thus exert a much larger impact on runoff and soil loss than what is commonly assumed. This should be better considered in soil conservation efforts but also in runoff and erosion modelling.