The effect of tillage and residue cover on runoff and soil loss from two land units

English:Land degradation, due to soil erosion, is a serious problem in many parts of the world. Productivity of large areas of cultivated land is decreasing due to severe soil degradation. A major factor responsible for the degradation of this natural resource is accelerated soil erosion. Water erosion is responsible for the biggest share of this degradation, contributing about 50-60%. This shows that soil erosion by water is the most important form of human induced land degradation. Every year, erosion undermines the sustainable use of land and land resource and threatens the livelihood of those depending on agriculture and beyond. Choosing the most appropriate tillage practices for a particular soil often decreases soil erosion and increases available water for crops. A conservation tillage practice such as no-tillage is generally credited with reducing soil losses when compared with conventional tillage. Field experiments were conducted on two land units. The first experiment was conducted at the University of the Free Sate (UFS) experimental site (South Africa) on a sandy soil with 8.4% clay in the topsoil under simulated rainfall conditions. The second field study was conducted at the Alemaya University (AU) experimental site (Ethiopia) on a clay soil with 45.1% clay in the topsoil. At both sites, the experiment consisted of three tillage practices, namely no-tillage, stubble mulch (traditional tillage for the experiment at AU) and conventional tillage with mouldboard ploughing, combined with four rates of wheat (Triticum aestivum L.) residue, namely 0, 2, 4 and 8t/ha. The experiment at AU was conducted for two consecutive main rainfall seasons. The results of the experiment at the UFS showed that the type of tillage had a significant effect on the initial infiltration rate of soil whereas the final infiltration rate was affected by the residue amount. Runoff and soil loss were also affected by the residue rate. Both runoff and soil loss decreased significantly with an increase in residue rate. When averaged over the four rates of residue cover, no-tillage had the highest runoff and soil loss followed by the stubble mulch tillage. Conventional tillage had the lowest runoff and soil loss. Given the type of soil, which was sandy without structure, conventional tillage practice appeared to have created structure which increased the infiltration rate and consequently decreased runoff and soil loss. A substantial decrease in runoff and soil loss was obtained when conventional tillage practice was combined with residue cover. On conventional tillage practices with higher residue cover rates, such as 4 and 8t/ha, the infiltration rate remained close to the rain application rate, thus controlling runoff and soil loss. Generally it was observed that a residue cover rate of 2t/ha was sufficient to effectively reduce runoff and soil loss on all the three tillage practices. The results of the experiment at the AU showed similar effects of tillage and residue cover on runoff and soil loss as that of the UFS. When averaged over the four rates of residue cover, no-tillage had a higher runoff and soil loss compared with the traditional and conventional tillage practices. It was also observed that rainfall characteristics in general and rainfall intensity in particular were found to be among the important factors affecting runoff and soil loss. The amount of residue cover required to effectively control runoff and soil loss was dependent on the rainfall intensity. Similar to the UFS site, a residue cover rate of 2t/ha was sufficient to effectively reduce runoff and soil loss for most of the erosive storms, with the exception of a single high intensity storm for which residue rates of 4t/ha and higher was required to control erosion. Soil loss was very well related to the rainfall erosivity index and accordingly classes of erosivity indices were defined where low to high soil losses may be expected.Comparison of results from the two land units showed that, generally, both tillage and residue cover affected runoff and soil loss in a similar way, but to a different degree. The general tendency reported in literature towards the superiority of no-tillage compared with conventional tillage could not be found in this study. At the UFS site, conventional tillage was found to be more effective than no-tillage in reducing runoff and soil loss. Although results for more seasons are required to draw a final conclusion for the AU site regarding the effectiveness of tillage, it was found that no-tillage was less effective in conserving water and soil compared to traditional and conventional tillage. It was therefore recommended that farmers should use tillage practices consisting of loosening of the soil, combined with maintaining at least 2t/ha or 62% cover of wheat residue. An attempt was made to predict runoff from rainfall characteristics (amount and intensity) for the AU site. Empirical relationships, established between runoff and rainfall amount, with the inclusion of all rainstorms and for erosive storms only, from the first year's data were used to predict runoff for the second year data. The predicted values agreed well with the measured ones for both conditions. The indices of agreement for the two approaches were 0.79 and 0.89, for the equation based on only erosive storms and on all storms respectively.Another approach was followed for the prediction of runoff from rainfall intensities and infiltration rates of the soil. A procedure was developed, based on area under the curve method, for the first year's data and it was then used to predict runoff for the second year's data. The predicted runoff values, using the area under the curve method, were comparable to the measured values for the same season. The agreement was good as shown by the high index of agreement (D-index = 0.92) between the two sets of values. This procedure was also used to estimate runoff at the UFS site, from the simulated rainfall intensity and infiltration rate obtained from the double ring infiltrometer. It was found that runoff could be estimated using this procedure when the predicted values were corrected for the depth of tillage.