The paper describes the results of the evaluation of erosion on arable slopes (in the thalweg part of the ancient microravines, no more than 400 m in length). The radiocesium method has been applied, augmented by the methods of the topographic survey, remote sensing and GIS analysis. It has been shown that layer-by-layer soil sampling in the thalwegs of the modern streams (which are clearly visible on arable slopes) allows obtaining the dependence of soil runoff (in centimeters of layer) from the activity of Cesium-137 presented in the arable horizon. Conversion of Cesium-137 activities onto a common time scale (by taking into account the radioactive decay) made it possible to increase the analyzed data series (in comparison to the previously published data) and to obtain the linear dependence with the correlation coefficient of 0.98 (significant at confidence level p=0.05). The 1 cm soil layer was washed out during 26 years on the watershed surface, according to the obtained dependence. About 6 cm layer over the same period was washed out in the thalwegs of the ravines in the lower part of the slope (in its concave part that is accumulation zone). We have found that very high resolution satellite imagery, or aerial photographs, of field surveys are needed for the correct implementation of the GIS analysis when modeling erosion activity of the streambeds. These supporting data have to be collected during the period of snowmelt or rainfall to understand the behavior of streams in the accumulation zones identified by negative values of profile curvature of the relief.