Soil erosion is a major aspect of the human role in modifying the face of the Earth. Much surface erosion is caused by surface runoff. This in turn is often controlled by the infiltration capacities of surface materials. These can be affected by land cover and land use, such as the introduction of grazing by domestic stock, the replacement of grassland with shrubland, and deforestation. In addition, there are other structural properties of soils that affect their erodibility, including the nature of their aggregates and their humus contents. Soil compaction by agricultural activities reduces the rate of water infiltration into the soil, which may change the soil moisture status and accelerate surface runoff and soil erosion. The equivalent of many decades of natural erosion may take place during a single year in areas cleared for construction. The presence of roads also increases sediment yield, especially in tropical areas with intense rainfall. Because fires remove vegetation and expose the ground they too tend to increase rates of erosion. There is some evidence that irrigation can lead to soil erosion. Soil erosion has a long history in the Mediterranean lands, but rates are currently changing due to land abandonment and to the establishment of olives and other orchards. Numerous techniques have been applied to try and reduce the amount of sediment that is moved, and for the study of soil erosion rates, some of them relating to short time scales and small spatial scales (e.g. plot studies), some of them to medium spatial and temporal scales (e.g. reservoir sedimentation data), but others taking a longer or larger perspective (e.g. sediment accumulation in lakes). There are many examples of humans accelerating mass movements by vegetation removal, loading of debris onto slopes, changing groundwater conditions, excavating the bases of slopes, and cutting away the toes of debris flows. Under increased greenhouse gas concentrations some GCMs exhibit enhanced global precipitation intensity. This could accelerate erosion. Likewise, fire activity may increase in a warmer and drier environment and this could cause greater erosion and debris flow generation. Geomorphologists have started to model changes in soil erosion and mass movements that may occur as a consequence of changes in rainfall amounts and intensity, glacial retreat, and the melting of permafrost.