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Surface runoff, or overland flow, is that part of precipitation that is left after inception or wetting of plants and other surfaces, infiltration into the topsoil, and surface storage in little pits and rills. Runoff refers to the water leaving an area of drainage and flowing across the surface to points of lower elevation. Runoff means that rainfall intensity exceeds the soil’s infiltration rate; a thin water layer forms on the surface that begins to move because of the influence of slope and gravity; flowing water accumulates in depressions, and forms small rills, which merge to form larger streams and rivers.
Surface runoff is a main cause of water erosion, after heavy raindrops have already caused the detachment of soil particles (splash erosion). The amount of surface runoff, and thus of erosion, is determined by the infiltration of water into the soil. The higher the infiltration from a single storm, the less surface runoff occurs. Infiltration is hindered by compacted or sealed surfaces. Therefore, high rainfall intensities often result in high runoff rates. A dense vegetation cover is the best protection against compacting and accelerates infiltration. Intensive cultivation combined with insufficient provision of organic matter can leave soil prone to crusting and compaction, and consequently to erosion. Heavy machinery and poor timing of agricultural operations often cause soil compaction.
Unsaturated overland flow is common in dry lands, where rainfall intensities are high and the soil infiltration capacity is reduced. Runoff can detach and transport large amounts of soil, as well as nutrients, phosphates, and pesticides. Erosion is one of the most serious consequences of runoff, particularly on agricultural land and other areas with low vegetation density. Land cover change and intensive land use lead to a decline of physical and hydrological soil properties, making the soil even more susceptible to erosion. With runoff it is possible that significant amounts of fertilizers and pesticides reach rivers or leach into groundwater. Nutrient and phosphorus pollution originating from agricultural operations are the main causes of water quality decline in lakes and rivers, adding up to more than 50 percent of the nutrient load in European rivers.
In urbanized watersheds, the infiltration capacity of the soil is greatly reduced because of impervious surfaces, such as pavements, streets, and buildings. Thus, runoff with higher peaks and larger volumes occurs, which in turn reduces groundwater recharge, thus lowering the water table. This can lead to the worsening of droughts because of low groundwater recharge and low dry season flows in rivers, and floods, which are particularly critical for agriculture.
In 2006, one in six people worldwide lived in the potential path of a 100-year flood, or roughly one billion people. That number is expected to double by 2050 due to climate change, deforestation, rising sea levels, and population growth in flood-prone areas. For example, in 2002, monsoon rains resulted in floods that submerged half of Bangladesh. Floods transport sediment and silt, which affect homes and crops. Floodwaters gave way to outbreaks of water-borne diseases that affect the most vulnerable groups-those already living below the poverty line. Poor people loose their basis of living in such events and often become dependent on humanitarian assistance.
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