<P> The other context of agricultural issues involves the transport of agricultural chemicals (nitrates, phosphates, pesticides, herbicides etc .) via surface runoff . This result occurs when chemical use is excessive or poorly timed with respect to high precipitation . The resulting contaminated runoff represents not only a waste of agricultural chemicals, but also an environmental threat to downstream ecosystems . </P> <P> Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream . The frequency with which this occurs is described by a return period . Flooding is a natural process, which maintains ecosystem composition and processes, but it can also be altered by land use changes such as river engineering . Floods can be both beneficial to societies or cause damage . Agriculture along the Nile floodplain took advantage of the seasonal flooding that deposited nutrients beneficial for crops . However, as the number and susceptibility of settlements increase, flooding increasingly becomes a natural hazard . In urban areas, surface runoff is the primary cause of urban flooding, known for its repetitive and costly impact on communities . Adverse impacts span loss of life, property damage, contamination of water supplies, loss of crops, and social dislocation and temporary homelessness . Floods are among the most devastating of natural disasters . </P> <P> Mitigation of adverse impacts of runoff can take several forms: </P> <Ul> <Li> Land use development controls aimed at minimizing impervious surfaces in urban areas </Li> <Li> Erosion controls for farms and construction sites </Li> <Li> Flood control and retrofit programs, such as green infrastructure </Li> <Li> Chemical use and handling controls in agriculture, landscape maintenance, industrial use, etc . </Li> </Ul>

The of the land affects the amount of runoff water