Surface runoff

Typically snowmelt will peak in the spring[8] and glacier melt in the summer,[9] leading to pronounced flow maxima in rivers affected by them.In these regions, even on less infertile cracking clay soils, high amounts of rainfall and potential evaporation are needed to generate any surface runoff, leading to specialised adaptations to extremely variable (usually ephemeral) streams.[15] When the soil is saturated and the depression storage filled, and rain continues to fall, the rainfall will immediately produce surface runoff.Erosion of silty soils that contain smaller particles generates turbidity and diminishes light transmission, which disrupts aquatic ecosystems.On the high central plateau of Madagascar, approximately ten percent of that country's land area, virtually the entire landscape is devoid of vegetation, with erosive gully furrows typically in excess of 50 meters deep and one kilometer wide.[32] Surface runoff occurring within forests can supply lakes with high loads of mineral nitrogen and phosphorus leading to eutrophication.Such land derived runoff of sediment nutrients, carbon, and contaminants can have large impacts on global biogeochemical cycles and marine and coastal ecosystems.Many world regulatory agencies have encouraged research on methods of minimizing total surface runoff by avoiding unnecessary hardscape.[41] Erosion controls have appeared since medieval times when farmers realized the importance of contour farming to protect soil resources.In the 1960s some state and local governments began to focus their efforts on mitigation of construction runoff by requiring builders to implement erosion and sediment controls (ESCs).This included such techniques as: use of straw bales and barriers to slow runoff on slopes, installation of silt fences, programming construction for months that have less rainfall and minimizing extent and duration of exposed graded areas.[42] Flood control programs as early as the first half of the twentieth century became quantitative in predicting peak flows of riverine systems.Some of the techniques commonly applied are: provision of holding ponds (also called detention basins or balancing lakes) to buffer riverine peak flows, use of energy dissipators in channels to reduce stream velocity and land use controls to minimize runoff.Methods commonly applied are: requirements for double containment of underground storage tanks, registration of hazardous materials usage, reduction in numbers of allowed pesticides and more stringent regulation of fertilizers and herbicides in landscape maintenance.The U.S. Clean Water Act (CWA) requires that local governments in urbanized areas (as defined by the Census Bureau) obtain stormwater discharge permits for their drainage systems.[44][45] Essentially this means that the locality must operate a stormwater management program for all surface runoff that enters the municipal separate storm sewer system ("MS4").These models considered dissolution rates of various chemicals, infiltration into soils, and the ultimate pollutant load delivered to receiving waters.One of the earliest models addressing chemical dissolution in runoff and resulting transport was developed in the early 1970s under contract to the United States Environmental Protection Agency (EPA).The benefit of the Monte Carlo analysis is not to decrease uncertainty in the input statistics but to represent the different combinations of the variables that determine potential risks of water-quality excursions.SELDM provides a method for rapid assessment of information that is otherwise difficult or impossible to obtain because it models the interactions among hydrologic variables (with different probability distributions), resulting in a population of values representing likely long-term outcomes from runoff processes and the potential effects of various mitigation measures.SELDM also provides the means for rapidly doing sensitivity analyses to determine the possible effects of varying input assumptions on the risks for water-quality excursions.