Overland flow

Surface runoff. Hydrologists have identified two processes for generating surface runoff over land. The first, saturated overland flow (SOF), is generated when precipitation (or snowmelt) occurs over a saturated soil since water has nowhere to infiltrate, it then runs off over land. SOF typically occurs only in humid environments or where the water table rises to intersect with a stream. Horton overland flow (HOF or infiltration-limited overland flow) occurs when precipitation intensity exceeds the infiltration capacity of the soil in a non-saturated environment. In this case, only the excess precipitation (that exceeding the infiltration capacity) runs off over the surface. Both types of overland runoff generate relatively rapid flows that constitute the surface water contribution to the hydrograph (Fig. 6-6). 

Fig. 8-7 Three principal ratios control the style of runoff generation prevalent in a landscape (1) ratio of rainfall intensity to the infiltration capacity of the soil (2) ratio of bedrock conductivity to soil conductivity and (3) the topographic index defined by the ratio of the upslope drainage area to the ground slope. HOF = Horton overland flow SOF = saturation overland flow SSS = subsurface stormflow GWR = groundwater flow.

Surface water information, including drainage patterns (overland flow, topography, channel flow pattern, tributary relationships, soil erosion, and sediment transport and deposition), surface water bodies (flow, stream widths and depths, channel elevations, flooding tendencies, and physical dimensions of surface water impoundments structures surface water/ groundwater relationships), and surface water quality (pH, temperature, total suspended solid, salinity, and specific contaminant concentrations)... 

More research and demonstration activity should be devoted to water harvesting, which can be considerably useful not only in reducing irrigation requirements but also in the reduction of overland flow and consequently in the protection of soils from water erosion, as well as in leaching soils from salts accumulated with irrigation water. The solution of tied ridges, or diked furrows, to be obtained either by animal energy or when possible with the use of mechanical equipment, has been... 

In general, soluble and nom-eactive contaminants are found mainly in dissolved form in runoff water. For example, a large percentage (up to 90%) of the most soluble herbicides present in the soil layer may be partitioned in overland flowing water. A substantial portion of dissolved nitrogen (8-80%) and phosphorus (7-30%) also may be transported in runoff water (Menzel et al. 1978 Hubbard et al. 1982 ... 

Quick responses are typical for the most vulnerable zone between 1,000 and 1,800 m. They are produced by high rainfall intensity in combination with steep gradients and thin soils. In many cases an extensive network of streams ensures a high specific discharge. The processes of bedload mobilization and transport are stimulated by overland flow, which is an important component of runoff generation in this zone. 

Tyrrel, S. F. and Quinton, J. N. (2003). Overland flow transport of pathogens from agricultural land receiving faecal wastes. Soc. Appl. Microbiol. Symp. Ser. 32, 87S-93S. 

Model computations begin with an estimate of the radioactive aerosol content of the infiltrating solution. This may be based on either direct input from vegetal storage in the absence of overland flow or the sheet flow aerosol concentration computed on the basis of exchange reactions during overland runoff. The known value of the surface soil content prior to infiltration is also used to determine the total content of the... 

The beginning radionuclide concentration values for individual plates are initial conditions determined by the soil concentration profiles. Thus, a continuous tabulation of the concentration profiles is maintained during simulation. This is of primary importance during rain periods when no overland flow occurs since it will tend to alter the surface soil concentration. Decreases in the surface layer radioaerosol concentration arising from infiltration will cause a corresponding decrease in subsequent surface runoff aerosol transport. 

Walsh, R. P. D., Boakes, D., Coelho, C. O. A., Goncalves, A. J. B., Shakesby, R. A., and Thomas, A. D. (1994). Impact of fire-induced hydrophobicity and postfire forest fitter on overland flow in northern and central Portugal. In Viegas, D. X., ed., Second International Conference on Forest Fire Research, Coimbra, Portugal, 21-24 November 1994. 

Pearce, R.A., M.J. Trlica, W.C. Leininger, D.E. Mergen, and G. Frasier (1998). Sediment movement through riparian vegetation under simulated rainfall and overland flow. J. Range Manage., 51(3) 301-308. 

According to Fetter (1988), the following three scenarios can be used to describe the relationship between precipitation and infiltration rates (1) when the total precipitation rate is lower than the equilibrium infiltration capacity all of the precipitation reaching the land surface should infiltrate (2) when the precipitation rate is higher than the equilibrium capacity and less than the initial infiltration capacity (at the start of the precipitation event) all of the precipitation will infiltrate initially and (3) when the precipitation rate is greater than the initial infiltration capacity there is an immediate accumulation of water on land. In general, when the precipitation rate exceeds infiltration capacity, overland flow of water occurs, commonly referred to as Horton overland flow (HOF) (Horton, 1933, 1940). 

Figure 3.4 Hypothetical storm hydrograph for a period of 2 days (with evenly distributed precipitation) showing how changes in overland flow are responsive to precipitation and correlate well with changes in total stream flow. Conversely, groundwater inputs (baseflow) remain essentially constant during this period. (Modified from Fetter, 2001.)...

When precipitation rate exceeds infiltration capacity, overland flow of water occurs, commonly referred to as Horton overland flow. 

Human exposure to any particular chemical through drinking-water requires a source of that chemical and a pathway from the source of contamination to the consumer. Pathways for transport of chemicals may be through natural features such as aquifers, surface water bodies, soils and rock, or overland flow, or through human-made components of water supply systems,... 

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