Overland flow, runoff

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.

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. 

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. 

Different rainfall weighting methods can substantially affect estimates of new/old waters in storm runoff in basins with large contributions of new water (McDonnell et al., 1990). Use of sequential rain values is probably the best choice in very responsive catchments or in catchments with high proportions of overland flow. When rain intensities are low and soils drain slowly, current rain may not infiltrate very rapidly and thus use of the cumulative approach (i.e., running average) is probably more realistic (Kendall et al., 2001a). 

As rainfall decreases, overland flow also decreases, but subsurface runoff continues for several days after the storm as water gradually drains from the soil. This explanation assumes that three water sources of differing chemical characteristics contribute to stream flow, in contrast to the assumption of two water sources made by other authors, for example, Pinder and Jones (45). 

Suspended-sediment concentrations increased sharply on rises and reached a maximum at or, commonly, shortly before peak discharge (Figures 6 and 7). Peak sediment concentrations consistently occurred at times of minima in silica concentration. This is to be expected if minimum silica and maximum sediment concentrations coincide with maximum contribution of overland runoff to stream flow, for at that time maximum dilution of silica-rich subsurface flow by direct runoff would occur and maximum erosive capability of direct runoff would exist. If this reasoning is correct, peak stream discharge would coincide with peak contribution from overland flow only when rainfall was intense or the surface soil was of very low permeability. Normally, peak direct runoff would precede peak stream discharge because of the significant contribution of subsurface flow at maximum stream discharge. 

Because the minimum in silica concentration and maximum in sediment concentration normally precede peak discharge, peak overland flow probably also precedes peak discharge. Therefore, runoff that has spent an appreciable period of time in soil pores contributes a major part of the stream flow at peak discharge. The rather small decrease in silica concentration during a stream rise supports this interpretation. Because little silica can be obtained by interaction between water and stream sediment... 

If it can be shown that silica minima and sediment maxima mark peak overland flow elsewhere and that the end of overland flow is marked by a leveling off in silica concentration after peak discharge in stream flow, then monitoring of these parameters should be very helpful in separating storm runoff into the various components of flow. 

Surface Runoff and Soil Erosion. Surface runoff is overland flow ... 

Most runoff from TA-V flows west onto TA-III as overland flow and in natural and manmade surface drainage features. The remainder flows into two storm sewers in the northern portion of TA-V. Both storm sewers discharge to open channels within, and just north of, TA-III. Drainage from TA-III is to the west onto undeveloped portions of KAFB and then into playas on undeveloped state land. At present there is no requirement for monitoring runoff from TA-V because the runoff flovwng from that area is not discharged to Waters of the United States (SNL 1998c). 

The hydrologic cycle, depicted in Fig. 1, involves five fundamental steps evapotranspiration, transport of water vapor, condensation/precipitation, overland flow/ percolation, and channel runoff. Evapotransporation is tlie vaporization of water through the combined processes of evaporation from surface water and transporation from plants. Sublimation of frozen water in glaciers is relatively small but is effectively included in the general category. The amount of water vapor that can be held in the atmosphere is a function of the ambient temperature and can be approximated using Eqs. (1) and (2)... 

The preferred on-site hydrology method is the rational method, requiring a minimum concentration time of 10 minutes. Often the concentration time for the contributing on-site pavement runoff is less than 10 minutes. The initial concentration time is determined by an overland flow method until the runoff is concentrated in a curbed section. Channel flow using the roadway-curb cross-section should be used to determine velocity and flow time to the first inlet. The channel flow velocity and flooded width are calculated by Manning s formula ... 

Formulation and initial placement influence the susceptibility of organophosphorus insecticides to transport in surface runoff, as well as their degradation by abiotic and microbial processes. Formulation affects the kinetics of insecticide release into soil water and overland flow, as well as sorption to soil solids and plant surfaces. Spray adjuvants affect initial placement by influencing the amount of insecticide depositing on foliar and soil surfaces. Initial placement determines the relative importance of such processes as volatilization, photolysis, biodegradation, and leaching out of the zone of interaction with overland flow. 

After organophosphorus insecticides are deposited on or incorporated into the soil matrix, they distribute themselves between the air, soil air, soil water and soil solid phases. Insecticides applied to foliar surfaces partition between the plant cuticle, air and any water present on the leaf surface (e.g., dew). Phase distribution affects the amount of organophosphorus insecticide available for transfer into overland flow, as well as the mode of transport in surface runoff. 

Dissolved insecticides are transferred from soil solution to surface runoff through the concurrent mechanisms of molecular diffusion, raindrop impact induced turbulent diffusion, and shear stress induced mass transfer (63, 64). In addition, shallow interflow may contribute dissolved chemicals to surface runoff as it returns to the surface downslope or seeps into rills and furrows 65). Most studies of dissolved chemical transport into overland flow have employed inorganic tracers such as bromide, gypsum (CaS04 2H20) and 66, 67). The behavior of organophosphorus insecticides, however, is considerably more complex due to association with particulate and colloidal natural organic matter. 

Solute concentrations in surface runoff are typically significantly lower than those in soil water, even when measured just below the soil surface 68). Molecular diffusion of solutes occurs in response to the concentration gradient between the soil solution and runoff water. Although turbulent diffusion enhances the transfer of solutes from soil solution into overland flow, molecular... 

As water flows over the soil surface, solute mass is transferred via mechanical dispersion from pore spaces into overland flow (64, 69). Shear stresses generated by overland flow accelerate removal of solutes from porous media into surface runoff (64, 72). The dispersion coefficient, D, resulting from overland shear flow increases in proportion to soil permeability, k, and the square of shear velocity, w D oc ku (64, 72). Flow regime is an important determinant of the mechanism responsible for interfacial frictional resistance between the porous medium and overland flow. 

Once dissolved pesticides have been extracted from the soil matrix into overland flow or reach the surface through shallow interflow, they are transported toward the field outlet with surface runoff For one-dimensional overland flow, dissolved insecticide transport can be expressed as 76) ... 

While the occurrence of most processes affecting organophosphorus insecticide concentrations in surface runoff are well documented, many are relatively poorly understood on a mechanistic level. Key processes requiring further study include sorption to plant and soil surfaces, and transfer from soil water into overland flow. Sorption to foliar surfaces determines the amount of applied insecticide available for washoff onto the soil surface or into surface runoff The kinetics of desorption from plant cuticular materials and the effect of adjuvants on foliar sorption processes need additional research. 

The excess of evaporation from the oceans is made up for by runoff from the land. Although this flux is much smaller than precipitation and ET, it is a major link in many cycles and is of particular importance to humans in terms of water supply. Runoff can be broadly categorized into subsurface, or groundwater, flow and surface flow, consisting of overland runoff and river discharge. 

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