Overland runoff

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. 

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). 

Overland Runoff The fraction of rainfall or irrigation water that flows over a land surface from higher to lower elevations, known as overland runoff, is an additional pathway for contaminant transport. Runoff occurs when the amount of rain or irrigation water is greater than the soil infiltration capacity. The formation of a crust on the soil surface is a major contributor to runoff formation in arid and semiarid zones, because it decreases the infiltration capacity. The soil crust is a thin layer (0-3 mm) with a high density, fine porosity, and low hydraulic conductivity compared to the underlying soil. This skin forms as a result of falling raindrops or sodification of soil clays. 

Overland runoff may be expressed as a conservation of mass in a flow domain, where the excess rainfall or irrigation rate, R, is equal to the difference between the rainfall and infiltration rates ... 

The extent of transport of dissolved contaminants in overland runoff is controlled by the topography and morphology of the land (also affected by anthropogeiuc activity), the depth of chemical incorporation into soil, and the time between rainfall initiation and surface runoff commencement. In addition to these factors, transport of adsorbed contaminants on suspended particles is affected by rainfall intensity, which favors soil erosion. 

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... 

River water results mainly from two contributions surface runoff of falling rainwater and groundwater upflow. Surface overland runoff is the consequence of precipitation on saturated or impervious surfaces its behavior is cyclical because it depends on the wet and dry spells on Earth. 

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. 

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. 

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. 

Approximately one-half of the water contained in land is in the upper 1 km of the Earth, and the other half is in the next 4 km, as deep groundwater. Much of the surface water comes from precipitation. When the precipitation rate exceeds the infiltration rate into the soil, an excess of water builds up on the soil surface and moves overland as surface runoff, contributing to the streams, rivers, and other surface water sources. Part of the surface water is a contribution from spring water (i.e., groundwater that flows out to the surface owing to a physical condition that obstructs its flow under the Earth, or when the water table intersects the surface). 

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). 

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 ... 

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