Horton 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.

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). 

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

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