E6 Surface Runoff and Flow Generation

Topic

Surface runoff is the flow of water moving across the ground surface until it eventually reaches the basin's watercourses. It is generated by two main mechanisms: Horton runoff—previously mentioned in the context of infiltration—occurs when the rainfall rate exceeds the soil's infiltration capacity; saturation runoff, on the other hand, occurs when the soil is already fully saturated and the water table reaches the surface, generating runoff even at low rainfall rates. The combination of climate, soil texture, and the terrain's antecedent moisture condition determines which of these two mechanisms dominates during any given rainfall event.

The unit hydrograph is a transfer function specific to each basin; it describes how the basin's discharge responds to one millimeter of effective runoff distributed uniformly over a unit rainfall duration. It is precisely this function that transforms the effective precipitation falling on the basin into the discharge ultimately measured at the basin's outlet.

Convolving this unit hydrograph with the effective precipitation from an actual event produces the flood hydrograph described elsewhere in this series; the time of concentration—defined as the time required for rainfall at the basin's most distant point to reach the outlet—determines the duration of that unit hydrograph.

Separating the various components of a hydrograph makes it possible to distinguish between direct flow (the faster surface runoff that responds within hours) and baseflow (the groundwater discharge described in the section on aquifers, which responds much more slowly, over days or even months).

Furthermore, analyzing the recession curves of this baseflow allows for the quantification of both the feeding aquifer's storage and its discharge rate—properties previously linked to the aquifer's transmissivity and storage coefficient. The expected behavior of this entire process of flow generation includes flood hydrographs with a time-to-peak that is inversely proportional to the basin size and directly proportional to the intensity of the rainfall that triggered them. Thus, spanning everything from the two generation mechanisms with which this document begins to the proportionality of the time-to-peak with which it concludes—and encompassing the unit hydrograph, time of concentration, the separation of direct flow from baseflow, surface runoff, and flow generation—these processes ensure that every raindrop falling on a basin eventually transforms into the flow measured at that basin's outlet.

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