It is generally recognized that runoff from a given watershed is influenced by two major groups of factors categorized as climatic and physiographic. These factors are evident in the form of Equation 6.1 with the climatic factors, precipitation and evapotranspiration, on the left side and the variables on the right side being related to physiographic factors. However, this is not an exclusive listing of the variables, but rather it is a synthesis of the influence of climatic and physiographic factors on the allocation of precipitation. A number of interactive processes are involved, and the identifying climatic and physiographic factors are largely a descriptive convenience.
Seasonal variability is a distinctive characteristic of the climatic factors influencing runoff. While the quantity of precipitation has an obvious dominant role, other precipitation characteristics are important in determining the proportion of precipitation that becomes runoff. The form, intensity, duration, space and time distribution, frequency of occurrence, and the direction of storm movement have identifiable influences on runoff.
Evapotranspiration is clearly a climatic factor because ETp is driven by the daily and seasonal regime of radiant energy. However, in the runoff process the quantity of interest is the actual moisture vaporized. Quantifying ETa requires consideration of numerous related characteristics that are indirectly climatic expressions. The presence or absence of vegetation is an obvious related influence on ETa, but when vegetation is present the scope of considerations expands significantly as seasonal changes in ETa influence the moisture allocated to runoff (Czikowsky and Fitzjarrald, 2004). Furthermore, vegetation intercepts precipitation, but interception in turn depends on the species, composition, age, and density of the vegetation, and interception varies with the season of the year and the intensity of the rainfall event. Since plant transpiration relies on water drawn from the soil, soil moisture storage and the antecedent condition of soil moisture influence the quantity of ETa.
Physiographic factors are expressions of the physical characteristics of the watershed and the stream channel. Compared to the climatic factors, these influences are relatively consistent over time but some display variations over time. One group of watershed characteristics that influence runoff are related to watershed geometry and include the size, shape, slope, orientation, elevation, and stream density of the basin (Ward, 1995). Other physical basin characteristics are land use and cover, surface infiltration, soil type, permeability and capacity of groundwater formations, and the presence of lakes and swamps. Stream channel characteristics that affect runoff are related to channel hydraulic properties including its size, shape, roughness, and length. These channel features determine the channel storage capacity and largely influence the timing of runoff rather than the quantity (Mosley and McKerchar, 1993; Ward and Robinson, 2000).
Interaction of the climatic and physiographic factors produces a runoff pattern that is characteristic for a particular watershed. In general, large
watersheds behave differently from small watersheds, but the dominant factor is not necessarily the size of the watersheds. Two watersheds of the same size commonly display a different runoff response. The reality is that one factor may be dominant in one watershed and may be inconsequential in a nearby watershed. The interplay of influences is complex and responsive to both climatic and physiographic changes that may occur in a watershed.
Figure 6.1 portrays the components of the watershed runoff process. Basin precipitation is partitioned following Equation 6.1, and precipitation exceeding ETa arrives at the stream channel by various flow paths. Storages are omitted in Figure 6.1 to emphasize the water flow paths. Space and time variations in the precipitation-streamflow response reflect the contrasting flow paths precipitation may follow in arriving at the stream channel. In the most general sense, when precipitation reaches the land surface to initiate the runoff process it is retained there, it infiltrates into the soil, or it moves to the stream channel. A sequence of intermediate steps underlies the actual runoff process.
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