Factors affecting runoff may be divided into those associated with the precipitation, and the watershed characteristics.
Factor # 1. Precipitation:
Since by far the largest portion of precipitation occurs as rain, and the rainfall directly affects soil erosion, the characteristics of rainstorm and intensity, duration and frequency of rainfall are of interest in describing the runoff.
(a) Characteristics of Rainstorm:
Storm patterns play an important role in determining the extent of runoff. It is generally observed that no two rainstorms have exactly the same time-intensity relationship. Based on their characteristics the following four storm patterns (Fig. 3.2), representing a common arrangement of rainfall intensities within a storm, have been described.
(i) Uniform intensity,
(ii) Advanced pattern,
(iii) Intermediate pattern, and
(iv) Delayed pattern.
The advanced pattern of rainfall brings higher intensities when the infiltration rate is the greatest, thus causing some reduction in the runoff peaks. On the other hand, the delayed pattern causes higher runoff peaks, as the high intensities occur when the infiltration is at a minimum and depression storage has been largely satisfied.
(b) Intensity, Duration and Frequency of Rainfall:
One of the most important rainfall characteristics is the rainfall intensity, usually expressed in centimeter per hour. Rainfall intensity influences both the rate and the volume of runoff. An intense storm exceeds the infiltration capacity by a greater margin than does a gentle rain.
Thus, the total volume of runoff is greater for the intense storm even though the total precipitation for the two rains is the same. Very intense storms, however, are not necessarily more frequent in areas having a high total annual rainfall. Also storms of high intensity generally last for fairly short periods and cover small areas.
Such storms may produce no runoff due to high initial infiltration capacity. Storm covering large areas are seldom of high intensity but may last for several days and, therefore, result in runoff.
The infrequent combination of relatively high intensity and long duration gives large total amount of rainfall. These storms do much erosion damage and may cause devastating floods. Intense storms of varying duration are common in almost every part of our country.
However, the probability of these heavy rainfalls varies with the locality. For designing a water control structure, it is necessary to determine the probable recurrence of storms of different intensity and duration. The return period or the recurrence interval refers to the average period of time within which the magnitude of rainfall for a given duration will be equaled or exceeded once on the average.
Intensity of rainfall and duration can be determined with the help of automatic rain gauges. Such a rainfall characteristics for 42 rain gauge stations of India were analysed by Ram Babu et al. (1979), who observed the following intensity-duration-return period relationship.
I = K Ta/(t + b)n â€¦(3.1)
where, I = intensity of rainfall, cm/hr
T = return period desired, years
t = duration of rainfall desired, hours
K, a, b, n = Constants (Table 3.2)
For design considerations, where the failure of structure would not lead to a serious loss of property or life, the return period of a storm causing peak flow has been related with type of structure (Table 3.3).
Factor # 2. Watershed Characteristics:
Generally, a watershed refers to a body of soil with definite boundaries around, above and below it. The watershed factors affecting runoff are type of catchment, physical nature of soil, degree and length of slope, distribution and kind of vegetal cover, and its size and shape. Both runoff volume and rate increases as watershed size increases.
However, both rate and volume per unit of watershed area, decreases as the runoff area increases. Thus, watershed size may help to determine the season in which high runoff may be expected to occur.
Long narrow watersheds are likely to have lower runoff rates than more compact watersheds of the same size. This is because the former does not concentrate as quickly as it does from the compact area and also long watersheds are less likely to be covered uniformly by intense storms.
Topographic features, such as slope of upland areas, the degree of development and gradients of channels, and the extent and number of depressed areas affect rates and volume of runoff.
Watersheds having extensive flat areas or depressed areas without surface outlets have lower runoff than areas with steep, well defined drainage patterns. Geology, extent and type of vegetation determine to a large extent the infiltration capacity of soil and hence the runoff volume.