In most cases, the prevalence of heavy or intense precipitation in the river basin is the primary cause of flooding. The meteorological variable of interest is the intensity of rainfall. In case of low-intensity rainfall, the rainwater infiltrates the soil and takes a relatively long period of time to reach the stream (by the subsurface or groundwater flow). If much rain prevails in a short time, intensity may be high enough to exceed the infiltration capacity of the soil (the ability of a soil to absorb water). The residual (falling water that is not absorbed by the soil) becomes surface runoff. Obviously, the potential for flooding is then great. Hence, next to heavy rainfall the absorption capacity is significant. This capacity depends on soil type and the amount of moisture already present in the soil. For example, sandy soils absorb much of the rain, but clay soils have very low infiltration capacities. When rainfall continues, soil moisture increases and the water holding capacity of the soil decreases. Whenever and wherever the rate of rainfall (or snowmelt) exceeds the infiltration rate at the surface, the excess water begins to accumulate at the surface. Even low-intensity rainfall will then produce surface runoff. Surface runoff is most likely to occur in the low portions of the catchment, near the streams, where the initial soil moisture content is highest. It is evident that the chance on flooding depends on the combination of rainfall and catchment characteristics. For example, high-intensity rainfall with limited duration may heavily affect a brook but will have little impact on large rivers. The latter are more sensitive to widespread rainfall over an extended period, especially when the area of rainfall is large enough to affect the feeding tributaries*.
These activities can have a noticeable effect on flow volume and peak magnitude, and timing of the peak for precipitation events that are not extreme in terms of duration and amount. However, when the amount and duration of precipitation increases (i.e., extreme rainfall events in terms of intensity and/or duration), the influence of the soil-plant system and human interferences in the catchment basin on runoff volume diminishes. Many studies indicate that the greatest increases in peak and flow volumes, and the occurrence of flooding take place under wet antecedent conditions.
The flow characteristics change in relation to the severity of disturbance of the ecological system and to the percentage of catchment area affected. Changes in streamflow become less evident downstream because of the combined changes in volume, peak and timing at the different upstream areas. For example, flow volume can increase as a result of a disturbance, but the magnitude of the peak discharge downstream can be reduced if upstream peakflows are desynchronised.
Reports on flow response to land treatments exhibit little consensus. Several studies have shown increases in flow volumes and peak discharge following deforestation. However, other studies indicate little effect or even reductions. For example, a study indicating a reduction in peak discharge following forest cutting, also showed that peak discharges were delayed by several hours. The delayed peaks were attributed to a larger soil disturbance due to a rougher surface with greater storage.
