The flow at downstream locations is determined by the quantity, location, and timing of water reaching a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases. Some precipitation evaporates, some slowly percolates through soil, some may be temporarily frozen as snow or ice, and some may quickly runoff from roofs, rock, pavement, saturated or frozen ground, among other surfaces. For light rain on dry, level ground, the proportion of incident precipitation that immediately reaches a drainage channel ranges from zero to as high as 170 percent for warm rain on accumulated snow.
The majority of precipitation records are based on water depth measurements taken within a predetermined time frame. The number of measurements during the entire observation period that exceed a particular precipitation threshold can be used to calculate the frequency of that threshold. By dividing each measured depth by the time between observations, intensity is converted from individual data points.
If the duration of the rainfall event was shorter than the fixed time interval for which measurements are reported, this intensity will be lower than the actual peak intensity. Unlike orographic precipitation, convective precipitation events (thunderstorms) typically have shorter durations. For the purpose of flood prediction, the duration, intensity, and frequency of rainfall events are crucial. In small drainage basins, precipitation of short duration is more important for flooding.