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Competency Area 2: Soil hydrology AEM

PO 10. Understand the characteristics of rainfall and the concept of return periods.

The characteristics of rainfall are the amount, the intensity, the duration, the frequency or return period, and the seasonal distribution.

The amount is of course important to the overall hydrologic cycle and replenishment of the soil water, and the amount is an accumulation or product of the intensity times the duration. For example, the amount may be the same for a high intensity short duration rainfall as it is for a low intensity long duration rainfall.

However, the intensity and duration can have a large influence on whether the rainfall infiltrates or becomes surface runoff. Higher rainfall intensity produces larger size raindrops which have more impact energy, and thus higher intensity storms can damage delicate vegetation and bare soil.  High intensity storms can literally displace soil particles, causing soil crusting or starting the soil erosion process. High intensity storms may also overwhelm the soils ability to infiltrate the rainfall at the same rate, causing infiltration-excess runoff.

The duration refers to the length of time rainfall occurs.  A high intensity rainfall for a short duration may affect tender seedlings, but it will not likely have much effect on soil erosion and runoff. Rainfall of longer duration can significantly affect infiltration, runoff, and soil erosion processes.

The frequency, or more specifically, the return period refers to how often rainfall occurs at a particular amount or intensity and duration. For example, rainfall return periods are referred to as 100 year-1 hour rainfall or 100 year-24 hour rainfall to define the probability that a given amount will fall within a given time period.

The seasonal distribution of rainfall refers to the time of year when various rainfall amounts occur. The seasonal distribution determines when surface runoff or deep percolation are most likely to occur or if irrigation is needed. Since the seasonal distribution of rainfall varies in different parts of the country, practices used or recommended in one part of the country may not necessarily be appropriate in another. For example, tillage practices to conserve moisture in Western Kansas are not likely necessary in New York. Or applying anhydrous ammonia in the fall in the upper Midwest may be appropriate, whereas following this same practice in the Northeast often leads to substantial nitrate leaching and little fertility benefit to the next year's crop.