Rainfall Losses

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“The flood hydrologist is primarily concerned with:

1. "Interception by vegetation and subsequent evaporation or retardation when reaching the ground surface.
2. "Evaporation from ground surface during prolonged rainfall events or when accumulated in frozen form from snowfall, which is generally termed “sublimation.”
3. "Retention in surface depressions that act as miniature reservoirs that do not release their waters until their storage capacity is exceeded, and then only in relation to a stage versus discharge relationship comparable to an uncontrolled spillway on a reservoir.
4. "Infiltration into the receiving soil, rock, or combination thereof."

"Any of the constituents of the Earth’s mantle have a capability to absorb water, whether it is a concrete airport parking and loading area or the most sandy of soils comprising some areas of the arid West. The first three of the above loss processes are usually low when compared with infiltration when rainfall intensities are sufficient to produce severe flood events such as the PMF. Under such conditions, the first three are often grouped with part of the infiltration loss and termed “initial losses,” and assumed to have been satisfied by antecedent rainfall occurring prior to the onset of the PMS."

"To illustrate the phenomena that occurs in the soil when water is applied in the form of rain, consider a condition at the onset of a rainstorm where the soil is comparatively dry as a result of no precipitation having recently occurred. Initially, part of the precipitation is intercepted by vegetation and, after the vegetation has reached its capacity to retain water by surface tension, additional precipitation simply runs off the leaves, stems, etc., and falls to the ground. Also, part of the rainfall falls directly on the ground surface and enters the soil. In nature, some of this precipitation evaporates back into the atmosphere; however, in the hydrologic analysis of floods, interception and evaporation losses are so small compared to the magnitude of the precipitation that they are neglected."

Best Practices Resources

National Engineering Handbook: Chapter 7 - Hydrologic Soil Groups, NRCS

National Engineering Handbook: Chapter 9 - Hydrologic Soil-Cover Complexes, NRCS

National Engineering Handbook: Chapter 10 - Estimation of Direct Runoff from Storm Rainfall, NRCS

Flood Hydrology Manual, USBR

Citations:

Revision ID: 7346
Revision Date: 07/21/2023